U.S. patent application number 14/322104 was filed with the patent office on 2015-01-08 for microscale combustor-heat exchanger.
The applicant listed for this patent is Oregon State University. Invention is credited to Monte Kevin Drost, Mohammad Ghazvini, Vinod Narayanan, Brian K. Paul.
Application Number | 20150010874 14/322104 |
Document ID | / |
Family ID | 52133032 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150010874 |
Kind Code |
A1 |
Ghazvini; Mohammad ; et
al. |
January 8, 2015 |
MICROSCALE COMBUSTOR-HEAT EXCHANGER
Abstract
A miniaturized power generation device and method are provided.
In one configuration a microscale combustor and heat exchanger may
include several repeating unit cells each of which performs
combustion, recuperation, and heat exchange. Catalytic combustion
may occur inside at least one combustion and one recuperator
channel. Specific features may be added to reduce heat loss and
pressure drop.
Inventors: |
Ghazvini; Mohammad;
(Corvallis, OR) ; Narayanan; Vinod; (Corvallis,
OR) ; Drost; Monte Kevin; (Corvallis, OR) ;
Paul; Brian K.; (Corvallis, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Oregon State University |
Corvallis |
OR |
US |
|
|
Family ID: |
52133032 |
Appl. No.: |
14/322104 |
Filed: |
July 2, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61842547 |
Jul 3, 2013 |
|
|
|
Current U.S.
Class: |
431/170 ;
122/4D |
Current CPC
Class: |
F28D 21/0005 20130101;
F24H 1/0045 20130101; F28D 9/0093 20130101; F23C 13/02 20130101;
F23C 2900/03001 20130101; F24H 2210/00 20130101; F28F 3/08
20130101; F28F 2260/02 20130101 |
Class at
Publication: |
431/170 ;
122/4.D |
International
Class: |
F24H 1/00 20060101
F24H001/00; F23C 13/02 20060101 F23C013/02 |
Goverment Interests
GOVERNMENT LICENSE RIGHTS
[0002] This invention was made with government support under
DE-FC36-09GO19005 awarded by United States Department of Energy.
The government has certain rights in this invention.
Claims
1. A microscale combustor and heat exchanger, comprising: a
plurality of layers each comprising one or more respective channels
extending therethrough, the layers joined to one another to permit
gaseous communication between selected respective channels of the
layers, the plurality of layers comprising: a combustor layer
comprising at least one combustion channel having a catalyst
disposed therein; and a recuperator layer comprising at least one
recuperation channel having a catalyst disposed therein, the at
least one recuperation channel disposed in gaseous communication
with a respective one of the at least one combustion channel to
receive a combustion gas therefrom.
2. The microscale combustor and heat exchanger according to claim
1, wherein the combustor layer and recuperator layer are disposed
in stacked arrangement so that the at least one recuperation
channel and the at least one combustion channel are disposed over
one another with a common channel wall therebetween.
3. The microscale combustor and heat exchanger according to claim
2, wherein the at least one recuperation channel and at least one
combustion channel are oriented relative to one another such that
the catalyst containing portion of the at least one recuperation
channel is adjacent the catalyst containing portion of the at least
one combustion channel.
4. The microscale combustor and heat exchanger according to claim
1, comprising an additional recuperator layer comprising at least
one recuperation channel disposed therein disposed in gaseous
communication with a respective one of the at least one combustion
channel to receive a combustion gas therefrom.
5. The microscale combustor and heat exchanger according to claim
4, wherein the at least one recuperation channel of the additional
recuperator layer has a catalyst disposed therein.
6. The microscale combustor and heat exchanger according to claim
1, wherein one or more of the at least one recuperation channel and
at least one combustion channel comprises pin fins disposed
therein.
7. The microscale combustor and heat exchanger according to claim
1, wherein the plurality of layers includes a heat exchange layer
comprising at least one heat exchange channel disposed therein and
wherein the recuperator layer is disposed between the heat exchange
layer and the combustor layer.
8. The microscale combustor and heat exchanger according to claim
1, wherein one or more of the plurality of layers comprises a
shroud disposed between the one or more respective channels and an
edge of the layer.
9. The microscale combustor and heat exchanger according to claim
8, wherein the shroud includes a passageway extending through the
layer in which the shroud is disposed.
10. The microscale combustor and heat exchanger according to claim
8, wherein one or more of the plurality of layers comprises a
groove disposed therein between the shroud and an edge of the
layer.
11. The microscale combustor and heat exchanger according to claim
1, wherein one or more of the plurality of layers comprises a
groove disposed therein between the one or more respective channels
and an edge of the respective layer.
12. The microscale combustor and heat exchanger according to claim
1, wherein ratio of the area of catalyst in the combustion channel
to the area of catalyst in recuperation channel is the range of
1/10 to 9/10.
13. The microscale combustor and heat exchanger according to claim
1, wherein ratio of the amount of catalyst in the combustion
channel to the amount of catalyst in recuperation channel is the
range of 1/10 to 9/10.
14. A microscale combustor and heat exchanger, comprising: a
plurality of layers each comprising one or more respective channels
extending therethrough, the plurality of layers comprising a
combustor layer having at least one combustion channel disposed
therein, and a heat exchange layer having at least one heat
exchange channel disposed therein; and a casing disposed around and
enclosing the plurality of layers and comprising an inner wall
defining a cavity disposed therein, the cavity dimensioned to
provide a gap between at least a portion of the inner wall and the
plurality of layers, wherein the at least one heat exchange channel
is disposed in gaseous communication with the gap.
15. The microscale combustor and heat exchanger according to claim
14, wherein the plurality of layers includes a recuperator layer
comprising at least one recuperation channel disposed therein, and
wherein the recuperator layer is disposed between the heat exchange
layer and the combustor layer.
16. The microscale combustor and heat exchanger according to claim
15, wherein the combustor layer and the recuperator layer are
disposed in stacked arrangement so that the at least one
recuperation channel and at least one combustion channel are
disposed over one another with a common channel wall
therebetween.
17. The microscale combustor and heat exchanger according to claim
15, wherein one or more of the at least one combustion channel, at
least one recuperation channel, and at least one heat exchange
channel comprises pin fins disposed therein.
18. The microscale combustor and heat exchanger according to claim
15, wherein the at least one recuperation channel has a catalyst
disposed therein.
19. The microscale combustor and heat exchanger according to claim
14, wherein the at least one heat exchange channel includes an
inlet in gaseous communication with the gap and wherein the
recuperator and combustor layers each include respective working
fluid outlet ports in gaseous communication with the gap.
20. The microscale combustor and heat exchanger according to claim
19, wherein the inlet of the heat exchange channel is disposed at
an edge of the heat exchange layer.
21. The microscale combustor and heat exchanger according to claim
14, wherein one or more of the plurality of layers comprises a
groove disposed therein between the one or more respective channels
and an edge of the respective layer.
22. The microscale combustor and heat exchanger according to claim
21, wherein the groove comprises a through hole extending through
the layer in which the groove is disposed.
23. The microscale combustor and heat exchanger according to claim
14, wherein one or more of the plurality of layers comprises two
concentric grooves disposed therein, the grooves disposed between
the one or more respective channels and an edge of the respective
layer.
24. The microscale combustor and heat exchanger according to claim
14, wherein the at least one combustion channel has a catalyst
disposed therein.
25. The microscale combustor and heat exchanger according to claim
14, wherein at least one of the plurality of layers comprises an
upper surface and a lower surface each of which surfaces includes a
respective channel disposed therein.
26. A microscale heat exchanger, comprising: a plurality layers
each comprising one or more respective channels extending
therethrough, the plurality of layers comprising one or more heat
exchange layers having at least one heat exchange channel disposed
therein, the plurality of layers disposed in stacked arrangement so
that at least one channel from each of two or more layers is
disposed adjacent one another with a common wall therebetween
through which heat may be exchanged, wherein the plurality of
layers comprises a shroud disposed between the one or more
respective channels and associated respective edges of the
layers.
27. The microscale heat exchanger according to claim 26, wherein
the plurality of layers includes a combustor layer comprising at
least one combustion channel.
28. The microscale heat exchanger according to claim 26, wherein
one or more of the plurality of layers comprises a groove disposed
therein between the shroud and an edge of the layer.
29. The microscale heat exchanger according to claim 26, wherein
the shroud includes a passageway extending through the layer in
which the shroud is disposed.
30. The microscale heat exchanger according to claim 26, wherein
one or more of the plurality of layers comprises a groove disposed
therein between the shroud and an edge of the layer.
31. The microscale heat exchanger according to claim 30, wherein
the groove comprises a through hole extending through the layer in
which the groove is disposed.
32. A microscale combustor, comprising: a plurality of layers each
comprising one or more respective channels extending therethrough,
the layers joined to one another to permit gaseous communication
between selected respective channels of the layers, the plurality
of layers comprising a combustor layer comprising at least one
combustion channel having a catalyst disposed therein; and an
electric heater disposed in thermal communication with the at least
one combustion channel.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S.
Provisional Application No. 61/842,547, filed on Jul. 3, 2013, the
entire contents of which application(s) are incorporated herein by
reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to a miniaturized
power generation device, and more particularly, but not
exclusively, to a microscale combustor and heat exchanger (.mu.CHX)
that may include several repeating unit cells each of which
performs combustion, recuperation, and heat exchange.
BACKGROUND OF THE INVENTION
[0004] Advances in fabrication technology have helped spur research
in miniaturized thermal devices for distributed power generation
and heat and mass transport process intensification. These devices
have the advantages of being lightweight and compact, thereby
yielding high energy and power densities. With a decrease in
channel dimensions, the surface area per unit volume of fluid
flowing in a microchannel increases thereby increasing the heat
transfer rate. The increased surface area per unit volume also
enhances completion of heterogeneous surface chemical reactions
within microchannels. Therefore, catalytic combustion and heat
transfer in microchannels can result in increased efficiency and
reduced size of thermal power generation and exchange devices. In
addition, the rate of heat generation from combustion and device
temperatures can be controlled to some extent by tailoring
heterogeneous reactions to occur at specific locations within the
channel walls. Thus, high temperatures that cause the formation of
NO.sub.X can potentially be mitigated. Because of these advantages,
several microscale combustor geometries for different fuels have
been presented in literature over the past decade. As the
combustion channel size decreases, homogeneous (gas-phase)
reactions face thermal and radical quenching. Depending on the heat
loss and the gas mixture velocity, flame extinction of gas phase
hydrogen combustion can occur in channels as large as 1000 .mu.m.
Combustion in channels smaller than this limit can occur via
heterogeneous (surface) reactions that are promoted by catalysts.
Heterogeneous combustion can further trigger homogeneous combustion
in the bulk of the fluid.
[0005] Combustion within microchannels has been documented in
several numerical and experimental studies. For example, Boyarko et
al. tested hydrogen-oxygen mixture combustion in a platinum
microtube and found that the 400 .mu.m and 800 .mu.m tubes used in
their experiments were below quenching size under most atmospheric
pressure test conditions. (Boyarko, G. A., Sung, C. J., and
Schneider, S. J., 2005, "Catalyzed combustion of hydrogen-oxygen in
platinum tubes for micro-propulsion applications," Proc. Combust.
Inst., 30, pp. 2481-2488.) In both numerical simulations as well as
experiments, Boyarko et al. observed that there was a minimum
threshold heat flux necessary for ignition. When the ignition heat
flux was increased further, the gas temperature got so high that a
choked flow resulted inside the tube. Zhou et al. modeled conjugate
heat transfer within a 500 .mu.m channel made of different wall
materials (quartz glass, alumina ceramic, copper) to investigate
the effect of wall thermal conductivity on
homogeneous/heterogeneous combustion of hydrogen-air mixture.
(Zhou, J., Wang, Y., Yang, W., Liu, J., Wang, Z., and Cen, K.,
2009, "Combustion of hydrogen-air in catalytic micro-combustors
made of different material," Int. J. Hydrog. Energy, 34, pp.
