U.S. patent application number 14/981555 was filed with the patent office on 2016-04-21 for tube for a heat exchanger.
This patent application is currently assigned to BORGWARNER EMISSIONS SYSTEMS SPEAIN, S.L.U.. The applicant listed for this patent is BORGWARNER EMISSIONS EYSTEMS SPAIN, S.L.U.. Invention is credited to Rafael JULIANA, Manuel LORENZO.
Application Number | 20160109188 14/981555 |
Document ID | / |
Family ID | 52354735 |
Filed Date | 2016-04-21 |
United States Patent
Application |
20160109188 |
Kind Code |
A1 |
JULIANA; Rafael ; et
al. |
April 21, 2016 |
TUBE FOR A HEAT EXCHANGER
Abstract
The present invention relates to a tube for a heat exchanger
comprising a region with dimples according to a specific pattern
such that it increases the degree of heat exchange with low
pressure drop. The use of a tube for a heat exchanger according to
the invention in heat exchangers for EGR (Exhaust Gas
Recirculation) systems is of particular interest. The present
invention is also considered of interest for the environment as it
increases the efficiency of the heat exchangers for which it is
designed.
Inventors: |
JULIANA; Rafael; (VIGO,
ES) ; LORENZO; Manuel; (Porrino, ES) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BORGWARNER EMISSIONS EYSTEMS SPAIN, S.L.U. |
Vigo |
|
ES |
|
|
Assignee: |
BORGWARNER EMISSIONS SYSTEMS
SPEAIN, S.L.U.
Vigo
ES
|
Family ID: |
52354735 |
Appl. No.: |
14/981555 |
Filed: |
December 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14877415 |
Oct 7, 2015 |
|
|
|
14981555 |
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Current U.S.
Class: |
165/181 |
Current CPC
Class: |
F28F 1/04 20130101; F28D
7/163 20130101; F28F 1/40 20130101; F28F 1/02 20130101; F28F 3/042
20130101; F28F 3/044 20130101; F28F 1/025 20130101; F28F 13/12
20130101; F28F 2001/027 20130101 |
International
Class: |
F28F 1/04 20060101
F28F001/04; F28F 1/40 20060101 F28F001/40 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2014 |
EP |
14382382.1 |
Claims
1. A tube for a heat exchanger having a flattened configuration
comprising a first essentially planar face and a second essentially
planar face parallel to and spaced from the first face; both faces
being connected by side walls where at least one face has a
plurality of dimples configured according to a pattern that is
repeated along the longitudinal direction X-X' of the tube, where
this pattern is configured by means of a plurality of groups of
dimple alignments covering the width of the planar face, aligned
according to one and the same direction Y-Y' arranged obliquely
between 40.degree. and 50.degree. degrees with respect to the
longitudinal direction X-X', and where it at least comprises: a
first group which is formed by two dimples, a first elongated
dimple extending according to the oblique direction Y-Y' aligned
with a first point dimple such that a first flow channel is
established between the elongated dimple and the point dimple, a
second group which is formed by four point dimples, a second point
dimple, a third point dimple, a fourth point dimple and a fifth
point dimple, essentially aligned according to the oblique
direction Y-Y', such that a second flow channel is established
between the second point dimple and the third point dimple, a third
flow channel is established between the third point dimple and the
fourth point dimple, a fourth flow channel is established between
the fourth point dimple and the fifth point dimple, a third group
which is formed by two dimples, a sixth point dimple aligned with a
second elongated dimple, extending according to the oblique
direction Y-Y', such that a fifth flow channel is established
between the sixth point dimple and the second elongated dimple, a
fourth group which is formed by four point dimples, a seventh
dimple, an eighth dimple, a ninth dimple and a tenth dimple,
essentially aligned according to the oblique direction Y-Y', such
that a sixth flow channel is established between the seventh point
dimple and the eighth point dimple, a seventh flow channel is
established between the eighth point dimple and the ninth point
dimple, an eighth flow channel is established between the ninth
point dimple and the tenth point dimple, where the position of the
first flow channel is such that the flow going through same in the
operating mode is mainly transported towards the third flow channel
and the fourth flow channel.
