Note: Descriptions are shown in the official language in which they were submitted.
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COMBUSTION CHAMBER/VENTURI COOLING FOR A LOW NOx EMISSION
COMBUSTOR - PCT
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an apparatus and method
for cooling the Combustion chamber and venturi used in a gas
turbine engine for reducing nitric oxide emissions.
Specifically an apparatus is disclosed for cooling the
combustion chamber/venturi to lower nitric oxide (NOx) emissions
by introducing preheated cooling air into the premix chamber for
use in the combustion process.
2. Description of Related Art
The present invention is used in a dry, low NOx gas turbine
engine typically used to drive electrical, generators. Each
Combustor includes an upstream premix fuel/air chamber and a
downstream combustion chamber separated by a venturi having a
narrow throat constriction that acts as a flame retarder. The
invention is concerned with improving the cooling of the
combustion chamber which includes the venturi walls while at the
same time reducing nitric oxide emissifons.
U.S. Patent 4,292,801 describes a gas turbine Combustor
that includes upstream premix of fuel and air and a downstream
combustion chamber.
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U.S. Patent 5,117,636 and U.S. Patent 5,285,631 deal with
cooling the combustion chamber wall and the venturi walls. The
patents state that there is a problem with allowing the cooling
air passage to dump into the combustion chamber if the passage
exit is too close to the venturi throat. The venturi creates a
separation zone downstream of the divergent portion which causes
a pressure difference thereby attracting cooling air which can
cause combustion instabilities. However, it is also essential
that the venturi walls and combustion chamber wall be adequately
cooled because of the high temperatures developed in the
combustion chamber.
The present invention eliminates the problem discussed in
the prior art because the cooling circuit for the venturi has
been adjusted such that the cooling air no longer dumps axially
aft and downstream of the venturi throat into the combustion
zone. In fact, cooling air flows in the opposite direction so
that the air used for cooling the combustion chamber and the
venturi is forced into the premix chamber upstream of the
venturi, improving the efficiency of the overall combustion
process while eliminating any type of cooling air recirculation
separation zone aft of the venturi as discussed in the U.S.
Patent 5,117,636.
Recent government emission regulations have become of great
concern to both manufacturers and operators of gas turbine
combustors. Of specific concern is nitric oxide (NOx) due to
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its contribution to air pollution.
It is well known that NOx formation is a function of flame
temperature, residence time, and equivalence ratio. In the
past, it has been shown that nitric oxide can be reduced by
lowering flame temperature, as well as the time that the flame
remains at the higher temperature. Nitric Oxide has also been
found to be a function of equivalence ratio and fuel to air
(f/a) stoiChiometry. That is, extremely low f/a ratio is
required to lower NOx emissions. Lowering f/a ratios do not
come without penalty, primarily the possibility of "blow-out".
"Blow-Out" is a situation when the flame, due to its
instability, can no longer be maintained. This situation is
common as fuel-air stoichiometry is decreased just above the
lean flammability limit. By preheating the premix air, the
"blow-out" flame temperature is reduced, thus allowing stable
combustion at lower temperatures and consequently lower NOx
emissions. Therefore, introducing the preheated air is the
ideal situation to drive f/a ratio to an extremely lean limit to
reduce NOx, while maintaining a stable flame.
In a dual-stage, dual-mode gas turbine system, the
secondary combustor includes a venturi configuration to
stabilise the combustion flame. Fuel (natural gas or liquid)
and air axe premixed in the combustor premix chamber upstream of
the venturi and the air/fuel mixture is fired ox Combusted
downstream of the venturi throat. The venturi configuration
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accelerates the air/ fuel flow through the throat and ideally
keeps the flame from flashing back into the premix region. The
flame holding region beyond the throat in the venturi is
necessary for continuous and stable fuel burning. The
combustion chamber wall and the venturi walls before and after
the narrow throat region are heated by the combustion flame and
therefore must be cooled. In the past, this has been
accomplished with back side impingement cooling which flows
along the back side of the combustion wall and the venturi walls
where the cooling air exits and is dumped into combustion
chamber downstream of the venturi.
The present invention overcomes the problems provided
by this type of air cooling passage by completely eliminating
the dumping of the cooling air into the combustion zone
downstream of the venturi. The present invention does not
permit any airflow of the venturi cooling air into the
downstream combustion chamber whatsoever. At the same time the
present invention takes the cooling air, which flows through an
air passageway along the combustion chamber wall and the
venturi walls and becomes preheated and feeds the cooling air
upstream of the venturi (converging wall) into the premixing
chamber. This in turn improves the overall low emission NOx
efficiency.