3535-3545.) They observed that heterogeneous reaction became
dominant as thermal conductivity of the material increased. Chen et
al., who simulated combustion in a 1-mm channel with different wall
materials, reached a similar conclusion. (Chen, G. B., Chen, C. P.,
Wu, C. Y., and Chao, Y. C., 2007, "Effects of catalytic walls on
hydrogen/air combustion inside a micro-tube," Appl. Catal. A-Gen.,
332, pp. 89-97.) They found that heterogeneous reaction was
dominant in the beginning of the tube, followed by homogeneous
reaction downstream of the tube. Lower wall conductivity was
observed to lead to a larger temperature gradient on the surface
causing homogeneous combustion to shift upstream. At the highest
studied velocity (20 m/s), no homogeneous reaction was observed in
channel heights lower than 200 .mu.m in diameter. In a follow-on
study, Chen et al. investigated the effect of differences in
catalyst configurations in the same geometry as that used in their
previous study. (Chen, G. B., Chao, Y. C., and Chen, C. P., 2008,
"Enhancement of hydrogen reaction in a micro-channel by catalyst
segmentation," Int. J. Hydrog. Energy, 33, pp. 2586-2595.) A
multi-segment catalyst was compared with a single segment catalyst
of the same total length. The multi-segment catalyst configuration
showed better conversion due to the occurrence of homogeneous
reaction in the regions between segments. Karagiannidis and
Mantzaras used a 2-D model to simulate transient hetero-homogeneous
combustion of methane over platinum catalyst within a 1000
micrometer channel. (Karagiannidis, S., and Mantzaras, J., 2010,
"Numerical investigation on the start-up of methane-fueled
catalytic microreactors," Combust. Flame, 157, pp. 1400-1413.) For
the pressures in the range of 1 bar-5 bar, they found that ignition
and steady state microreactor residence times decreased with an
increase in pressure. Combustors with lower thermal conductivity
walls had smaller ignition times.
[0006] Recuperation has been used alongside combustion in order to
preheat the gas mixture by several groups. Lloyd and Weinberg
fabricated a spiral counterflow combustor, often referred to as a
"Swiss roll" type combustor to improve the efficiency of combustion
processes. (Lloyd, S. A., and Weinberg, F. J., 1974, "A burner for
mixtures of very low heat content," Nature, 251 (5470), pp. 47-49.)
Peterson et al. developed a microscale hydrogen combustor with
counterflow heat recuperator. (Peterson, R. B., and Vanderhoff, J.
A., 2000, "A catalytic combustor for microscale applications,"
Combust. Sci. Technol. Comm., 1, pp. 10-13.) They observed that
preheating helped to keep a sustained homogeneous reaction. In
addition to a microscale combustor, an efficient heat exchanger is
required in order to transfer heat produced by the reaction to a
working fluid. The heat transfer to the working fluid will alter
the wall temperature distribution, which will in turn affect the
combustion process. Janicke et al. used hydrogen combustion over a
platinum covered surface to heat a gas stream cross-flow to the
combustion gas flow in a microscale heat exchanger. (Janicke, M.
T., Kestenbaum, H., Hagendorf, U., Schuth, F., Fichtner, M., and
Schubert, K., 2000, "The controlled oxidation of hydrogen from an
explosive mixture of gases using a microstructured reactor/heat
exchanger and Pt/Al2O3 catalyst," J. Catal., 191 (2), pp.
282-293.)
[0007] There have been several studies involving stacked
microchannel arrays for various applications. The review articles
by Fan and Luo and Khan and Fartaj provide some of the recent
examples of stacked microchannel devices including heat exchangers
and chemical reactors. (Fan, Y., and Luo L., 2008, "Recent
applications of advances in microchannel heat exchangers and
multi-scale design optimization," Heat Transf. Eng., 29(5), pp.
461-474. Khan, M. G., and Fartaj, A., 2011, "A review on
microchannel heat exchangers and potential applications", Int. J.
Energy Res., 35, pp. 553-582.) In stacked microscale reactors, one
layer could have several parallel microchannels wherein a reaction
occurs while exchanging heat with a working fluid that flows in an
adjacent layer. Such an arrangement has been used for methane steam
reforming where a fuel combusts in the combustor layers and
transfers the produced heat to the reformer sheets. Ryi et al.
tested methane steam reforming with hydrogen catalytic combustion
in an integrated microchannel reactor. (Ryi, S. K., Park, J. S.,
Choi, S. H., Cho, S. H., and Kim S. H., 2005, "Novel micro fuel
processor for PEMFCs with heat generation by catalytic combustion,"
Chem. Eng. J., 113, pp. 47-53.) The designed device consisted of
cover plate, a base plate and 50 plates (25 alternating combustor
and reformer plates) with microchannels. Inconel plates were used
to fabricate the microchannel sheets and stainless steel sheets
were used for the cover and base plates. Each sheet had 22
microchannels in parallel with 500 .mu.m in diameter, 250 .mu.m in
depth and 17 mm in length. Pt--Sn/Al.sub.2O.sub.3 and
Rh--Mg/Al.sub.2O.sub.3 were impregnated by wash-coating in the
combustor and reformer for catalytic reactions respectively. Hwang
et al. developed a similar combined combustor and methane reformer
device and were able to achieve 95% conversions and hydrogen
production rate of 0.78 mol/h in the reformer. (Hwang, K. R., Lee,
C. B., Lee, S. W., Ryi, S. K., and Park, J. S., 2011, "Novel
micro-channel methane reformer assisted combustion reaction for
hydrogen production," Int. J. Hydrog. Energy, 36, pp. 473-481.)
Their device consisted of a variety of chemically etched metal
plates, such as half-etched straight channel plates (10 sheets),
fully etched 3D mixing channel plates (2 sheets), and
cover/holder/separator plates (5 sheets). Hydrogen and/or methane
were used as the fuel in the combustor sheets to provide heat for
methane reformation. A Pt-coated mesh catalyst was used as an
igniter at the inlet of the combustor until a flame was
generated.
[0008] Mettler et al. used CFD simulations to model stacks of
different sizes and characterize the effects of scaling up of
microchemical systems. (Mettler, M. S., Stefanidis, G. D., and
Vlachos, D. G., 2011, "Enhancing stability in parallel plate
microreactor stacks for syngas production," Chem. Eng. Sci., 66,
pp. 1051-1059.) They studied syngas production from methane using a
parallel-plate reactor with alternating combustion and steam
reforming channels. The author compared stacks of 3 units to 15
units, each comprised a combustion channel and reformer channel.
They found that heat losses caused extinction of combustion in the
outer channels and consequently reduced the efficiency of the
smaller stack. Whereas extinction of combustion occurred in the
outer channels even for the larger stack, the interior channels
sustained combustion, resulting in a higher efficiency. They also
recommended stack materials with thermal conductivities higher than
100 W/m-K for a more stable device.
[0009] Very recently Zhang et al. synthesized a Pt-based catalyst,
and investigated the behavior of hydrogen catalytic combustion at
low temperatures of the hydrogen/dry air mixture. (Zhang, C.,
Zhang, J., and Ma, J., 2012, "Hydrogen catalytic combustion over a
Pt/Ce0.6Zr0.4O2/MgAl2O4 mesoporous coating monolithic catalyst,"
Int. J. Hydrog. Energy, 37, pp. 12941-12946.) They found that for
low temperature catalytic combustion of hydrogen, the initial
reaction temperature, H.sub.2 concentration, and flow rates were
very important parameters. They tried hydrogen combustion at
mixture temperatures of 298 K and 263 K and their results show that
higher H.sub.2 concentration was helpful in initiating and
sustaining catalytic combustion. For the 263 K combustion, the
authors could not achieve conversions higher that 40% for low
hydrogen concentrations and although they could start the catalytic
combustion, they described the largest challenge to be avoiding
product water from freezing.
[0010] Additionally, several reaction mechanisms are available in
literature on hydrogen oxidation. Although the rates were
determined for macroscale channels, these reaction rate
coefficients can be used for microscale simulations since they are
surface reactions. Warnatz et al. studied stagnation flow of
hydrogen-oxygen mixture over a platinum surface and developed a
reaction mechanism for H.sub.2/O.sub.2 combustion. (Warnatz, J.,
Allendorf, M. D., Kee, R. J., and Coltrin, M. E., 1994, "A model of
elementary chemistry and fluid mechanics in the combustion of
hydrogen on platinum surfaces," Combust. Flame, 96, pp. 393-406.)
Warnatz et al.'s as well as three other homogeneous reaction
mechanisms as well as three heterogeneous reaction schemes were
tested by Appel et al. (Appel, C., Mantzaras, J., Schaeren, R.,
Bombach, R., Inauen, A., Kaepperli, B., Hemmerling, B., and
Stampanoni, A., 2002, "An experimental and numerical investigation
of homogeneous ignition in catalytically stabilized combustion of
hydrogen/air mixtures over platinum," Combust. Flame, 128, pp.
340-368.) When combustion in a 7 mm high channel with platinum
covered walls was considered, they found differences from 8 to 66
percent between the modeling results using these schemes and their
own experimental results for ignition characteristics. Their study
showed that Warnatz's homogeneous reaction mechanism and
Deutschmann's heterogeneous reaction mechanism give the best
predictions, within 8% of the experimental results. (Deutschmann,
O., Schmidt, R., Behrendt, F., and Warnatz, J., 1996, "Numerical
modeling of catalytic ignition," Proc. 26th Symposium
(International) on Combustion/The Combustion Institute, Pittsburgh,
Pa., pp. 1747-1754.)
[0011] However, a need remains in the art for a compact and
efficient microchannel heat exchangers for low temperature
applications, in particular, for example, ones which provides
combustion of fuels and heat exchange from the combustion gases to
a working fluid.
SUMMARY OF THE INVENTION
[0012] In one of its aspects the present invention relates to a
miniaturized power generation device, such as a microscale
combustor and heat exchanger (.mu.CHX). The .mu.CHX may include
several repeating unit cells each of which performs three unit
operations: combustion, recuperation, and heat exchange.
Heterogeneous catalytic combustion may occur on the walls of
microchannels in the presence of a platinum catalyst. In one
particular configuration, the present invention may include a
distributed catalyst arrangement which deters extinction of the
reaction due to a cold gas stream and which provides a high
hydrogen conversion (greater than 95 percent) for a range of
operating conditions.
[0013] For instance, in one of its aspects, the present invention
may provide several microchannels that are connected in parallel in
order to meet the thermal power requirements of the desired
application. The parallel microchannels may be linked together by
inlet and outlet headers that distribute the flow uniformly amongst
the microchannels. When only one working fluid is involved, a
single layer of parallel microchannels could be sufficient, as in
the case of a heat sink or simple chemical reactors. However, for
heat or mass exchangers and more complex chemical reactors, a
stacked up, multi-layer parallel microchannel architecture may be
needed. In the design of such microchannel devices, it may be
sufficient to optimize the performance of a single microchannel
"unit cell" (for example, two microchannels separated by a
non-permeable wall if the device is a heat exchanger) and to ensure
that the flow distribution between the microchannel unit cells and
between the stacked layers is uniform. Typically, the unit cells
and headers may also be designed with pressure drop constraints in
mind.
[0014] In one exemplary application, the devices and methods of the
present invention may find use in an automotive cryo-adsorbant
storage system for hydrogen. In such an exemplary application,
hydrogen gas that exits a cryo-adsorbant storage tank needs to be
heated to a minimum temperature of 233K (-40.degree. C.) prior to
entering the fuel cell. During cold start conditions, heat exchange
with ambient air or with the fuel cell coolant is insufficient to
provide this minimum temperature, thereby requiring an additional
source of thermal energy. This thermal energy can be provided by
combusting a small portion of a cold hydrogen stream in a device
capable of transferring heat of reaction back to the cold stream.
To maintain a high on-board efficiency and low storage system
weight and volume, it is desirable for the device to be small,
lightweight and operate at a high efficiency. Another exemplary
application is building or distributed heating. Exemplary
configurations of compact devices based on parallel microchannel
architecture are presented herein.
[0015] For example, the present invention may provide a microscale
combustor and heat exchanger, comprising a plurality of layers each
having one or more respective channels extending therethrough. The
layers may be joined to one another to permit gaseous communication
between selected respective channels of the layers. The plurality
of layers may include a combustor layer comprising at least one
combustion channel having a catalyst disposed therein and a
recuperator layer comprising at least one recuperation channel
having a catalyst disposed therein. The recuperation channel may be
disposed in gaseous communication with a respective combustion
channel to receive a combustion gas therefrom. The combustor layer
and recuperator layer may be disposed in stacked arrangement so
that the at least one recuperation channel and the at least one
combustion channel are disposed over one another with a common
channel wall therebetween. The plurality of layers may also include
a heat exchange layer having at least one heat exchange channel
disposed therein, the recuperation layer may be disposed between
the heat exchange layer and the combustor layer.
