U.S. patent application number 10/725565 was filed with the patent office on 2005-02-10 for combustion system.
Invention is credited to Doebbeling, Klaus, Gutmark, Ephraim, Paschereit, Christian Oliver.
Application Number | 20050032014 10/725565 |
Document ID | / |
Family ID | 9949079 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050032014 |
Kind Code |
A1 |
Doebbeling, Klaus ; et
al. |
February 10, 2005 |
Combustion system
Abstract
A combustion system for a heat generator, the burner 1 being
connected to a combustion chamber 6 by means of an outlet 10,
wherein the outlet 10 comprises a multiply stepped transional
structure 11 in the direction of flow of fluid 3 in the burner 1 so
as to create turbulence in the fluid flow.
Inventors: |
Doebbeling, Klaus;
(Windisch, CH) ; Gutmark, Ephraim; (Cincinnati,
OH) ; Paschereit, Christian Oliver; (Berlin,
DE) |
Correspondence
Address: |
CERMAK & KENEALY LLP
P.O. BOX 7518
ALEXANDRIA
VA
22307
US
|
Family ID: |
9949079 |
Appl. No.: |
10/725565 |
Filed: |
December 3, 2003 |
Current U.S.
Class: |
431/350 |
Current CPC
Class: |
F23D 14/46 20130101;
F23D 2210/00 20130101; F23C 2900/07002 20130101; F23D 14/02
20130101; F23C 7/002 20130101 |
Class at
Publication: |
431/350 |
International
Class: |
F23D 014/46 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2002 |
GB |
0228319.0 |
Claims
1. A combustion system for a heat generator, comprising: a premix
burner, and outlet, and a combustion chamber, the premix burner
being connected to the combustion chamber by the outlet; wherein
the outlet comprises a multiply-stepped transitional structure in
the direction of flow of fluid so as to create turbulence in the
fluid flow.
2. A combustion system as claimed in claim 1, wherein the
transitional structure comprises three to five steps.
3. A combustion system as claimed in claim 2, wherein the
transitional structure comprises four steps.
4. A combustion system as claimed in claim 1, wherein the length to
height ratio of the steps is from 1:1 to 10:1.
5. A combustion system as claimed in claim 4, wherein the length to
height ratio of the steps is from 1:1 to 7:1.
6. A combustion system as claimed in claim 1, wherein the outlet is
in the form of comprises a nozzle.
7. (Canceled)
Description
[0001] The present invention relates to a combustion system for
generating a hot gas, and in particular to a premix burner
connected to a combustion chamber.
[0002] Many premix burners rely on swirling to produce efficient
mixing of reactants. However, interaction between the complex flow
patterns within the swirling fluid and acoustic resonant modes in
the combustion chamber can lead to undesired thermoacoustic
pulsations or vibrations. These pulsations are associated with
coherent vortical flows in the combustion chamber. The vortical
flows introduce periodicity into the mixing process, which may lead
to periodic heat release and resonant coupling with the combustor
acoustic resonant modes. Vortical mixing of the reactants also
tends to be limited to large scale mixing with the result that
mixing in regions between vortices in the vortical flow tends to be
poor.
[0003] Thermoacoustic vibrations are problematic in combustion
processes, since they can lead to high-amplitude pressure
fluctuations, as well as to a limitation in the operating range of
the burner in question and to increased emissions from the burner.
Many combustion chambers do not possess adequate acoustic damping
to account for such thermoacoustic vibrations.
[0004] In conventional combustion chambers, the cooling air flowing
into the combustion chamber acts to dampen noise and therefore
contributes to the damping of thermoacoustic vibrations. However,
in modern gas turbines, an increasing proportion of the cooling air
is passed through the burner itself in order to achieve low
emissions. The cooling air flow within the combustion chamber is
thus reduced, resulting in reduced damping of the thermoacoustic
vibrations in the chamber.
[0005] Another method of damping is the coupling of Helmholtz
dampers in the combustion chamber, preferably in the region of the
combustion chamber dome or in the region of the cold air supply.
However, such dampers require a considerable amount of space in
order to allow them to be accommodated in the combustion chamber.
Since modern combustion chambers tend to be relatively compact, it
is usually impossible to incorporate Helmholtz dampers in the
combustion chamber without substantial re-design of the
chamber.
[0006] A further method of controlling thermoacoustic vibrations
involves active acoustic excitation. In this process, a shear layer
which forms in the outlet region of the burner is acoustically
excited. A suitable phase lag between the thermoacoustic vibrations
and the excitation vibrations makes it possible to achieve damping
of the combustion chamber due to the superimposition of the
vibrations and the excitation. However, a considerable amount of
energy is expended in generating such acoustic excitation.
[0007] A further means of providing damping in the combustion
chamber is to modulate the fuel mass flow in the burner. Fuel is
injected into the burner with a phase shift relative to measured
signals in the combustion chamber so that additional heat is
released at a minimum pressure This reduces the amplitude of the
thermoacoustic vibrations. However, this technique also leads to
high emissions due to the increased fuel.
[0008] A further alternative is to inject air into the burner via
nozzles to disturb and break up the vortical flow. However, the
required additional pipes and plumbing complicates the design of
the combustor. Furthermore, the required additional air flow
reduces the overall efficiency.
[0009] In a similar technique, the vortical flow is broken up by
baffles which are located inside the burner in order to disturb the
vortical flow. However, the inclusion of such baffles increases the
constructional outlay of the burner, which is disadvantageous.
[0010] An object of the present invention is to provide a
combustion system in which the above disadvantages are
overcome.