2. The tube according to claim 1, where in the second group formed
by four point dimples essentially aligned according to the oblique
direction Y-Y', the first pair of dimples formed by the second
point dimple and third point dimple is aligned according to the
oblique direction Y-Y', the second pair of dimples formed by the
fourth point dimple and the fifth point dimple is aligned according
to the oblique direction Y-Y'; and the alignment of the first pair
and the alignment of the second pair are misaligned a specific
distance despite being parallel.
3. The tube according to claim 1, where in the fourth group formed
by four point dimples essentially aligned according to the oblique
direction Y-Y', the first pair of dimples formed by the seventh
dimple and the eighth dimple is aligned according to the oblique
direction Y-Y', the second pair of dimples formed by the ninth
dimple and the tenth dimple is aligned according to the oblique
direction Y-Y'; and the alignment of the first pair and the
alignment of the second pair are misaligned despite being
parallel.
4. The tube according to claim 1, where the first flow channel, the
third flow channel and the fifth flow channel are essentially
aligned.
5. The tube according to claim 1, where the first flow channel, the
fourth flow channel and the eighth flow channel are essentially
aligned.
6. The tube according to claim 1, where the second flow channel,
the fifth flow channel and the sixth flow channel are essentially
aligned.
7. The tube according to claim 1, where the second point dimple,
the fifth point dimple, the seventh point dimple and the tenth
point dimple are widened in the direction transverse to the oblique
direction Y-Y'.
8. The tube according to claim 1, where the tube has one and the
same pattern on the first face and the second face.
9. The tube according to claim 1, where it comprises on a first
flattened face one or more patterns distributed along the length of
the tube and comprises on a second flattened face, the opposite
face, one or more patterns distributed along the length of the
tube, and where the pattern of the first face and the pattern of
the second face are distributed longitudinally such that they
extend with the same length, the pattern of the first face and the
pattern of the second face are positioned such that the end of the
first elongated dimple located closest to the central axis of the
tube of the first face and the end of the second elongated dimple
located closest to the central axis of the tube of the second face
overlap one another according to a projection on a plane parallel
to both faces.
10. A heat exchanger comprising at least one tube, the tube having
a flattened configuration comprising a first essentially planar
face and a second essentially planar face parallel to and spaced
from the first face; both faces being connected by side walls where
at least one face has a plurality of dimples configured according
to a pattern that is repeated along the longitudinal direction X-X'
of the tube, where this pattern is configured by means of a
plurality of groups of dimple alignments covering the width of the
planar face, aligned according to one and the same direction Y-Y'
arranged obliquely between 40.degree. and 50.degree. degrees with
respect to the longitudinal direction X-X', and where it at least
comprises: a first group which is formed by two dimples, a first
elongated dimple extending according to the oblique direction Y-Y'
aligned with a first point dimple such that a first flow channel is
established between the elongated dimple and the point dimple, a
second group which is formed by four point dimples, a second point
dimple, a third point dimple, a fourth point dimple and a fifth
point dimple, essentially aligned according to the oblique
direction Y-Y', such that a second flow channel is established
between the second point dimple and the third point dimple, a third
flow channel is established between the third point dimple and the
fourth point dimple, a fourth flow channel is established between
the fourth point dimple and the fifth point dimple, a third group
which is formed by two dimples, a sixth point dimple aligned with a
second elongated dimple, extending according to the oblique
direction Y-Y', such that a fifth flow channel is established
between the sixth point dimple and the second elongated dimple, a
fourth group which is formed by four point dimples, a seventh
dimple, an eighth dimple, a ninth dimple and a tenth dimple,
essentially aligned according to the oblique direction Y-Y', such
that a sixth flow channel is established between the seventh point
dimple and the eighth point dimple, a seventh flow channel is
established between the eighth point dimple and the ninth point
dimple, an eighth flow channel is established between the ninth
point dimple and the tenth point dimple, where the position of the
first flow channel is such that the flow going through same in the
operating mode is mainly transported towards the third flow channel
and the fourth flow channel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/877,415 filed Oct. 7, 2015, which is
incorporated herein in its entirety by reference thereto.