[0016] In addition, the present invention may provide a microscale
combustor and heat exchanger comprising a plurality of layers each
having one or more respective channels extending therethrough. The
plurality of layers may include a combustor layer having at least
one combustion channel disposed therein, and a heat exchange layer
having at least one heat exchange channel disposed therein. In
addition, a casing may be provided around and enclosing the
plurality of layers and may include an inner wall defining a cavity
disposed therein; the cavity may be dimensioned to provide a gap
between at least a portion of the inner wall and the plurality of
layers, with at least one heat exchange channel disposed in gaseous
communication with the gap. A recuperator layer may be provided
having at least one recuperation channel disposed therein, and the
recuperation layer may be disposed between the heat exchange layer
and the combustor layer. The at least one heat exchange channel may
include an inlet in gaseous communication with the gap and the heat
exchange and combustor layers may each include respective working
fluid outlet ports in gaseous communication with the gap. Further,
one or more of the plurality of layers may include a groove or two
concentric grooves disposed therein between the respective channels
and an edge of the respective layer. The at least one combustion
channel, at least one recuperation channel, and/or at least one
heat exchange channel may include pin fins disposed therein, and
the at least one combustion channel and/or at least one
recuperation channel may include a catalyst disposed therein.
[0017] In yet an additional aspect, the present invention may
provide a microscale heat exchanger, comprising a plurality layers
each comprising one or more respective channels extending
therethrough. The plurality of layers may include one or more heat
exchange layers having at least one heat exchange channel disposed
therein, and may be disposed in stacked arrangement so that at
least one channel from each of two or more layers is disposed
adjacent one another with a common wall therebetween through which
heat may be exchanged. The plurality of layers may include a shroud
disposed between the one or more respective channels and associated
respective edges of the layers. One or more of the plurality of
layers may include a groove disposed therein between the shroud and
an edge of the layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The foregoing summary and the following detailed description
of exemplary embodiments of the present invention may be further
understood when read in conjunction with the appended drawings, in
which:
[0019] FIG. 1 schematically illustrates a unit cell of an exemplary
microscale combustor and heat exchanger (.mu.CHX) in accordance
with the present invention;
[0020] FIG. 2A schematically illustrates temperature contours
within the .mu.CHX unit cell of FIG. 1;
[0021] FIG. 2B illustrates hydrogen mass fraction along the length
of the combustor and recuperator channels of the .mu.CHX unit cell
of FIG. 1;
[0022] FIG. 3A-3D schematically illustrate an exemplary combustor
shim, an exemplary upper recuperator shim, an exemplary lower
recuperator shim, and exemplary heat exchange channel shims,
respectively, in accordance with the present invention, each
comprising eight channels;
[0023] FIGS. 4A-4C schematically illustrate an exploded view of an
exemplary physical configuration of eight unit cells, of the type
shown in FIG. 1, comprising five different layers, with FIG. 4A
illustrating the flow of fuel/air mixture (H.sub.2/air) through the
layers, FIG. 4B illustrating the flow of exhaust through the
layers, and FIG. 4C illustrating the flow of nitrogen through the
layers;
[0024] FIG. 5 illustrates velocity contours (scale in m/s) for one
half of the microchannels within the combustor layer shown in FIG.
3A;
[0025] FIG. 6 schematically illustrates an exemplary header in
accordance with the present invention for use with stacked unit
cells of the type shown in FIGS. 4A-4C in which the location of
inlets and outlets is shown;
[0026] FIGS. 7A, 7B schematically illustrate exemplary top layers
of H.sub.2/air and nitrogen headers in accordance with the present
invention showing the transition from circular tubing to respective
plena;
[0027] FIG. 8 illustrates pressure contours (Pa) of combustible gas
flow through the header of FIG. 6 in combination with the layers of
FIG. 4B;
[0028] FIG. 9 illustrates pressure contours (Pa) of the nitrogen
flow through the header of FIG. 6 in combination with the layers of
FIG. 4A;
[0029] FIG. 10 schematically illustrates an exploded view of a
further exemplary configuration of a microscale combustor and heat
exchanger in accordance with the present invention comprising five
layers and two unit cells having pin fins and showing the flow
direction of a working fluid, a fuel/air mixture, and exhaust
across one half of each layer;
[0030] FIG. 11A schematically illustrates the combustor layer of
FIG. 10;
[0031] FIG. 11B schematically illustrates the upper recuperator
layer of FIG. 10;
[0032] FIG. 11C schematically illustrates the lower recuperator
layer of FIG. 10;
[0033] FIG. 11D schematically illustrates the heat exchange layer
of FIG. 10;
[0034] FIGS. 12A, 12B schematically illustrate a groove without
holes and a groove with through holes, respectively;
[0035] FIG. 13 schematically illustrates a partial cross-sectional
view of the combustor layer of FIG. 11A taken along the sectioning
line 13-13;
[0036] FIGS. 14A, 14B schematically illustrate velocity contours
(scale in m/s) for one half of the microchannels within the
combustion and heat exchange layers, respectively, of the
microscale combustor and heat exchanger of FIG. 10;
[0037] FIGS. 15A, 15B schematically illustrate velocity profiles
for combustion and heat exchange layers, respectively, at two
different longitudinal locations indicated by the dashed lines in
FIGS. 14A, 14B;
[0038] FIG. 16 schematically illustrates an exemplary header in
accordance with the present invention for use with stacked unit
cells of the type shown in FIG. 10 in which the location of inlets
and outlets is shown;
[0039] FIG. 17 schematically illustrates assembly of the stacked
unit cells of the type shown in FIG. 10 along with a casing;
[0040] FIG. 18 illustrates pressure drop in the heat exchanger side
(in Pa) of the device of FIG. 17, with one quarter of the flow
paths shown;
[0041] FIG. 19 illustrates velocity vectors at the mixing region of
the device of FIG. 17;
[0042] FIG. 20 schematically illustrates an exploded including the
top and bottom distribution layers of yet a further exemplary
microscale combustor and heat exchanger in accordance with the
present invention;
[0043] FIG. 21 schematically illustrates a combustor layer of the
device of FIG. 20 comprising 2 unit cells;
[0044] FIG. 22 illustrates a combustor shim, corresponding to the
combustor layer design of FIG. 3A, as fabricated by chemical
etching and laser cutting;
[0045] FIG. 23 schematically illustrates a test facility used for
testing a microscale combustor and heat exchanger of the present
invention;
[0046] FIG. 24 illustrates the measured variation of H.sub.2
conversion, efficiency, and heat loss ratio by residence time of a
fabricated prototype;
[0047] FIG. 25 illustrates the measured variation of H.sub.2
conversion, efficiency, and heat loss ratio by body temperature of
a fabricated prototype; and
[0048] FIGS. 26A, 26B schematically illustrate a microchannel of a
microscale combustor in accordance with the present invention
comprising an electric heater configured to heat a selective
location of the combustor, where the electric heater can be as
small as a wire (FIG. 26A) or as large as the size of the combustor
itself (FIG. 26B).
DETAILED DESCRIPTION OF THE INVENTION
[0049] In one of its aspects the present invention provides a
general device design which includes several repeating unit cells
100 each of which may perform three unit operations: combustion,
recuperation, and heat exchange, FIG. 1. Heat from combustion may
be transferred to a working fluid that can either be a gas (air) or
liquid (water). Heterogeneous catalytic combustion may occur on the
walls of microchannels 110, 140 in the presence of a platinum
catalyst 130, 132.
[0050] Two levels of numerical simulations are performed to realize
the design. The first level represents a single unit cell 100
comprising a combustion channel 110, two recuperator channels 140,
and two heat exchange channels 120, FIG. 1. At the unit cell level,
a two-dimensional numerical model with detailed surface chemistry
is used to realize a design with a high unit cell thermal
efficiency. It is shown that with the help of a novel distributed
catalyst arrangement 130, 132, extinction of the reaction due to
the cold gas stream is prevented and a high hydrogen conversion
(greater than 95 percent) is achieved for a range of operating
conditions. The second level of simulations is at the physical,
device scale, comprising multiple unit cells connected together
with appropriate fluidic headers FIGS. 3A-7B. Multiple unit cells
are created from several layers 410, 420, 300, 430, 440 (hereafter
"410-440"), with multiple such unit cells then stacked in parallel
with appropriate headers to provide a multi-unit .mu.CHX 600, FIGS.
4A-4C, 6. Three dimensional simulations of fluid flow are performed
to ensure uniformity in flow distribution while maintaining low
pressure drop through the .mu.CHX 600, FIGS. 8, 9. Fabrication
constraints are incorporated into the device level design and
simulations.
[0051] In one exemplary application, the multiple unit cell
microscale combustor and heat exchanger (.mu.CHX) 600 can operate
at temperatures as low as 200 K. One particularly useful
application of the .mu.CHX 600 is cryo-adsorbent hydrogen storage
systems for fuel cell cars under cold start conditions. In this
exemplary application, desired operating conditions for the .mu.CHX
600 are shown in Table 1. A fraction of the incoming cold hydrogen
gas would be premixed with air to provide the thermal energy rate
needed to increase the temperature of the rest of the hydrogen flow
from 200 K to 233 K. The hydrogen flow rate to the fuel cell could
vary between 0.5 g/s to 2 g/s and the working pressure could vary
between 5 to 20 bars. For the cold start condition, the
environment, from which air is drawn for the combustion process, is
assumed to be at 233 K (-40.degree. C.).
TABLE-US-00001 TABLE 1 Desired operating conditions for the .mu.CHX
P.sub.H2,in m P.sub.c,.sub.in T.sub.H2,in (K) T.sub.H2,out (K)
(bar) (g/s) T.sub.c,in (K) (bar) 200 233 5-20 0.5-2 233 1
Unit Cell Level Design
[0052] Each unit cell 100 of the multi-unit .mu.CHX 600 was
designed to perform, at the minimum, the unit operations of
combustion and heat exchange to the working fluid (e.g., a cold
hydrogen stream), by having a combustion channel 110 surrounded on
both sides by heat exchange channels 120. The combustion channel
110 may include inner catalyst beds 130 disposed on respective
inner surfaces 112 of the combustion channel 110, and may include
outer catalyst beds 132 disposed on respective outer surfaces 114
of the combustion channel 110, which catalysts beds 130, 132 may
include a noble metal, such as platinum, palladium, rhodium, and/or
other suitable material, for example. The catalyst can be in the
form of a thin layer (coating) deposited on the channels walls, or
the catalyst can be in the form of insets put inside the channels
(or attached to the walls). The catalyst (coating or insert) may
comprise a porous material.
[0053] Results from simulations (described below) for such a unit
cell design showed that the flow of a very cold working fluid
(e.g., 200 K hydrogen gas) in the heat exchange channels 120
reduced the temperature of the catalyst beds 130, 132 and prevented
catalytic combustion. Therefore, recuperator channels 140 were
provided between the combustion and heat exchange channels 110, 120
to provide a thermal buffer therebetween, FIG. 1. Thus, the unit
cell 100 may include a central combustor microchannel 110
surrounded by two recuperator channels 140 on either side. The heat
exchange channels 120, in turn, may be provided on either side of
the recuperator channels 140. In addition to providing a thermal
buffer, the recuperator channels 140 also permit recuperator gases
disposed therein to pre-heat the incoming air-fuel (e.g.,
air-H.sub.2) mixture prior to exiting the unit cell 100, thereby
providing a region of recuperation "R", FIG. 1. (The preheat region
"R" optionally may or may not exist based on the application.) The
added length to the recuperator channels 140 was adjusted based on
the simulation results so that the recuperator gases remain at the
minimum temperature of 373 K (100.degree. C.)--a requirement that
was imposed in order to avoid condensation.