[0011] The invention provides a combustion system for a heat
generator, comprising a premix burner and a combustion chamber, the
premix burner being connected to the combustion chamber by means of
an outlet, wherein the outlet comprises a multiply stepped
transitional structure in the direction of flow of fluid so as to
create turbulence in the fluid flow.
[0012] In contrast to the sharp-edged transition between the premix
burner and the combustion chamber the combustion system designed
according to the invention has a gradual transition between the
premix-burner and the combustion chamber, said transition having a
segmented line-up of rectilinearly designed side wall portions
forming a multiply stepped transitional structure. The term
"multiply stepped transition" is intended to mean basically any
transitional geometry which widens in steps the flow cross section
within the premix burner, which is dimensioned smaller than that
within the combustion chamber, successively to the combustion
chamber cross section.
[0013] In a preferred embodiment of the invention, the transitional
structure comprises three to five steps, and preferably four.
[0014] By a gradual transition being provided between the premix
burner and the combustion chamber, the widening of the fuel/air
mixture entering the combustion chamber is increased considerably,
the result of this being, even in the case of a gradual transition,
that a marginal flow having cross vortices is formed, which,
however, impinges onto the combustion chamber wall at a
reapplication point which is very much nearer in the direction of
the premix burner than in the case of a sharp-stepped transition.
This has an advantageous effect on the combustion process in two
respects. Thus, on the one hand, the marginal flow having cross
vortices is reduced, and therefore the intensity and number of the
cross vortices formed are also reduced, with the result that the
combustion chamber pulsation generated by thermoacoustic vibrations
can be decisively damped. On the other hand, by virtue of the
markedly greater widening of the fuel/air mixture propagated within
the combustion chamber, the dead space caused by shading-off
effects is reduced to a minimum, with the result that virtually the
entire combustion chamber volume is available for the combustion of
the fuel/air mixture and ensures complete combustion of the
fuel.
[0015] The invention will now be described in detail with reference
to the accompanying drawings, in which:
[0016] FIG. 1 is a cross-sectional view of a burner according to
the invention attached to a combustion chamber;
[0017] FIGS. 2a and 2b are graphs showing the effect of the
invention on pressure fluctuations.
[0018] In FIG. 1, a heat generator has a burner 1 with a swirl
generator 2. The swirl generator 2 generates a swirl 3 with an
axial flow component facing toward a downstream burner outlet 4.
Mixing takes place in an area 5 of the generator 2, so as to ensure
adequate mixing of fuel and combustion air. The axial flow
cross-section of the area 5 widens in the direction of the outlet
4; this configuration facilitates attainment of a constant swirl 3
in the area 5 with an increasing combustion air mass flow in the
direction of the longitudinal axis B of the burner 1. The generator
2 comprises two hollow partial cones (not shown) arranged offset to
one another. The offset of the respective centre axes of the
partial conical bodies creates two tangential air channels 6. A
combustion air flow 7 flows, with a relatively high tangential
velocity component, through the two tangential channels 6 into the
area 5, thus generating the swirl 3. Fuel is introduced into the
burner 1 via a fuel inlet 8 in the form of a nozzle.
[0019] The burner 1 is attached to a combustion chamber 9 via an
outlet 10 through which the swirl 3 passes. The swirl 3 contains
vortical flow, which causes flow instabilities including
thermoacoustic vibrations which result in low performance of the
combustion chamber.
[0020] The outlet 10 is provided with a series of steps 11, 11a and
11b. The steps 11, 11a and 11b induce multiple inflection points
into the swirl 3 as a result of the sudden change of velocity of
the flow at the steps 11, 11a and 11b. Multiple sources of
turbulence are thus formed. This increased turbulence serves to
break up the existing vortical flow in the swirl 3, thus
stabilising the flow. As a result the performance of the combustion
chamber 6 is improved. Furthermore, the increased turbulence
results in better small scale mixing. It should be noted, however,
that emissions are not noticeably increased as a result of the
increased turbulence.
[0021] The preferred range of the ratio of the length to the height
of the steps 11, 11a, 11b is 1:1-7:1, but can be as large as 10:1.
The number of steps depends on the expansion ration at the outlet
10, on the re-attachment length, and the selected length to height
ratio. The number of steps is usually between three and five.
However, one single step can be effective. This is particularly so,
if the step height is the same as the amplitude of the dominant
vortices.
[0022] FIG. 2a shows the effect of the burner according to the
invention on pressure fluctuations according to variation in Lambda
number. Line 12 is effectively a baseline, i.e. it represents a
burner which has not been modified in any way. Line 13 represents a
burner having steps 11, 11a and 11b with a length to height ratio
of 1:1. Line 14 represents a nozzle with extended steps, i.e. steps
extended beyond a recirculation zone. This configuration can lead,
however, to a destabilisation in combustion.
[0023] FIG. 2b shows the effect of the burner according to the
invention on pressure fluctuations according to variation in power.
Line 12a is effectively a baseline, i.e. it represents a burner
which has not been modified in any way. Line 13a represents a
burner having steps 11, 11a and 11b. Line 14a represents a burner
with extended steps.
[0024] It will be appreciated that variations of the embodiment
described above are possible. Alternative configurations of pre-mix
burners are well-known to persons skilled in the art. Similarly, it
would be possible to replace the conical swirl generator 2 with a
cylindrical swirl generator. It is also known to arrange a
displacement body, tapering towards the outlet 10, inside the swirl
generator; this could provide a further alternative embodiment of
the invention.
[0025] The number and depth of the steps could also be varied.
* * * * *