OBJECT OF THE INVENTION
[0002] The present invention relates to a tube for a heat exchanger
comprising a region with dimples according to a specific pattern
such that it increases the degree of heat exchange with low
pressure drop.
[0003] The use of a tube for a heat exchanger according to the
invention in heat exchangers for EGR (Exhaust Gas Recirculation)
systems is of particular interest. The present invention is also
considered of interest for the environment as it increases the
efficiency of the heat exchangers for which it is designed.
BACKGROUND OF THE INVENTION
[0004] Heat exchangers are devices promoting heat transfer between
two fluids through an exchange surface which prevents the mixing of
both fluids. The amount of transferred heat is proportional to the
exchange surface and also proportional to the exchange rate per
unit area.
[0005] The heat exchange rate per unit area depends primarily on
flow conditions around the exchange surface and on fluid
properties. Given that convective heat transfer mechanisms are much
more efficient than diffusive heat transfer mechanisms, solutions
used in the state of the art seek to enhance convection in the
vicinity of the exchange surface.
[0006] One of the means for increasing convection in the vicinity
of the exchange surface is the use of fins. The most widely used
fins are projections in the form of plates emerging from the
surface. Fins have a dual effect on the flow flowing in the
vicinity of the exchange surface, the first effect being an
increase in the effective surface in contact with the fluid since
the exchange surface remains the same except for the area where the
fin starts, plus the area on both sides of the fin; and the second
effect being an increase in turbulence in the vicinity of the
exchange surface.
[0007] Increasing turbulence enhances convection effects since the
occurrence of vortices having a small characteristic diameter
generates movement structures favoring mass exchange between the
vicinity of the exchange surface and regions further away where the
main flow is located, the final result being increased convective
heat transport between the exchange surface and the fluid.
[0008] This clear advantage has the drawback that although heat
exchange increases, pressure drop also increase. When the heat
exchange area is formed by tubes through which a first fluid flows
and said tubes are immersed in a second fluid with which it
exchanges heat and which in turn is housed in a shell, pressure
drop occur both between shell inlet and outlet and between tube
inlet and outlet.
[0009] A high pressure drop involves the need for higher propulsion
energy. The pressure drop allows establishing, together with heat
exchange rate, the overall degree of efficiency of the heat
exchanger. An efficient heat exchanger therefore requires not only
a high heat exchange rate but also low pressure drop.
[0010] A high heat exchange rate allows reducing the volume
necessary for housing the exchange surface for specific heat
requirements, for example.
[0011] A particular case of tubes for a heat exchanger that uses
fins are those known in this state of the art as hybrid tubes, fin
and tube or fin and plate. Hybrid tubes are tubes formed by means
of sheet metal the shape of which is of a planar configuration,
i.e., according to the section thereof, it has two parallel planar
segments laterally closed by means of respective curved
segments.
[0012] The inside of this tube having a planar configuration has a
second part formed by a sheet with several bends forming fins where
this sheet is trapped between the two planar surfaces.
[0013] Such tube is very efficient as regards exchange rate but has
specific problems due to the thermal expansion effect of the inner
sheet, for example, which can cause fatigue in areas where this
expansion generates stresses. This is the case of the tube
embedment area, for example, where expansion of the inner part is
limited by the embedment.
[0014] An alternative to such tubes is a tube also having a planar
configuration containing dimples on at least one of its faces.
Dimples are not as efficient as fins since they cannot be compacted
as much nor can they increase the exchange area to the same extent,
but they have the advantage of not having the expansion problems
indicated above and of increasing tube rigidity.
[0015] The conventional way of manufacturing such tubes is based on
a drawn metal sheet that is subsequently bent according to a
longitudinal direction until the tube is closed. The tube is closed
by means of a longitudinally extending weld bead joining the edges
facing one another, for example.