[0054] Geometrical arrangements and thermofluidic parameter values
that ensure high efficiency and conversion were determined
Efficiency is used to identify the overall performance of the unit
cell 100, and is defined as the ratio of the amount of heat
transferred to the working fluid (e.g., cold hydrogen stream) to
the chemical energy of input hydrogen in the combustible gas
mixture,
.eta. = m . wf ( h out - h in ) wf m . H 2 .DELTA. h reaction / M H
2 ( 1 ) ##EQU00001##
where M.sub.H.sub.2 is the molar mass of hydrogen,
.DELTA.h.sub.reaction is the molar enthalpy of reaction, and {dot
over (m)}.sub.H.sub.2 and {dot over (m)}.sub.wf are the inlet
hydrogen and working fluid mass flow rate, respectively. Enthalpy
of reaction is calculated at the volumetrically-averaged
temperature in the catalyst section of the combustor. Hydrogen
conversion is defined as the ratio of the amount of hydrogen
combusted to that of input hydrogen,
Conversion H 2 = Y H 2 , in - Y H 2 , out Y H 2 , in . ( 2 )
##EQU00002##
As part of the design considerations, to produce a high specific
power within the unit cell 100, an equivalence ratio of unity was
considered. The equivalence ratio, .phi., is the ratio of the molar
fuel-to-air ratio at the desired test conditions to that at
stoichiometric conditions. The large equivalence ratio also results
in reduced pressure drop for the same thermal power generated when
compared with lower equivalence ratio mixtures.
[0055] In the exemplary design, the height of the combustion and
heat exchange channels 110, 120 were 300 .mu.m each, and height of
the recuperator channel 140 was 150 .mu.m. The width of all
channels 110, 120, 140 in the unit cell was 2 mm while the length,
L, of the unit cell 100 was kept at 15 mm. Based on simulations,
this length L was found to provide sufficient area for heat
exchange between the recuperator and heat exchange channels 140,
120 while keeping the pressure drop low. All outer walls of the
recuperator and heat exchange channels 140, 120 were considered
insulated (as indicated by the cross-hatching in FIG. 1).
[0056] Initially the catalyst beds 130, 132 were located entirely
on the inner walls 112 of the combustion channel 110 alone;
however, simulations showed that almost half of the catalyst length
was not being efficiently utilized because of the low reactant
mixture temperature. Despite the thermal buffer provided by the
recuperator channels 140, the cold hydrogen gas flow in the heat
exchange channels 120 tended to decrease the mixture temperature
rapidly past the initial part of the catalyst bed 130. As a result,
hydrogen conversion of only around 80 percent was typically
achieved.
[0057] In order to obtain higher conversions while managing the
amount of catalyst used, a novel catalyst bed arrangement was used.
The basic premise of the new arrangement was that higher gas
mixture and catalyst bed temperatures resulted in higher reaction
rates and hence more complete hydrogen conversion. To achieve this
larger catalyst temperature, the catalyst bed was redistributed
such that of the catalyst bed was shifted from the end of the inner
wall 112 of the combustion channel 110 and placed in the
recuperator channels 140 on the outer surface 114, (i.e., outer
catalyst bed 132). The location of the outer catalyst beds 132
within the recuperator channels 140 coincided with the location of
the inner catalyst beds 130 in the combustion channel 110, FIG. 1.
Simulations showed that the optimum ratio for the length of the
inner catalyst bed 130 to that of the outer catalyst bed 132 was
1.5 so that 3/5 of the length was located in the combustion channel
130 and of the combined bed length was located in the recuperator
channel 140, though other ratios may be used in accordance with the
present invention. For example, the combustion channel 110 may have
larger catalyst area than the recuperator channel 140 or vice
versa, and the ratio of the catalyst area in each channel 110, 140
over the total catalyst area can be in the range of 1/10th to
9/10th. With this new arrangement of catalyst beds 130, 132,
hydrogen conversions of about 99% were achieved with the L=15 mm
and a total catalyst length of 12.5 mm for each inner/outer bed
130, 132 pair. Depending on the application and design, the optimum
ratio may be different.
[0058] The catalyst used in a microscale combustor and heat
exchanger 600 in accordance with the present invention may have any
site density. Catalyst site density is defined as the amount of
catalytically active site per unit area (cm.sup.2). Catalyst site
density may be adjusted based on the required reactivity of the
catalyst, e.g., the lower the site density, the lower the reaction
rate (and therefore produced power). In addition, the catalyst in
the combustor and recuperator channels 110, 140 may have different
site densities. In the case where the catalyst surface (catalyst
bed) in the combustor and recuperator channels 110, 140 have
different site densities, the total amount of catalyst can be
calculated by multiplying the site density value by the area. The
ratio of the amount of catalyst in each channel 110, 140 over the
summation of the catalyst sites (total catalyst amount in the
combustion channel 110+total catalyst amount in the recuperator
channel(s) 140) can be in the range of 1/10 to 9/10.
Numerical Simulations of the Unit Cell
[0059] Two-dimensional, steady-state simulations were carried out
on the mass, momentum, energy and species (both gas-phase and
surface species) balance equations for the unit cell geometry
indicated in FIG. 1. The reactant gases entering the channel 110
were modeled as comprised of a hydrogen and dry air (approximated
as a 21 percent by volume oxygen and 79 percent by volume nitrogen)
mixture. Specifically, combustion of hydrogen on platinum surface
was modeled using the reaction mechanisms and detailed scheme of
Deutschmann et al. The scheme, shown in Table 2, includes 7 gas
species, 5 surface species and 13 reaction steps, and has shown
good comparison with experimental results. The five surface species
in this scheme are H(s), O(s), OH(s), H.sub.2O(s) and Pt(s). Pt(s)
describes free surface sites that are available for adsorption. A
surface site density of .GAMMA.=2.7.times.10.sup.-9 mol/cm.sup.2
was used simulating polycrystalline platinum, and it was assumed
that the catalyst surface was initially covered by Pt(s).
TABLE-US-00002 TABLE 2 Surface reactions for hydrogen oxidation on
platinum. (Deutschmann et al.) Reaction A (S) .beta. E.sub.a
1--H.sub.2 + 2Pt(s) .fwdarw. 2H(s).sup.a 0.046.sup.b 0.0 0 2--2H(s)
.fwdarw. H.sub.2 + 2Pt(s) 3.7 .times. 10.sup.21 0.0 67.4-6.0 H(s)
3--O.sub.2 + 2Pt(s) .fwdarw. 2O(s) 0.07.sup.b 0.0 0 4--2O(s)
.fwdarw. O.sub.2 + 2Pt(s) 3.7 .times. 10.sup.21 0.0 213.2-60 O(s)
5--H + Pt(s) .fwdarw. H (s) 1.00.sup.b 0.0 0 6--O + Pt(s) .fwdarw.
O (s) 1.00.sup.b 0.0 0 7--OH + Pt(s) .fwdarw. OH (s) 1.00.sup.b 0.0
0 8--H.sub.2O + Pt(s) .fwdarw. H.sub.2O(s) 0.75.sup.b 0.0 0 9--H
(s) + O(s) OH (s) + Pt(s) 3.7 .times. 10.sup.21 0.0 11.5 10--H(s) +
OH(s) H.sub.2O(s) + Pt(s) 3.7 .times. 10.sup.21 0.0 17.4
11--H.sub.2O(s) + O(s) OH(s) + OH(s) 3.7 .times. 10.sup.21 0.0 48.2
12--OH (s) .fwdarw. OH + Pt(s) 1.0 .times. 10.sup.13 0.0 192.8
13--H.sub.2O(s) .fwdarw. H.sub.2O + Pt(s) 1.0 .times. 10.sup.13 0.0
40.3 .sup.aThe hydrogen adsorption (first reaction) is first order
with respect to platinum. .sup.bSticking coefficient.
[0060] The mass fraction of species at the inlet of the unit cell
100 was defined by the equivalence ratio. For brevity, the
governing equations and boundary conditions were set according to
previous work by two of the presently named inventors. (Ghazvini,
M., and Narayanan, V., 2011, "Performance characterization of a
microscale integrated combustor recuperator oil heat exchanger,"
Proc. AJTEC2011: ASME/JSME 2011 8th Thermal Engineering Joint
Conference, Honolulu, Hi., 2011). The numerical model was validated
against the combined experimental and numerical study on
hetero-/homogeneous combustion of hydrogen/air mixtures over
platinum in a single channel by Appel et al. (Appel, C., Mantzaras,
J., Schaeren, R., Bombach, R., Inauen, A., Kaepperli, B.,
Hemmerling, B., and Stampanoni, A., 2002, "An experimental and
numerical investigation of homogeneous ignition in catalytically
stabilized combustion of hydrogen/air mixtures over platinum,"
Combust. Flame, 128, pp. 340-368.) The current simulations were
seen to predict the experimental data in Appel et al. at least as
well as the parity between their own numerical simulations and
experiments.
[0061] The governing equations and boundary conditions were solved
in FLUENT.RTM. V14 (ANSYS, Inc., Cannonsburg, Pa., USA) in
conjunction with Chemkin-CFD.TM. (Reaction Design, San Diego,
Calif., USA) for the chemical reactions. A non-uniform mesh was
used to refine the near-wall regions. The total number of grids was
34,300 for the whole model domain. It was initially verified that
an orthogonal staggered grid of 606.times.20 grid points (in x and
y, combustion channel, or 12,120 grids) was sufficient to produce a
grid independent solution. Additionally, adaptive mesh refinement
was also applied in several locations based on the gradients of
mass imbalance for better convergence. The simulation convergence
was decided when the residuals approached steady values
asymptotically and when the relative residuals were smaller than
10.sup.-7 for continuity, momentum, energy and species, with the
exception of residuals for O and H species which were less than
10.sup.-3.
[0062] Based on the required mass flow rate of cold hydrogen in
Table 1, the first step in a unit cell simulation was to determine
the flow rate of cold hydrogen and flow rate of hydrogen in the
combustion channels 110 within each unit cell 100. Based on
previous work with oil as the heat exchange fluid, the initial
inlet mixture velocity to the combustion channel was picked to be 4
m/s. In that study, this flow rate provided sufficient residence
time for conversions in excess of 90 percent. The inlet velocity of
the cold hydrogen stream was set arbitrarily to a value. The exit
temperature of the cold hydrogen stream was checked against the
requirement in Table 1 (>233 K). Simultaneously, the hydrogen
conversion was verified to be in excess of 90 percent. If the exit
temperature was found to be lower, the flow rate of cold hydrogen
was lowered. If the hydrogen conversion was found to be lower, the
inlet mixture velocity of the combustion gases was lowered. Once
conditions that met both the cold hydrogen exit temperature as well
as conversion were achieved, the fraction of hydrogen stream
required for combustion to that being heated in the heat exchange
channels 120 was determined. Based on the ratio of the maximum mass
flow rate of the cold gas in the .mu.CHX to the mass flow rate in
each unit cell, a total number of 168 unit cells with the channel
width of 2 mm was found to be sufficient to provide heat to 2 g/s
of cold hydrogen. For the lower mass flow rate limit of 0.5 g/s in
Table 1, the same number of unit cells was retained, with a
corresponding decrease in velocity of the cold hydrogen and
combustion mixture gases. For example, at a pressure of 5 bar the
flow rate of cold hydrogen gas through each of the 168 unit cells
were 6.1 mg/s and 1.5 mg/s for a total flow rate of 2 g/s and 0.5
g/s, respectively.
[0063] A representative temperature contour plot within the unit
cell 100 for the conditions indicated in Table 1 and heat exchange
fluid (cold hydrogen) inlet flow rate of 2 g/s, inlet temperature
of 200 K, and pressure of 5 bars is shown in FIG. 2A. The
temperature contours indicate regions of high temperatures at the
catalyst beds 130, 132 due to heterogeneous combustion. Fluid and
surface temperatures reached a peak value near the first one-third
length of the inner catalyst bed 130 within the combustion channel
110 and then decreased downstream due to the transfer of heat to
the recuperator and heat exchange channels 140, 120. For this
condition, outlet hydrogen and exhaust gas temperatures were 235.7
K and 391.5 K, respectively. FIG. 2B illustrates the
cross-sectional averaged hydrogen mass fraction distribution in the
unit cell 100. The location of the catalyst beds 130, 132 in both
the combustion and recuperator microchannels 110, 140 are
represented in this plot for clarity. Hydrogen/air mixture enters
the combustion channel 110 with a hydrogen mass fraction of 0.0283,
corresponding with an equivalence ratio of unity. After going
through the inner catalyst bed 130 in the combustion channel 110,
the mass fraction reduces to 0.0054 indicating that the hydrogen
conversion is 81% within the combustion channel 110. The remaining
hydrogen reacts in the outer catalyst bed 132 in the recuperator
channel 140 such that the mixture exits the unit cell 100 with less
than 1% of the initial hydrogen content.