[0016] The presence of dimples increase turbulence and therefore
the heat transfer rate. Planar tubes formed by dimples having
various dimple patterns to increase the heat exchange rate are
known.
[0017] Patterns that are modified downstream to take into account
flow variations due to the influence of dimples located upstream
are also known. Nevertheless, such patterns make manufacturing the
tube difficult since the same mold cannot be used in stamping along
the entire length of the tube or actuators modifying the depth of
the dimples as the sheet stamping operation for obtaining the tube
progresses are required in machines. Although these patterns
increase the heat transfer rate, they also increase pressure
drop.
[0018] The present invention relates to a tube incorporating
dimples for increasing the heat transfer rate which allows
overcoming the drawbacks identified above using a dimple pattern
combining a high heat exchange rate and a low pressure drop.
[0019] Description of the Invention
[0020] The tube for a heat exchanger according to the present
invention is a tube having a flattened configuration comprising a
first essentially planar face and a second essentially planar face
parallel to and spaced from the first face. The flattened
configuration of the tube must therefore be interpreted by the
presence of the first planar face and the second planar face
arranged parallel to one another and both faces being connected by
side walls. Although the side walls can have any configuration, the
most common embodiments use a configuration in a semicircular
segment according to the section of the tube. At least one face has
a plurality of dimples configured according to a pattern that is
repeated along the longitudinal direction X-X' of the tube. The
longitudinal direction of the tube is the direction along which the
directrix of the tube extends.
[0021] The pattern is configured by means of a plurality of groups
of dimple alignments covering the width of the planar face. The
dimples usually have a fillet radius in the root so that stamping
does not generate excessive residual stresses or deformation does
not weaken the sheet. This fillet radius causes a certain spacing
between other dimples or the walls of the tube so the indication
that the dimples cover the width of the planar face must be
interpreted to mean that there can be a transition area or a
passage channel between the dimples located on the sides of the
planar face and the walls of the tube.
[0022] The plurality of groups of alignments complies with:
being aligned according to one and the same direction Y-Y' arranged
obliquely between 40.degree. and 50.degree. degrees with respect to
the longitudinal direction X-X, and where at least the tube
comprises: [0023] a first group which is formed by two dimples, a
first elongated dimple extending according to the oblique direction
Y-Y' aligned with a first point dimple such that a first flow
channel is established between the elongated dimple and the point
dimple, [0024] a second group which is formed by four point
dimples, a second point dimple, a third point dimple, a fourth
point dimple and a fifth point dimple, essentially aligned
according to the oblique direction Y-Y', such that a second flow
channel is established between the second point dimple and the
third point dimple, a third flow channel is established between the
third point dimple and the fourth point dimple, a fourth flow
channel is established between the fourth point dimple and the
fifth point dimple, [0025] a third group which is formed by two
dimples, a sixth point dimple aligned with a second elongated
dimple, extending according to the oblique direction Y-Y such that
a fifth flow channel is established between the sixth point dimple
and the second elongated dimple, [0026] a fourth group which is
formed by four point dimples, a seventh dimple, an eighth dimple, a
ninth dimple and a tenth dimple, essentially aligned according to
the oblique direction Y-Y', such that a sixth flow channel is
established between the seventh point dimple and the eighth point
dimple, a seventh flow channel is established between the eighth
point dimple and the ninth point dimple, an eighth flow channel is
established between the ninth point dimple and the tenth point
dimple, wherein the position of the first flow channel is such that
the flow going through same in the operating mode is mainly
transported towards the third flow channel and the fourth flow
channel.
[0027] The pattern contains elongated dimples and point dimples.
The elongated dimples have a preferred direction. When a group of
dimples is indicated as being aligned in a specific direction, it
must be interpreted as the elongated dimples extending with the
orientation of the alignment, and furthermore the axis along which
they extend also passes through the locations of the point dimples
except for possible minor deviations.
[0028] The pattern formed by each group which is formed by the
plurality of dimples has an oblique orientation Y-Y' towards one
and the same side. This orientation leads to the existence of speed
components that are also oblique but does not reorient the entire
flow in this direction. The strategic position of passage sites
with continuity between consecutively arranged groups of dimples
generates preferred passage channels oriented either in the
longitudinal direction or even in the oblique direction but with an
opposite inclination with respect to the oblique direction Y-Y'
according to the symmetry defined by the longitudinal axis
X-X'.
[0029] Alternating one oblique direction and another increases
local turbulence, in turn allowing significant flow through the
channels thus identified. The state of the art teaches that the
existence of a large number of dimples increases turbulence but
reduces the effective section of the tube since they are
projections that penetrate the tube, increasing resistance to
passage.
[0030] The special configuration according to the present invention
maintains a large number of dimples having the effect of a high
degree of turbulence, but it does not significantly reduce the
effective section since the formation of channels as established
according to the particular dimple pattern according to the
invention maintains a flow with low pressure drop since the flow
finds preferred passage channels.
[0031] The formation of preferred passage channels depends not only
on the relative position of the space between dimples downstream of
a specific channel but also on the influence of other surrounding
dimples. The final behavior of a specific dimple design cannot be
predicted due to this strong link between neighboring dimples both
laterally and in both the downstream and upstream flow direction,
and much less so when the use of a pattern which is periodically
reproducible along the length of the tube is of interest such as
the case where there are changes from one pattern to the next in
the downstream flow. The only way to determine if a pattern
complies with efficiency objectives, i.e., high heat transfer with
few losses, is by flow simulation in a tube incorporating the
proposed pattern. This simulation can be carried out by means of
numerical models (CFD (Computational Fluid Dynamics) simulations),
for example. The simulations allow carrying out numerical
experiments assessing heat transfer and flow behavior where, in
contrast, comparison with other known patterns never allows
extrapolating the results.
[0032] The pattern object of the present invention has been proven
to efficiently solve the described drawbacks. The combination of a
distribution of groups of dimples in one direction combined with
the channels formed between consecutively arranged sets has been
proven to have an perturbing influence on the surrounding flow
extending a smaller distance than those known in the state of the
art according to the direction perpendicular to the plane on which
the dimples are arranged. This allows the symmetrical arrangement
on both opposite faces of the tube to continue to maintain the
increase in heat transfer by pattern shown by the use of a pattern
on a single face; i.e., the flow modified by the pattern on one
face does not negatively interfere with the flow modified by the
dimple pattern of the opposite face, something that does occur with
other patterns when they are located facing one another.
[0033] The configuration of the pattern proposed by the invention
causes vortices to be generated downstream of the dimples which are
then homogenized as a result of the fluid flow established through
specific preferred channels that are generated with the spaces
between dimples. Particularly, an S-shaped or similarly shaped path
with vortices is formed, successfully increasing heat transfer such
that it makes up for the pressure drop resulting from the
dimples.
DESCRIPTION OF THE DRAWINGS
[0034] The foregoing and other features and advantages of the
invention will be more clearly understood based on the following
detailed description of a preferred embodiment provided only by way
of illustrative and non-limiting example in reference to the
attached drawings.
[0035] FIG. 1 shows a plan view of a tube incorporating along its
length X-X' an alignment of patterns according to one embodiment of
the invention.
[0036] FIG. 2 shows a Nusselt number variation map for determining
the increase in heat transfer on the surface incorporating the
plurality of patterns according to the embodiment shown in FIG. 1
in comparison with the heat transfer if this were to occur only by
conduction.
[0037] FIG. 3 shows a tube where both flattened faces have a
pattern such as that of the example shown in FIG. 1, i.e., repeated
along the length of the tube. This figure shows the two patterns
being superimposed on one another, the pattern of the visible face
being shown by means of continuous lines and the pattern of the
opposite non-visible face being shown by means of dotted lines.
DETAILED DESCRIPTION OF THE INVENTION
[0038] According to the first inventive aspect, the present
invention relates to a tube for a heat exchanger showing high
convective heat transfer without significant pressure drop. The
configuration of the tube is planar, i.e., formed by two laterally
connected parallel planar surfaces where there is a plurality of
dimples according to a specific pattern located on one or both
planar faces.