[0064] In the unit cell simulations, based on the requirements
stated in Table 1, two parameters were varied--the inlet pressure
and mass flow rate of the cold hydrogen gas. Table 3 summarizes the
achieved conversions and unit cell efficiencies for the extremities
in the range of the desired pressures and hydrogen mass flow rates
presented in Table 1. It can be seen that the exit temperature in
all cases is in excess of 233K and that the conversions and
efficiencies are in excess of 99% and 92% respectively. Pressure
drop values within the combustor and recuperator channels 110, 140
are also shown in Table 3. A larger cold hydrogen mass flow rate
requires a proportionally larger hydrogen/air mixture flow rate,
thereby increasing the pressure drop in the combustion and
recuperator channels 110, 140 with increase in cold hydrogen mass
flow rate. However, pressure drop is almost independent of the
working fluid pressure, because the mass flow rate of hydrogen is
kept the same for both 5 bar and 20 bar. The slight increase in
pressure drop with higher pressure is a result of the larger
specific heat value at higher pressures. With the temperature
difference between the inlet and exit fixed at 33 K, a larger Cp
resulted in an increase in the amount of heat rate needed and hence
a slightly larger flow of hydrogen-air mixture within the
combustion channel 110.
TABLE-US-00003 TABLE 3 Simulation results for the desired pressure
and hydrogen mass flow rate ranges shown in Table 1 H.sub.2
conversion (in {dot over (m)}.sub.H.sub.2 P.sub.H.sub.2
T.sub.out,H.sub.2 combustor) .eta. .DELTA.P (g/s) (bar) (K) (%) (%)
(Pa) 2 5 235.7 99.8 93.3 5736.3 20 236.1 99.5 92.9 5833.9 0.5 5
237.7 99.8 92.4 991.6 20 243.8 99.4 92.1 1014.4
Numerical Simulations for Comparison to Laboratory Measurements
[0065] One goal was to fabricate and characterize the performance
of a multi-unit cell .mu.CHX and validate the numerical simulations
with laboratory measurements. However, due to safety considerations
in the laboratory, cold nitrogen gas was used in place of cold
hydrogen gas as the heat transfer fluid. Since density and thermal
properties of nitrogen are considerably different from those of
hydrogen, additional simulations were performed using cold nitrogen
gas for the unit cell 100 of FIG. 1 to create simulation results
which could later be compared to laboratory measurements of a
prototype device.
[0066] There are two thermal resistances in the path for the
requisite amount of heat to be transferred within the unit cell 100
from the heat exchange wall 116 (separating the recuperator and
heat exchange channels 140, 120) to the cold gas. The first one
pertains to the convection resistance, R.sub.conv=1/(h.sub.cold
gasA), while the second one is the resistance due to heating of the
cold gas stream, R.sub.heat=1/({dot over (m)}Cp). For the range of
hydrogen flow rates considered, the flow is laminar and hence the
heat transfer coefficient, assuming fully developed flow, is about
1120 W/m.sup.2-K. The R.sub.conv and R.sub.heat for cold hydrogen
flow within each unit cell 100 are 35.7 K/W and 5,926 K/W
respectively. The R.sub.heat estimate is based on a working
pressure of 5 bar and for a flow rate of 2 g/s. Since R.sub.heat is
the dominant thermal resistance in transferring heat to the cold
gas, in order to preserve the same representative thermal
conditions as cold hydrogen flow, it is clear that the R.sub.heat
between hydrogen and nitrogen flows has to be matched. Hence, the
heat capacity rates (1/R.sub.heat) between the cold hydrogen and
cold nitrogen flows, as well as the temperature rise (see Table 1)
were kept identical. This meant that, because of the high specific
heat of hydrogen compared to nitrogen
(Cp.sub.H.sub.2=13.02Cp.sub.N.sub.2), the nitrogen mass flow rate
would be proportionally higher than hydrogen mass flow rate. This
larger flow rate resulted in a R.sub.conv of 35.0 K/W and a
R.sub.heat of 5,974 K/W for the cold nitrogen stream at a working
pressure of 5 bar and flow rate of 26.1 g/s (.about.13 times the
maximum cold hydrogen flow rate).
[0067] Table 4 shows the result of the simulations with nitrogen as
the working fluid. The working pressure was fixed at 5 bar for
these simulations. Nitrogen mass flow rate of 26.1 g/s and 6.57 g/s
have the same heat capacity of 2 g/s and 0.5 g/s of hydrogen,
respectively. By a comparison of results in Tables 3 and 4, it can
be seen that the hydrogen conversion remains largely unaffected by
changing the heat exchange fluid. This result is to be expected
since changing the heat exchange fluid only changes the boundary
condition on the combustion process. When the heat capacity rates
are matched, the temperature drop along the heat exchange channels
140 would remain similar for both cold hydrogen and cold nitrogen
cases, thereby causing little variation in the hydrogen conversion.
It can also be seen that the unit cell efficiency remains unchanged
between the two cases which is an indication that the convective
resistances on the recuperator and heat exchange channels 140, 120
are smaller than that of the thermal resistance along the heat
exchange channel 120 (1/{dot over (m)}Cp). Table 4 shows that,
similar to cold hydrogen flow, pressure drop in the combustor and
recuperator channels 110, 140 are higher when the working fluid
flow rate is higher.
TABLE-US-00004 TABLE 4 Simulation results for nitrogen as the
working fluid H.sub.2 conversion (in {dot over (m)}.sub.N.sub.2
P.sub.N.sub.2 T.sub.out,N.sub.2 combustor) .eta. .DELTA.P (g/s)
(bar) (K) (%) (%) (Pa) 26.1 5 233.6 99.9 93.1 5133.7 6.57 5 233.3
99.8 92.4 711.8
Device Level (Physical Layer) Design
[0068] As described above, a total of 168 unit cells 100 are needed
to increase the temperature of hydrogen flow of 2 g/s from 200 K to
233K. The same amount of heat (911 W) can be removed using 26.1 g/s
of nitrogen flow with the same inlet and outlet temperatures. In
the present device level design, cold nitrogen gas flow is used for
the working fluid. Because the flow rate of cold nitrogen is 13
times larger than that of cold hydrogen, the former presents a
limiting case in the design of the headers for uniform flow
distribution.
[0069] Only fluid flow is simulated at this level due to
computational requirements. Properties of nitrogen were estimated
at an average temperature of 216.5 K while the properties of
hydrogen-air fuel mixture were kept fixed at 400 K. Another
important index for performance is pressure drop. The performance
measure at the device level pertains to uniform flow distribution
amongst the unit cells and an overall low pressure drop within the
device.
[0070] The microscale combustor and heat exchanger 600 comprising
multiple unit cell stacks 400 was designed so it could be
fabricated using chemical etching and diffusion bonding, FIGS.
4A-4C, 6. (The device could also be readily manufactured using
additive manufacturing technologies such as 3D printing.) The
planned diffusion bonding manufacturing method imposed some
important limits on the design of individual layers 410-440 which
taken together constitute a unit cell stack 400. That is, per the
schematic in FIG. 1, it is clear that a single unit cell 100 would
have to span five layers in a physical realization of the unit-cell
design, that is: upper and lower heat exchange layers 410, 440,
upper and lower recuperator layers 420, 430, and a combustor layer
300, FIGS. 3A-4C. The central layer would comprise the combustor
microchannel layer 300, which would be surrounded on either side by
two recuperator layers 420, 430, each of which in turn would be
surrounded by respective heat exchange layers 410, 440.
[0071] Since the width of the microchannels 415, 425, 315, 435, 445
(hereafter "415-445") within each unit cell 100 of the design of
FIG. 1 is only 2 mm, several unit cell microchannels 415-445 may be
positioned alongside each other in each of the respective
individual heat exchanger, combustor, and recuperator layers
410-440, and the layers 410-440 may then be stacked together to
form a stack 400 of eight unit cells. That is, each layer 410-440
may include eight constituent channels such that the stack 400
comprises eight unit cells. In this regard, the upper and lower
heat exchange layers 410, 440 may each include eight heat exchange
channels 415, 445, FIG. 3D; the upper and lower recuperator layers
420, 430 may each include eight recuperator channels 425, 435,
FIGS. 3B, 3C; and, combustor layer 300 may include eight combustion
channels 315, FIG. 3A. Thus, in combination, the layers 410-440 may
provide eight unit cells of overall dimensions of 24 mm.times.73 mm
upon assembly with the remaining recuperator layers 420, 430 and
heat exchange layers 410, 440, FIGS. 4A-4C. The thickness of the
combustor layer 300 may be 600 .mu.m, with 300 .mu.m deep
combustion channels 315 formed via chemical etching. (In fact, all
dimensions presented in connection with FIG. 1 may be incorporated
in the design of FIGS. 3A-4C.) Two holes, H, may be provided on
diagonally opposite corners of the layer 410-440 to permit
alignment of the layers 410-440 during diffusion bonding. Following
the split catalyst arrangement of FIG. 1, catalyst beds 330, 421,
431 may be provided on the upper and/or lower recuperator layers
420, 430 and combustor layer 300, FIGS. 3A-3C.
[0072] In the combustor layer 300, four combustion channels 315 may
be located on each side of the combustor layer 300. Locations of
different inlets 312 for the combustion gas mixture are also shown
in FIG. 3A. The flow direction of each of the streams is shown for
one half of the unit cells, FIGS. 4A-4C. It can be seen that the
H.sub.2/air mixture and cold nitrogen streams flow in the same
direction while the recuperator gases flow counter to the others,
FIGS. 4A-4C. The cold nitrogen inlet ports 418, 428, 318, 438, 448
(hereafter "418-448") may be located on the sides of the layers
410-440 to permit heat gain from the surrounding ambient air, FIG.
4C. The combustion gas inlet ports 412, 422, 312, 432, 442
(hereafter "412-442") may be located well within the layers 410-440
in order to increase the preheating area. The layers 410-440 may
also include respective exhaust outlet ports 416, 426, 316, 436,
446 (hereafter "416-446") and nitrogen outlet ports 413, 423, 313,
433, 443 (hereafter "413-443").
[0073] In particular, combustion gas inlets 312 may be provided in
the combustor layer 300 to introduce a combustion gas into the
combustion channels 315, FIGS. 3A, 4A. The combustion gases may
pass through the other layers 410, 420, 430, 440 of the cell stack
400 via respective combustion gas ports 412, 422, 432, 442 which do
not communicate with respective channels 415, 425, 435, 445 of
their respective layers 410, 420, 430, 440, FIG. 4A. Upon entry
into the combustion channels 315, the combustion gases may flow
across the catalyst beds 330 towards the center of the combustor
layer 300. At the innermost end of the combustor channels 315
recuperator passageways 314 may be provided in gaseous
communication with the recuperation channels 425, 435 of the
adjoining upper and lower recuperator layers 420, 430. Combustion
gases may then flow from the recuperator layer 300 through the
recuperator passageways 314 to the recuperation channels 425, 435
of the upper and lower recuperation layers 420, 430. In order to
reach the recuperation channel 425 of the the upper recuperation
layer 420, recuperation passageways 414 may be provided at the
innermost ends of the recuperation passageways 425 of the upper
recuperator layer 420, FIGS. 3B, 4A. Gases entering the upper and
lower recuperation channels 425, 435 from the combustor layer 300
may then travel longitudinally along the recuperation channels 425,
435 from the center to the ends of the layers 420, 430 where the
gases may exit through exhaust ports 426, 436, FIG. 4B. The exhaust
gases may then exit the cell stack 400 via respective exhaust gas
ports 416, 316, 446 of the additional three layers 410, 300, 440.
It can be seen that the fuel/air mixture flows in the opposite
direction to the recuperator gas flow.
[0074] As to the working fluid, the upper and lower heat exchange
layers 410, 440 may each include respective working fluid inlets
418, 448 through which a working fluid may be introduced into the
respective heat exchange channels 415, 445, FIG. 4C. The upper and
lower heat exchange layers 410, 440 may be identical. The working
fluid may travel longitudinally along the length of the heat
exchange channels 415, 445 toward the center of the heat exchange
layers 410, 440 to exit the layers 410, 440 through working fluid
outlets 413, 443. The working fluid may then exit the cell stack
400 via respective working fluid ports 423, 313, 433 disposed in
the recuperator and combustor layers 420, 300, 430. It should be
noted that nitrogen has a much larger flow rate than combustion
mixture flow, thereby making flow distribution uniformity a much
bigger challenge. Nitrogen may first enter a rectangular outer
plenum 418A, 428A, 318A, 438A, 448A, wherein, because of the
relatively high velocity of the flow, recirculation occurs. Several
small passages 429, 319, 439 may be disposed in a separating wall
427, 317, 437 to let nitrogen enter into a second (inner) resting
plenum 428B, 318B, 438B to which the nitrogen gas layers are
connected, FIGS. 3A-4C.
[0075] The thickness of the five layer stack 400 of eight unit
cells may be about 3 mm. Twenty-one such stacks 400 of layers
410-440 may be stacked up with a top and a bottom header caps 602,
604, FIG. 6, to form a 168 unit cell microscale combustor heat
exchanger 600 that could provide the desired heat (911 W) to the
working fluid. The overall dimension of the layers 410-440,
excluding the top and bottom header caps 602, 604 may be 2.4
cm.times.7.3 cm.times.6.3 cm. Alternatively, a greater number of
unit cells could be located within each layer 410-440 to reduce the
height and increase the lateral dimensions of each layer
410-440.
[0076] At the layer level, the design ensured that two criteria
were satisfied: (1) uniform flow distribution between the
microchannels 415-445 within each layer 410-440 and (2)
manufacturability. Three-dimensional simulations of fluid flow were
used to verify the uniform flow among the recuperator, combustion,
and heat exchange channels 415-445. FIG. 5 presents the velocity
contours for one half of the combustor layer 300 (i.e., four of the
combustion channels 315). Table 5 provides the cross-sectional
averaged velocities at each of the four microchannels 315 shown in
FIG. 5 of the combustor layer 300, along with the four
corresponding heat exchange channels 415 of the upper heat exchange
layer 410. Also provided are the cross-sectional averaged
velocities for four heat exchange channels 415 of the upper heat
exchange layer 410 with nitrogen flowing through them. It is seen
that the flow is uniform to within 1 percent in both combustor and
upper heat exchange layers 300, 410 indicating uniform flow
distribution within each of the layers 300, 410. Note that the flow
distribution within the recuperator channels 425, 435 will follow
that of the combustor layer 300, since the flow from each
combustion channel 315 goes into a corresponding recuperator
channel 425, 435.
TABLE-US-00005 TABLE 5 Average velocity within each of the channels
within a single layer Average Velocities (m/s) Layers Channel 1
Channel 2 Channel 3 Channel 4 Combustor 2.55 2.56 2.57 2.56 layer
Heat 37.3 37.4 37.3 37.4 exchange layer
[0077] In addition to uniform flow distribution, the design needed
to accommodate the diffusion bonding manufacturing requirements.
One of these requirements was that the wall thicknesses had to be
sufficiently thick to provide a leak free seal between fluids. To
meet this requirement, walls that separated different fluids
(combustion mixture, exhaust, and cold nitrogen) were thickened to
1 mm. In addition, a conservative 5 mm of solid material was added
around the periphery of each layer 410-440 to ensure that fluids
did not leak out of the stack 400.
Header Design
[0078] Turning to the header, the location of the inlet and exit
manifolds 603, 605, 608, 616 of the microscale combustor and heat
exchanger 600 were designed to promote uniform flow distribution
amongst the different layers 410, 420, 300 430, 440, FIG. 6, where
a device comprising several stacks 400 of unit cells is shown. The
headering system should be designed in such a way that uniform flow
is permitted into each of the fluidic layers 410-440. The headering
system may be viewed as including top and bottom header caps 602,
604 along with the respective inlet ports 412-442, 418-448 and
outlet ports 416-446, 413-443 of the fluidic layers 410-440.
[0079] There are two different fluids flowing in the microscale
combustor and heat exchanger 600, therefore there needs to be at
least two inlets 605, 608 and two outlets 603, 616. In the current
design, for better flow distribution and lower pressure drops in
both streams, two inlets 605, 608 are included for each stream. In
the present design, hexagonal inlets 416-446 are used to provide
better flow distribution and to reduce the size of the headering
section, FIGS. 4A-5. The positioning of the exhaust and
hydrogen-air mixture manifolds 616, 605 provided for preheating of
the hydrogen-air mixture prior to the catalyst location 330 in the
combustion channels 315. Moreover, as mentioned before, nitrogen
inlet ports 418-448 are located at the outer edges of the
microscale combustor and heat exchanger 600 to permit heat gain
from the surroundings, and to mitigate the chances of thermal
quenching in the combustion channels 315.
[0080] Several three-dimensional simulations of fluid flow were
performed in FLUENT.RTM. V14 (ANSYS, Inc., Cannonsburg, Pa., USA)
in order to determine the proper headering design. While not
optimized, the designs presented here represent iterative efforts
at obtaining uniform flow distribution amongst layers 410-440. In
these simulations, 3 sets of 8-unit cell stacks 400 were placed on
top of each other to make a 24 unit cell .mu.CHX. The flow rate
through each unit cell was kept identical to the largest flow rate
5.75.times.10.sup.-5 for heat exchange layers 410, 440 and
6.70.times.10.sup.-6 for the combustor layer 300, and hence the
flow distribution for larger stacks should be similar to the one
presented herein.
[0081] Four additional layers 702, 704, 706, 708 on top and three
layers 705, 707, 709 at the bottom of the stacks 400 were necessary
to transition from circular tubing of the header caps 602, 604 to
the plena that distribute flow amongst different layers 410-440,
FIGS. 7A, 7B. The main constraint in designing the headers 602, 604
lay in the transmission of force through the stacks 400 when the
pressure is exerted during diffusion bonding. (Micro-texturing was
utilized to satisfy the bonding criterion.) Small unsupported gaps
between walls of the channels in two adjacent layers were found to
be acceptable provided that the ratio on the span to the layer
thickness is less than 7. In the current design, this ratio was
kept to 5 at the maximum; hence for layer thickness of 600 .mu.m
the span was ensured not to exceed 3 mm. The resulting design of
the upper four layers 702, 704, 706, 708 of inlet headering and
lower three layers 705, 707, 709 is shown in FIG. 7A, 7B.
[0082] Pressure contours for flow within the headers 602, 700,
combustion gas inlet ports 412-442, and combustion channels 315 is
shown in FIG. 8. This figure shows the left half of the combustor
layers 300 with the inlet, the manifold and combustion channels
(the recuperator and heat exchange channels 415, 445, 425, 435 are
not shown. Table 6 shows the average velocity in the 12 channels.
It can be seen than the flow is distributed uniformly between
layers and maximum difference is less than 4 percent.
TABLE-US-00006 TABLE 6 Average velocity magnitude in combustor
layers Channel velocity (m/s) Layer 1 2.61 2.59 2.57 2.6 Layer 2
2.57 2.51 2.51 2.57 Layer 3 2.56 2.53 2.53 2.55
[0083] Pressure contours of the nitrogen gas flow within the
headers 602, 701, nitrogen inlet ports 418-448, and the heat
exchange channels 415, 445 are shown in FIG. 9. As with the
combustion gas distribution structure 703 and inlet ports 412-442,
most of the nitrogen pressure drop occurs in the distribution
structure for nitrogen 701 and associated nitrogen inlet ports
418-448. Average velocities in all heat exchange channels 415, 445
in each layer are shown in Table 7. It can be seen than flow is
distributed uniformly and maximum difference is less than 6%.
TABLE-US-00007 TABLE 7 Average velocity magnitude in nitrogen
layers Layer Layer Layer Layer Layer Layer 1 2 3 4 5 6 Velocity
37.2 37.4 37.6 37.4 37.2 37.1 (m/s) 37.1 37.4 37.8 39.4 37.4 37.2
37.1 37.5 37.3 37.2 39.2 37.1 37.2 37.5 37.6 37.1 37.3 37.3
[0084] The design presented for the nitrogen headering is an
extreme case since the flow rates for nitrogen are 13 times that of
hydrogen, which will be the actual working fluid for the device.
The thermo-fluidic design presented herein does not consider
conjugate heat transfer effects, which could be significant
especially when the thermal conductivity of the material from which
the layers 410-440 is fabricated is large.
Experimental Results
[0085] A prototype of the .mu.CHX 600 of FIGS. 3A-9 was fabricated
and the performance characterized for comparison to the results of
the design modeling. The prototype comprised 16 unit cells with 8
unit cells in a layer and was capable of producing and transferring
about 100 W of thermal energy rate. The device included several
stainless steel shims corresponding to layers 410-440 that were
chemically etched to form the channels 415-445 of the design,
though other materials such as aluminum, or alloys including but
not limited to Inconel, Haynes.RTM. alloy, or other suitable
materials may be used. For example, a combustor shim 300A was
fabricated according to the designed combustor layer 300, FIG. 22.
Platinum catalyst was deposited selectively within regions of the
combustor shim 300A and recuperator shims according to the design
of FIGS. 1, 3A-3C to provide high conversion while minimizing the
use of the catalyst. The dimensions of the combustor shim 300A were
73 mm in length, 24 mm in width, and 0.75 mm in thickness. In a
similar manner, shims were fabricated corresponding to each of the
designed recuperator and heat exchange layers 410, 420, 430, 440.
The shims were bolted together along with the insulating fluidic
headers of the type shown in the designs of FIGS. 6, 7A, 7B.
[0086] In the experiments, due to safety considerations, cold
nitrogen gas was used as the heat transfer fluid instead of cold
hydrogen gas to be used in the actual application. FIGS. 4A-4C show
the flow path of H.sub.2/air mixture, exhaust and cold nitrogen
flows used in the experiment. The overall size of the 16 unit-cell
device was 24 mm.times.73 mm.times.11 mm with the weight of 116
g.
[0087] An experimental facility 230 was built to characterize the
performance of the device, labeled ".mu.CHX" in FIG. 23. Hydrogen
and air were premixed before entering the device at point 23A at
room temperature while nitrogen stream entered the device at point
23B at a low temperature after exchanging heat to liquid nitrogen
within a Dewar. Water vapor was removed from the exhaust gases
using a condenser 23C and desiccant filters 23D. Samples were taken
from the dry exhaust gases to quantify the amount of uncombusted
hydrogen in the exhaust. Hydrogen conversion was defined as the
ratio of the amount of hydrogen combusted to that of input hydrogen
as given by Eqn. (2) above.
[0088] Inlet and outlet temperatures, pressures and flow rates of
the streams were measured using digital data acquisition. The
gathered data was used to calculate the overall performance of the
.mu.CHX and was defined as the ratio of the amount of heat
transferred to nitrogen to the chemical energy of input hydrogen as
given by Eqn. (1) above, with the work fluid, wf, being nitrogen in
the present experiment.
[0089] Heat losses were calculated by subtracting the heat
transferred to nitrogen from the heat produced in the combustor by
the combusted hydrogen.
Q L = m . H 2 .DELTA. h reaction ( H 2 Conversion ) M H 2 - m . N 2
( h out - h in ) N 2 ( 3 ) ##EQU00003##
The temperature of the body of the device was measured at four
different locations. To determine the body temperature, readings
from two of the thermocouples that were located closest to the
catalyst section of the .mu.CHX were averaged.
[0090] Varied parameters include the inlet temperature of cold
nitrogen (150-273 K), mass flow rate of cold nitrogen, and mass
flow rate of the combustible gas mixture. The performance of the
device was characterized using three parameters: pressure drop
across the combustor and recuperator channels, hydrogen conversion,
and efficiency index. The equivalence ratio, which is defined as
the ratio of the molar fuel-to-air ratio at the test conditions to
that at stoichiometric conditions, was kept constant at unity.
[0091] One of the important factors in hydrogen conversion was the
residence time over the catalyst for reactions. Residence time is
defined as the time a hydrogen molecule has to react before leaving
the catalyst region.
t.sub.r=L.sub.Cat/ V (4)
where L.sub.Cat is the catalyst bed length and V is the averaged
velocity of hydrogen/air mixture in a channel. V is calculated by
dividing the total flow rate by the number of channels and channel
cross section area. FIG. 24 shows the effect of residence time on
H.sub.2 conversion and device efficiency. In these experiments, the
inlet nitrogen stream temperature was kept constant at -19.degree.