[0039] FIG. 1 shows a tube according to an embodiment having a
flattened configuration. In this embodiment, the pattern shows a
plurality of dimples (A, B, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
configured according to a pattern that is repeated along the
longitudinal direction X-X' of the tube. Direction X-X' is the
longitudinal direction of the tube.
[0040] The group of dimples can be repeated along the length of the
tube as many times as necessary.
[0041] The pattern is configured by means of a plurality of groups
of dimple alignments covering the width of the planar face, with
the understanding that there is a fillet radius between the side
dimples and the edge. The plurality of groups of alignments are
aligned according to one and the same direction Y-Y' arranged
obliquely between 40.degree. and 50.degree. degrees with respect to
the longitudinal direction X-X'. In this embodiment shown in FIG.
1, the angle of the oblique axis Y-Y' is 45 degrees.
[0042] Particularly, the pattern is formed by at least four
groups.
[0043] The first group is formed by two dimples (A, 1), a first
elongated dimple (A) extending according to the oblique direction
Y-Y' aligned with a first point dimple (1) such that a first flow
channel (C1) is established between the elongated dimple (A) and
the point dimple (1).
[0044] The flow channel is a gap between the dimples, i.e., the
first elongated dimple (A) and the first point dimple (1). A flow
channel will be understood hereinafter as such gaps between
dimples.
[0045] The flow can go through the position of a dimple since said
dimple does not reach the opposite wall of the tube. Nevertheless,
it imposes oblique speed components. The passage channels are not
subjected to this imposition that laterally diverts part of the
flow and allows recovering either a longitudinal trajectory or even
also an oblique trajectory but with an inclination opposite that of
the Y-Y' axis with respect to the X-X' axis.
[0046] The second group is formed by four point dimples, a second
point dimple (2), a third point dimple (3), a fourth point dimple
(4) and a fifth point dimple (5), essentially aligned according to
the oblique direction Y-Y', such that a second flow channel (C2) is
established between the second point dimple (2) and the third point
dimple (3), a third flow channel (C3) is established between the
third point dimple (3) and the fourth point dimple (4), a fourth
flow channel (C4) is established between the fourth point dimple
(4) and the fifth point dimple (5).
[0047] As indicated above, the second channel (C2), the third
channel (C3) and the fourth channel (C4) do not have to be fed
completely by the channels arranged upstream, but they can also
receive flow from the main flow of the tube. Such a complex
response of the flow to a specific pattern hence arises. In this
particular case, the way to align the different flow channels
formed between dimples determining different effects on the flow as
will be identified below when describing all the elements forming
the pattern is of special interest.
[0048] The third group is formed by two dimples (6, B), a sixth
point dimple (6) aligned with a second elongated dimple (B),
extending according to the oblique direction Y-Y', such that a
fifth flow channel (C5) is established between the sixth point
dimple (6) and the second elongated dimple (B).
[0049] The fourth group is formed by four point dimples, a seventh
dimple (7), an eighth dimple (8), a ninth dimple (9) and a tenth
dimple (10), essentially aligned according to the oblique direction
Y-Y', such that a sixth flow channel (C6) is established between
the seventh point dimple (7) and the eighth point dimple (8), a
seventh flow channel (C7) is established between the eighth point
dimple (8) and the ninth point dimple (9), an eighth flow channel
(C8) is established between the ninth point dimple (9) and the
tenth point dimple (10).
[0050] Having identified the third group and the fourth group, it
is important to highlight the similarity of these groups with the
first and second groups. Nevertheless, this similarity is not by
means of a ratio of symmetry since they maintain the inclination in
the same direction, nor is it by means of ratio of repetition since
the elongated dimple (B) in the third group is located downstream
instead of upstream or at the start of the alignment as occurs in
the first group.