C. Table 8 summarizes the parameters of this plot.
TABLE-US-00008 TABLE 8 Test conditions related to FIG. 24. Input
t.sub.r Power H.sub.2 Q.sub.L (ms) (W) Conversion .eta. (W)
r.sub.Q.sub.L 55.8 19.8 0.941 0.663 5.49 0.277 49.9 22.1 0.938
0.696 5.33 0.241 27.2 40.5 0.912 0.786 5.11 0.126 11.7 94.3 0.868
0.820 4.51 0.048 8.64 127.2 0.858 0.822 4.57 0.036
[0092] It can be seen that H.sub.2 conversion increases by about
10% with increasing residence time from 9-56 ms. However, the
efficiency of the device drops from 82% to 66% due to heat losses.
When the power generated in a device is small, the effect of heat
losses on the overall efficiency of the system can be significant.
The device was covered with Pyrogel.RTM. and Cryogel.RTM. (Aspen
Aerogels, Inc., Northborough, Mass., USA) insulations, and the body
temperature was kept approximately the same between all experiments
at about 186.degree. C. and therefore there was not much difference
between heat losses (in watts, see Table 8). However, the ratio of
the heat loss to total power ratio which is defined as,
r QL = Q L m . H 2 .DELTA. h reaction / M H 2 ( 5 )
##EQU00004##
increased with an increase in residence time. Since the total
length of the catalyst and the equivalence ratio were fixed, in
order to achieve different residence times the flow rate of the
hydrogen/air mixture had to be varied (see Eq. 4). Different
hydrogen/air flow rates resulted in different input power to the
system (Table 8). Therefore, that heat loss ratio was different for
different cases and is shown in FIG. 24. The higher the heat loss
ratio, the lower the efficiency of the .mu.CHX.
[0093] The body temperature of the combustor was a second important
factor in conversion and device efficiency since higher reaction
temperatures result in higher reaction rate and hence conversion.
FIG. 25 shows an increase in H.sub.2 conversion with an increase in
body temperature where body temperature was varied from 124.degree.
C. to 196.degree. C. while keeping the residence time the same (9
ms). In these experiments, the inlet nitrogen stream temperature
was kept fixed at -70.degree. C.
[0094] The heat loss values varied from 1.1 W for the body
temperature of 124.degree. C. to 4.6 W for the body temperature of
196.degree. C. Since the power input was high in these cases (120
W), although heat loss increased, the heat loss ratios were small
numbers and had insignificant effect on the overall efficiency of
the system. Therefore the efficiency also increased with increasing
the body temperature. The experimental results showed that hydrogen
residence time and body temperature had significant effects on the
overall efficiency of the device. Conversions as high as 94.1% and
efficiencies as high as 88.3% were achieved, FIGS. 24, 25.
Alternative Design
[0095] In another of its aspects, the present invention provides an
alternative exemplary configuration of a microscale combustor and
heat exchanger (.mu.CHX) 900, which retains the basic features of
the unit cell design 100 of FIG. 1, such as the distributed
catalyst beds 130, 132, as well as the features of a five-layer
structure of FIGS. 4A-4C with a central layer 850 providing the
combustion function, two surrounding recuperator layers 830, 870
performing the recuperation function, and two surrounding heat
exchange layers 810, 890 providing a heat exchange function, FIGS.
10, 16. Among the chief differences between the two .mu.CHX's 600,
900 are changes to the headering system (FIG. 10), the inclusion of
a shroud 500 of inlet gas/fluid used to absorb the heat coming out
of the layer stacks 800 (FIG. 17), and the inclusion of pin fins
811, 831, 851, 871, 891 (hereafter "811-891") in the heat
exchanger, recuperator, and combustor layers 810, 830, 850, 870,
890 (hereafter "810-890"), FIGS. 11A-11D. Heat loss from the
.mu.CHX 800 by convection is directly proportional to the body
temperature, and by radiation is proportional to the fourth power
of temperature. By designing a shroud 500 around the layer stacks
800, heat loss from the .mu.CHX 800 is minimized, FIG. 17. In this
design, the gas is present in the shroud 500 between a casing 902
and chemically etched/micromachined layer stacks 800 disposed
within the casing 902.
Alternative Layer Design
[0096] Turning to the layer structure in more detail, consistent
with the schematic in FIG. 1, a unit cell stack 800 spans five
layers 810-890: the central combustor layer 850 which is surrounded
on either side by two recuperator layers 830, 870, which in turn
are surrounded on either side by two heat exchange layers 810, 890,
FIG. 10. Each layer 810-890 may include two constituent channels
such that the stack 800 comprises two unit cells. Specifically, the
upper and lower heat exchange layers 810, 890 may each include two
heat exchange channels 815, FIG. 11D; the upper and lower
recuperator layers 830, 870 may each include two recuperator
channels 835, 875, FIGS. 11B, 11C; and, combustor layer 850 may
include two combustion channels 855. The combustor, recuperator and
heat exchange channels 815, 835, 855, 875, 895 (hereafter
"815-895") include pin fins 811-891, FIGS. 10, 11A-11D. It should
be noted that the two unit cells in the cell stack 800 take the
place of the eight unit cells of the cell stack 400. Following the
split catalyst arrangement of FIG. 1, catalyst beds 840, 860 may be
provided on the upper and/or lower recuperator layer 830, 870 and
the combustor layer 850 in the base of the channels 835, 855, 875
between and/or on the pin fins 831, 851, 871, FIGS. 10, 11A, 11B,
13. Both upper and lower sides of the combustor layer 850 and upper
and lower sides of the upper recuperator layers 830, 870 may be
partially covered with the catalyst beds 840, 860. FIG. 13 shows a
cross section of the combustor layer 850 with the locations of the
catalyst beds 860 between the pin fins 851. Circular pin fins 851
are shown in this design to distribute the flow uniformly and
provide support for bonding. However, other pin fin shapes and
other methods (for example parallel channels) may also be used for
this goal.
[0097] Combustion gas inlets 852 may be provided in the combustor
layer 850 to introduce a combustion gas into the combustion
channels 855, FIG. 11A. The combustion gases may pass through the
other layers 810, 830, 870, 890 of the cell stack 800 via
respective combustion gas ports 812, 832, 872, 892 which do not
communicate with the respective channels 815, 835, 875, 895 of
their respective layers 810, 830, 870, 890, FIG. 10. Upon entry
into the combustion channels 855, the combustion gases may flow
across the catalyst bed 860 around the pin fins 851 towards the
center of the combustor layer 850. At the innermost end of the
combustor channels 855 recuperator passageways 857 may be provided
in gaseous communication with recuperation channels 835, 875 of the
adjoining upper and lower recuperator layers 830, 870. Combustion
gases may then flow from the recuperator layer 850 through the
recuperator passageways 857 to the recuperation channels 875 of the
upper surface of the lower recuperation layer 870. In order to
reach the recuperation channel 835 of the upper surface of the
upper recuperation layer 830, recuperation passageways 837 may be
provided at the innermost ends of the recuperation passageways 835
of the upper recuperator layer 830, FIGS. 10, 11B. Gases entering
the upper and lower recuperation channels 835, 875 from the
combustor layer 850 may then travel longitudinally along the
recuperation channels 835, 875 from the center to the ends of the
layers 830, 870 where the gases may exit through exhaust ports 834,
874, FIGS. 10, 11A-11C. The exhaust gases may then exit the cell
stack 800 via respective exhaust gas ports 814, 854, 894 of the
additional three layers 810, 850, 890, FIG. 10. It can be seen that
the fuel/air mixture flows in the opposite direction to the
recuperator gas flow.
[0098] As to the working fluid, the upper and lower heat exchange
layers 810, 890 may each include respective working fluid inlets
816, 896 through which a working fluid may be introduced into the
respective heat exchange channels 815, 895, FIG. 10, 11D. The upper
and lower heat exchange layers 810, 890 may be identical. The
working fluid inlets 816, 896 may be located on the sides of the
layers 810, 890 to permit heat gain from the surrounding ambient
air. (In the case of building heating, the working fluid is the
surrounding air; hence there will be no heat gain.) The working
fluid travels longitudinally along the length of the heat exchange
channels 815, 895 toward the center of the heat exchange layers
810, 890 to exit the layers 810, 890 through working fluid outlets
813, 893. The working fluid may then exit the cell stack 800 via
respective working fluid ports 833, 853, 873 disposed in the
recuperator and combustor layers 830, 850, 870.
[0099] The multi-unit cell stack 800 may be designed such that it
may be fabricated using chemical etching and diffusion
bonding/laser welding. (The device could readily also be
manufactured using additive manufacturing technologies such as 3D
printing.) The manufacturing methods impose some important
constraints on the design of the layers 810-890 which are addressed
in this design. Etching does not provide sharp corners at the
bottom of the walls and the cross section of the channels will be
U-shaped. That reduces the cross section area and can increase
pressure drop and cause maldistribution. In order to reduce the
effect of curved corners, the width of the walls should be at least
twice the height of the channels 815-895. On the other hand,
bonding methods have other limitations. For example for diffusion
bonding there should not be an unsupported span, and force should
be transmitted from top to bottom in the regions that needs to be
diffusion bonded. In addition, diffusion bonding as well as laser
welding needs a minimum width for the bonding surface.
[0100] Each layer 810-890 may have a thickness of 600 .mu.m, and
thus the thickness of the five layer stack 800 may be about 3 mm.
Unlike the design of FIGS. 3A-4C, both sides of each layer 810-890
may be etched 150 .mu.m deep. For example, the top surface of the
combustor layer 850 and bottom surface of the upper recuperator
layer 830 may both be etched to have complementary combustion
channels 855 that are 150 .mu.m deep, such that when the combustor
layer 850 is sealed to the upper recuperator layer 830 the channels
on the lower surface of the upper recuperator layer 830 and on the
upper surface of the combustor layer 850 may be joined to form a
conjoined combustion channel 855 that is 300 .mu.m tall. Likewise,
the lower surface of the combustor layer 850 and upper surface of
the lower recuperator layer 870 may both be etched to have
complementary recuperation channels 875 that are 150 .mu.m deep,
such that when the combustor layer 850 is sealed to the lower
recuperator layer 870 the channels on the upper surface of the
lower recuperator layer 870 and on the lower surface of the
combustor layer 850 may be joined to form a conjoined recuperation
channel 875 that is 300 .mu.m tall. Therefore, the bottom side of
the combustor layer 850 will have features similar to the upper
surface of the bottom recuperator channel which can be seen in FIG.
11C. In a similar manner, the lower surface of upper heat exchange
layer 810 may be etched to include features similar to the upper
surface of the upper recuperator layer 830, and the lower surface
of lower recuperator layer 870 may be etched to include features
similar to the upper surface of the lower heat exchange layer
890.
[0101] There are two reasons for having 150 .mu.m deep features on
both sides instead of 300 .mu.m deep features on one side. The
first reason pertains to the constraints of the fabrication
process. Isotropic chemical etching limits the distance between
features to be at least twice the etching depth. For example, if
the etch depth is 300 .mu.m, the distance between features has to
be at least 600 .mu.m. However, in the designs in FIGS. 11A-11D,
the distance between the pins 811-891 may be as small as 400 .mu.m
in order to permit uniform flow distribution. Therefore an etching
depth of 150 .mu.m is chosen. The other reason is that, as shown in
FIG. 1, both sides of the combustor and upper recuperator layers
850, 830 are to include catalyst beds 840, 860. Without an etched
surface on the back side of the layers 850, 830, the catalyst
deposition would be done on a smooth surface, which does not permit
good adherence of the catalyst to the layers 850, 830. Even if the
flat surface were etched, catalyst would cover the entire flat
surface making bonding of the layers 850, 830 impossible due to the
catalyst layer on one layer 850, 830 being in contact with the pin
831, 851 on the adjoining layer. Existance of features such as pins
831, 851, on the lower sides of the layers 830, 850 lets the
catalyst solution flow around the features keeping the bonding
contact surface clean, FIG. 13. Other fabrication techniques may
require different design considerations, or the fabrication
technique may not put any constrain on the features or the channel
height, in this case the two sides of the layers 810-890 may have
features with different heights.
[0102] In addition to uniform flow distribution, the design needed
to accommodate the bonding manufacturing requirements. One of these
requirements was that the layer wall thicknesses had to be
sufficient to provide a leak free seal between fluids. To meet this
requirement, walls that separated different fluids (combustion
mixture, recuperator, and cold nitrogen) were thickened to at least
2 mm. In addition, a conservative 3 mm of solid material was added
around the periphery of each layer 810-890 to ensure that fluids
did not leak out of the stack 800.