[0051] In other words, as a result the alignment of all the groups
follows the same inclination but the particular alignments of the
channels existing between dimples according to the flow direction
do not. Although a channel is formed by the passage between two
dimples, the channel is considered oriented where the orientation
is the direction perpendicular to the line connecting the centroids
of both point dimples. Generally, the orientation of the channel
will be the direction perpendicular to the line connecting both
dimples where said connection is between the regions closest to one
another.
[0052] In the pattern shown in FIG. 1, it is observed that the
position of the first flow channel (C1) is such that the flow going
through same in the operating mode is mainly transported towards
the third flow channel (C3) and the fourth flow channel (C4). This
branching has an effect on the way in which half the pattern
arranged downstream formed by the third and fourth groups is
fed.
[0053] It has been pointed out that the four groups have an oblique
orientation and the channels formed between dimples establish speed
components with transverse components giving rise to oblique flows
but with an inclination opposite the Y-Y' direction. In this
embodiment, it is observed that in the second group formed by four
point dimples essentially aligned according to the oblique
direction Y-Y', the first pair of dimples formed by the second
point dimple (2) and third point dimple (3) is aligned according to
the oblique direction Y-Y', the second pair of dimples formed by
the fourth point dimple (4) and the fifth point dimple (5) is
aligned according to the oblique direction Y-Y'; and the alignment
of the first pair and the alignment of the second pair are
misaligned a specific distance despite being parallel.
[0054] This misalignment is shown in FIG. 1 by means of two close
parallel lines Y-Y'. The technical effect is a change in
orientation of the third channel (C3) causing a higher degree of
inclination in the direction opposite the inclination of the axis
Y-Y' so that a higher flow rate is directed towards the fifth
channel (C5) which is located close to the side opposite the side
which tends to divert the inclination of the axis Y-Y' imposed on
all the groups of dimples. This fifth channel (C5) mainly feeds the
passage through the remaining channels located downstream resulting
in greater flow uniformity through all the dimples such that the
largest possible exchange area is used.
[0055] This same technical effect is achieved with the
configuration of the fourth group of point dimples, which are
essentially aligned according to the oblique direction Y-Y'. The
first pair of dimples formed by the seventh dimple (7) and the
eighth dimple (8) is aligned according to the oblique direction
Y-Y', the second pair of dimples formed by the ninth dimple (9) and
the tenth dimple (10) is aligned according to the oblique direction
Y-Y'; and the alignment of the first pair and the alignment of the
second pair are misaligned despite being parallel.
[0056] This configuration makes homogenizing the flow easier when
the pattern is periodically repeated downstream.
[0057] Generating preferred paths according to an oblique direction
with an inclination opposite that established by the axis Y-Y' is
enhanced by the condition that the first flow channel (C1), the
third flow channel (C3) and the fifth flow channel (C5) are
essentially aligned. This preferred path provides a flow reaching
even the side opposite the side on which the inclination of the
axis Y-Y' offers the bias thereof.
[0058] This configuration with the alignment of three channels, the
first flow channel (C1), the third flow channel (C3) and the fifth
flow channel (C5), also contributes to flow homogenization when the
pattern is periodically repeated downstream.
[0059] The described pattern can also be configured as shown in
FIGS. 1 and 3, such that there is an alignment between the first
channel (C1), the fourth channel (C4) and the eighth channel (C8).
These three channels form a preferred path for longitudinal guiding
of the flow so that it does not tend to keep excessively towards
the outside of the pattern, the side wall of the tube. In the
alignment of these three channels (C1, C4, C8), the elongated
dimple (B) of the third group is interposed between the fourth
channel (C4) and the eighth channel (C8). Nevertheless, this
interposition does not prevent the effect of the alignment since,
as indicated above, the flow reaching a channel is fed not only by
the flow from channels arranged upstream but also by the main flow
of the tube.
[0060] Given that the preferred path formed by the alignment of
channels is configured between dimples, this path is undulated and
enhances vorticity generation, having as an immediate consequence
an increase in the degree of heat transfer with the surface of the
tube.