[0103] At the layer level, the design again ensured that two
criteria were satisfied: (1) uniform flow distribution between the
microchannels 815-895 within each layer 810-890, and (2)
manufacturability. Three-dimensional simulations of fluid flow were
used to verify the uniform flow among the channels 815-895. FIGS.
14A, 14B present the velocity contours for one half of the
combustor and heat exchanger layers 850, 810, 890, respectively.
FIGS. 15A, 15B show the velocity magnitude plots for combustor and
heat exchange layers 850, 810, 890, respectively, taken along the
dashed lines in FIGS. 41A, 41B. It can be seen that uniform flow
distribution is achieved.
Shrouding and Headering
[0104] Heat loss from a combustor by convection is directly
proportional to the body temperature. By designing a shroud 500
around the cell stack 800, heat loss is minimized. Specifically,
the cell stacks 800 may be stacked together to provide four unit
cells which may then be placed in an enclosing casing 901
comprising lower and upper casing portions 902, 904 which are
larger than the stacks 800, FIGS. 16, 17. Thus, the space between
the relatively smaller stacks 800 and the larger lower casing
portion 902 may provide a gap between the stacks 800 and lower
casing portion 902 to provide the shroud 500, FIG. 17. The shroud
500 represents one option for distribution of the working fluid
around the cell stacks 800 and inside the heat exchange layers 810,
890. The distance between the sides of the cell stacks 800 and the
casing 901 internal sides may be found by numerical simulations.
The distance on the long side (along the length) may be 1 mm and on
the short side may be 3 mm, for example. (Since the casing 901 is
separated from the hot portions of the cell stack 800, the casing
901 can be made of lower temperature materials including, but not
limited to aluminum, wood, plastic, and so forth.)
[0105] The heat exchange fluid may flow in the shroud 500 around
all the layers 810, 890 that have openings 816, 819 for the intake
of the working fluid. In this regard there may also be a space
between a top distribution layer 920 and the layer stacks 800 and
also between the lower casing portion 902 and the layer stacks 800
which lets the working fluid flow around the layer stacks 800.
Therefore in addition to the sides, there is working fluid on top
and bottom of the layer stacks 800. It can be seen from FIGS. 10,
17 that the inlets 816, 896 of the heat exchange layers 810, 890
are extended to the edge so that the working fluid can enter the
layers 810-890 from the casing 901. The rest of the working fluid
stream enters the stacks 800 via the working fluid outlets 813,
833, 853, 873, 893 (hereafter "813-893") at the center of the
layers 810-890 and mixes with the hot working fluid. Flow path for
different streams are shown in FIGS. 10, 17. The casing 901 may
include working fluid inlets 908, combustion mixture inlets 905,
recuperator gas outlets 916, and working fluid outlet 903 to assist
in creating the different flow streams, FIG. 16. In addition, upper
and lower distribution layers 920, 920A, 922 disposed between the
casing 901 and stacks 800 may include working fluid inlets 928,
combustion mixture inlets 925, 925A working fluid outlet 923, 923A,
and recuperator gas outlets 926, as well as alignment pin holes H,
to further assist in directing the flow streams, FIG. 17. For
example, the gas enters from the bottom distribution layer 922 to
the center plenum (i.e., working fluid outlets 813-893) and mixes
with the hot streams coming out of the heat exchange layers 810,
890.
[0106] The flow distribution arrangement between the shroud 500 and
the heat exchange channels 815, 895 causes only a fraction
(typically 1/5th) of the fluid, to enter the heat exchange channels
815, 895. Fluid going through these channels 815, 895 gets heated
by heat transfer from the recuperator channels 835, 875 and exits
at temperatures typically in excess of 100.degree. C. The hot fluid
then mixes with the bypassed fluid in the casing, and the desired
air temperature is attained at the exit 903 of the .mu.CHX 900. The
distance between the lower casing portion 902 and the bottom of the
cell stacks 800 may be adjusted, such as by raised spacers which
may be part of the recuperator gas outlets 926, such that the
desired mixture ratio is achieved (e.g., 600 .mu.m in this case).
This flow distribution arrangement is important for two reasons:
(i) by allowing only a fraction of the inlet working fluid to enter
the heat exchange channels 815, 895, the overall body temperature
within the combustion, recuperation and heat exchange channels
815-895 is maintained relatively high, thereby resulting in high
reaction rates and high fuel conversion; and, (ii) the pressure
drop through the .mu.CHX 900 is reduced when compared with the
entire fluid flowing through the heat exchange channels 815,
895.
[0107] To further supplement the insulative effect of the shroud
500, one or two sets of concentric grooves 818, 838, 858, 878, 898
(hereafter "818-898") may be positioned around the periphery of
each of the layers 810-890 exterior to the respective channels
815-895 to further reduce the heat transfer out of the cell stack
800, FIGS. 10-11D. The grooves 818-898 located around the
combustor, recuperator and heat exchange channels 815-819 to
separate these channels 815-819 from the shroud 500. The grooves
818-898 may help reduce the axial conduction of heat from the
combustor region to the shroud 500. In the absence of these grooves
818-898, the working fluid in the shroud 500 may cool down the
combustor region thereby reducing the reaction rate and hence fuel
conversion. Since the layers 810-890 may comprise a thermally
conductive material that permits conduction of heat from one or
more of the channels 815-895 to the shroud 500, inclusion of the
grooves 818-898 containing material, such as a gas or liquid, (or
absence of the material, such as a vacuum) that is less thermally
conductive than the layers 810-890 may deter heat conduction to the
shroud 500. Furthermore, the grooves 818-898 may include through
holes 819, 839, 859, 879, 899 to further decrease the heat transfer
rate by reducing the amount of material in the layers 810-890 that
may add to the heat transfer area (FIGS. 11A-12B). Thus, the
combination of the shroud 500 and the grooves 818-898 cooperate to
reduce heat loss (high device efficiency) and high conversion,
respectively, and is a salient feature of this exemplary
configuration.
[0108] Several three-dimensional simulations of fluid flow were
performed in FLUENT.RTM. V14 (ANSYS, Inc., Cannonsburg, Pa., USA)
in order to determine the proper headering design. Pressure
contours for flow within the heat exchange layers is shown in FIG.
18. This figure shows the flow field for a quarter of the device
shown in FIG. 17 with nitrogen as the working fluid. The velocity
vector in the mixing region, shown in FIG. 19, indicates that the
upward flow through the mixing region does not enter the channels
and cause flow reversal.
[0109] A comparison between the pressure drop between the designs
of FIGS. 4A-4C and FIG. 10 is summarized in Table 9. In this table
the pressure drops at layer and stream levels are shown. The stream
level pressure drops includes pressure drops in the layers and
headers. It can be seen that mass flow rates are kept the same for
all three different designs. Nitrogen was used as the working fluid
in these simulations. The pressure drop in the fuel/air stream is
much lower than that of the working fluid, because the flow rates
are much smaller. In the layer level, pressure drop in the
combustor layer 850 of FIG. 10 (0.200 kPa) is higher than that of
the combustor layer 300 of FIGS. 4A-4C (0.036 kPa), due to the
existence of pin fins 811-891. At the header level, on the other
hand, the pressure drop of the fuel/air stream is much smaller for
the FIG. 10 design (0.201 kPa) compared to the FIGS. 4A-4C design
(1.4 kPa). Only 1 Pa pressure drop exists in the headering section
of the FIG. 10 design.
[0110] An important achievement here is the reduction of pressure
drop in the heat exchange stream. The pressure drop in the FIG. 10
design is 7 times less in the layer level and more than 10 times
less in the stream level. Therefore a significant improvement is
achieved with regard to pressure drops.
TABLE-US-00009 TABLE 9 Comparison of pressure drops between
different designs Pressure drop (kPa) mass flow Section Gen II Gen
III rates (kg/s) Heat exchanger layer 37 5 5.75E-05 Heat exchange
171 13 2.53E-03 stream Combustor layer 0.036 0.200 6.70E-06
Fuel/air stream 1.4 0.201 1.33E-05
Other Possible Design
[0111] In yet a further possible design 200 in accordance with the
present invention is incorporation of the casing 901 and shroud 500
of FIGS. 10, 17 within the layers 250, FIGS. 20, 21. With this
design 200, the size may be smaller; however, pressure drop may be
higher. FIG. 20 shows the location of a shroud 210 disposed
directly within the layers 235, 250 which replaces the role of the
casing. The shroud 210 may be provided in the form of one or more
passageways that extend through each layer 250. The combustor,
recuperation, and heat exchange channels 240 are thus encased
within a shroud 210 of inlet gas/fluid that is to be heated using
the device 200. FIG. 21 shows an exemplary combustor layer 235 for
this new design 200. A circumferential grove 220 may exist to
reduce heat transfer from the shroud 210 to the channels 240.
Electric Heating Option
[0112] In another aspect of it aspects, the present invention may
provide a microscale combustor 2600, 2601 which includes an
auxiliary heat source to heat a selected portion of the combustion
channel 2610, 2620 to assist in initiating a catalytic reaction
within the combustion channel 2610, 2620. For example, the
combustion channel 2610, 2620 may be included a part of a device
having the configuration shown in FIG. 1. The heat source may be
provided in the form of a wire 2604 disposed in thermal
communication with a wall 2613 of the combustion channel 2610, with
the wire electrically connected to a power source 2602 such that
the flow of electricity through the wire 2604 heats the wire 2604
and the wall 2613 and any catalyst disposed proximate thereto.
Alternatively the heat source may be provided within a portion 2624
of the wall 2623 of the combustion channel 2620, with the wall
portion 2624 electrically connected to a power source 2622 such
that the flow of electricity through the wall portion 2624 heats
the wall portion 2624 and any catalyst proximate thereto. Thus, the
present invention may provide a microscale combustor comprising
plurality of layers each comprising one or more respective channels
extending therethrough. The layers may be joined to one another to
permit gaseous communication between selected respective channels
of the layers, with the plurality of layers comprising a combustor
layer having at least one combustion channel with a catalyst
disposed therein. In addition, one or more electric heaters may be
provided at a selected location to heat at least a portion of the
combustion channel to initiate the catalytic reaction. The electric
heater may include a wire or may extend along and/or through an
entire surface of the combustion channel, for example. The electric
power may be provided from external sources or from a rechargeable
battery located close to or attached to the combustor.
[0113] These and other advantages of the present invention will be
apparent to those skilled in the art from the foregoing
specification. For example, while the exemplary configurations
illustrate parallel flow among the channels of the layers, cross
flow channel configurations may also be utilized in accordance with
the present invention. Accordingly, it will be recognized by those
skilled in the art that changes or modifications may be made to the
above-described embodiments without departing from the broad
inventive concepts of the invention. It should therefore be
understood that this invention is not limited to the particular
embodiments described herein, but is intended to include all
changes and modifications that are within the scope and spirit of
the invention as set forth in the claims.
[0114] The contents of all publications cited throughout the text
of this disclosure are hereby incorporated herein by reference. In
addition, the following symbols used throughout this disclosure
have the following meanings:
[0115] A Pre-exponential factor (Arrhenius equation)
(mol-cm-K-s)
[0116] Cp Specific heat at constant pressure (J/kgK)
[0117] E.sub.a Activation energy (Arrhenius equation) (kJ/mol)
[0118] h Enthalpy (J/kg)
[0119] H.sub.c Combustion channel height (m)
[0120] H.sub.o Working fluid channel height (m)
[0121] H.sub.R Recuperator channel height (m)
[0122] M Molar mass (kg/kmol)
[0123] {dot over (m)} Mass flow rate (kg/s)
[0124] P Pressure (Pa)
[0125] T Temperature (K)
[0126] Y.sub.g Mass fraction of g.sup.th gaseous species
[0127] Greek Symbols
[0128] .beta. Temperature exponent (Arrhenius equation)
[0129] .phi. Species equivalence ratio in a reaction
[0130] .GAMMA. Surface site density (mol/cm.sup.2)
[0131] .eta. Efficiency of the unit cell
[0132] Subscripts
[0133] 0 Value at the inlet
[0134] in Inlet
[0135] g Index for species
[0136] out Outlet
* * * * *