[0061] Additionally, the pattern according to this same embodiment
greatly enhances the creation of preferred paths on the side
opposite the side on which the inclination of the axis Y-Y' offers
the bias thereof, establishing the alignment of the second channel
(C2), the fifth channel (C5) and the sixth channel (C6).
[0062] In this same example, the second point dimple (2), the fifth
point dimple (5), the seventh point dimple (7) and the tenth point
dimple (10) are widened in the direction transverse to the oblique
direction Y-Y'. The technical effect of this widening is the
prolongation of the dimples towards the sides of the tube so that
the main flow does not seek preferred paths for passage through
these sides and is concentrated on the segment with dimples for
increasing heat exchange.
[0063] In this same embodiment, the pattern shown in FIG. 1 is
repeated on the opposite face. It has been proven by means of
numerical simulation that pressure drop due to the inclusion of a
dimple pattern on one face, when comparing said pressure drop with
that obtained in an embodiment in which only one face of the tube
has a dimple pattern, is not greater than two-fold even when the
projection according to a plane transverse to the tube shows that
the dimples have a depth reducing the section on both faces to one
third. In other words, the heat transfer:pressure drop ratio is
more favorable if there is a dimple pattern on both faces of the
tube than if there is a pattern on only one face.
[0064] This means that flow perturbations generated by the dimples
on one face do not have a negative influence on the pressure drop
in the other face. This influence will be even less with sections
in which the depth of the dimples is proportionally less than that
indicated.
[0065] FIG. 2 depicts the Nusselt number value on the surface of
the tube, a high Nusselt number value being observed homogenously
for all the dimples. This indicates that, for the position
corresponding to a dimple, there is a large increase in heat
transfer on the surface in comparison with the heat transfer that
occurs only by conduction, and all this with reduced pressure
drop.
[0066] Said Nusselt number value is obtained by means of the
following ratio:
Nu = h ef L ref k ##EQU00001##
where L.sub.ref is the hydraulic diameter of the tube, k is the
conductivity of said tube and h.sub.ef is the film coefficient
value, or convective heat transfer coefficient according to the
ratio:
h ef = q T w - T ref ##EQU00002##
where q is the thermal power transferred in the convective process,
T.sub.w is the wall temperature of the tube and T.sub.ref is a
reference temperature value which corresponds to the temperature at
the hot gas inlet of the tube.
[0067] In the example shown in FIG. 2, 6.75 mm has been used as the
value of the hydraulic radius L.sub.ref.
[0068] In the example shown in FIG. 2, 673.15 K has been used as
the reference temperature value T.sub.ref.
[0069] The figure therefore shows the distribution of Nusselt
number values as a function of the heat transfer that occurs with
the passage of a hot gas flow through the tube, the direction of
which is also shown in the figure.
[0070] A dimple pattern of these characteristics results in a
highly efficient tube when it is part of a heat exchanger,
according to the transferred thermal power values as demonstrated
by the Nusselt number values represented.
[0071] FIG. 3 shows a tube where both flattened faces have a
pattern such as that of the example shown in FIG. 1 which is
repeated along the length of the tube. This figure shows the two
patterns being superimposed on one another, the pattern of the
visible face is shown by means of continuous lines and the pattern
of the opposite non-visible face is shown by means of dotted lines.
To make identifying the dimples on both faces easier, the reference
numbers of the visible face are located on each dimple whereas the
numbers of the dimples located on the non-visible face are located
outside the area occupied by the depiction of the tube.
[0072] The pattern of the first face and the pattern of the second
face are distributed longitudinally such that they extend the same
length. In this case, it is not only because the dimples have the
same size, but because the distribution shown on both faces takes
up the same length.
[0073] The pattern of the first face and the pattern of the second
face are positioned such that the end of the first elongated dimple
(A) located closest to the central axis of the tube of the first
face and the end of the second elongated dimple (B) located closest
to the central axis of the tube of the second face overlap one
another according to a projection on a plane parallel to both
faces. In this manner, flow perturbations due to the presence of
dimples on one face has been shown to not significantly affect the
flow modified by the presence of dimples on the opposite face.
* * * * *