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BRIEF SUMMARY OF THE INVENTION
An improved apparatus for cooling a combustion chamber wall
having a flame retarding venturi used in low nitric oxide
emission gas turbine engines that includes a gas turbine
combustor having a premixing chamber and a secondary combustion
chamber and a venturi, a cooling air passageway concentrically
surrounding said venturi walls and said combustion chamber wall.
A plurality of cooling air inlet openings into said cooling air
passageway are disposed near the end of the combustion chamber.
The combustion chamber wall itself is substantially
cylindrical and includes the plurality of raised ribs on the
outside surface which provide additional surface area for
interaction with the flow of cooling air over the combustion
cylinder liner. The venturi walls are also united with the
combustion chamber and include a pair of convergent/divergent
walls intricately formed with the combustion chamber liner that
includes a restricted throat portion. The cooling air passes
around not only the cylindrical combustion chamber wall but both
walls that form the venturi providing cooling air to the entire
combustor chamber and venturi. As the cooling air travels
upstream toward the throat, its temperature rises.
The cooling air passageway is formed from an additional
cylindrical wall separated from the combustion chamber wall that
is concentrically mounted about the combustion chamber wall and
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a pair of conical walls that are concentrically disposed around
the venturi walls in a similar configuration to form a complete
annular passageway for air to flow around the entire combustion
chamber and the entire venturi. The downstream end of the
combustion chamber and the inlet opening of the cooling air
passageway are separated by a ring barrier so that none of the
cooling air in the passageway can flow downstream into the
combustion chamber, be introduced downstream of the combustion
chamber, or possibly travel into the separated region of the
venturi. In fact the cooling air outlet is located upstream of
the venturi and the cooling air flows opposite relative to the
combustion gas flow, first passing the combustion chamber wall
and then the venturi walls. The preheated cooling air is
ultimately introduced into the premix chamber, adding to the
efficiency of the system and reducing nitric oxide emissions
with a stable flame.
The source of the cooling air is the turbine compressor
that forces high pressure air around the entire combustor body
in a direction that is upstream relative to the combustion
process. Air under high pressure is forced around the
combustor body and through a plurality of air inlet holes in
the cooling air passageway near the downstream end of the
combustion chamber, forcing the cooling air to flow along the
combustor outer wall toward the venturi, passing the throat of
the venturi, passing the leading edge of the venturi wall where
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there exists an outlet air passageway and a receiving channel
that directs air in through another series of inlet holes into
the premix chamber upstream of the venturi throat. With this
flow pattern, it is impossible for cooling air to interfere
with the combustion process taking place in the secondary
combustion chamber since there is no exit or aperture
interacting with the secondary combustion chamber itself. Also
as the cooling air is heated in the passageway as it flows
towards the venturi and is introduced into the inlet premix
chamber upstream of the venturi, the heated air aides in
combustor efficiency to reduce pollutant emissions.
The outer Combustor housing includes an annular outer band
that receives the cooling air through outlet apertures upstream
of the venturi. The air is then directed further upstream
through a plurality of inlet air holes leading into the premix
chamber allowing the preheated cooling air to flow from the air
passageway at the leading venturi wall into the premix area.
The combustion chamber wall includes a plurality of raised
rings to increase the efficiency of heat transfer from the
combustion wall to the air, giving the wall more surface area
for air contact. Although a separate concentric wall is used
to form the air cooling passageway around the combustion
chamber and the venturi, it is possible in an alternative
embodiment that the outer wall of the combustor itself could
provide that function.
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It is an obj ect of the present invention to reduce nitric
oxide (NOx) emissions in a gas turbine combustor system while
maintaining a stable flame in a desired operating condition
while providing air cooling of the combustor chamber and
venturi.
It is another object of this invention to provide a low
emission combustor system that utilizes a venturi for providing
multiple uses of cooling air for the combustor chamber and
venturi.
And yet another object of this invention is to lower the
"blow-out" flame temperature of the combustor by utilizing
preheated air in the premixing process that results from cooling
the combustion chamber and venturi.
In accordance with these and other objects, which will
become apparent hereinafter, the instant invention will now be
described with particular reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a side elevational view in cross-section of
a gas turbine combustion system that represents the prior art,
which shows an air cooling passage that empties into and around
the combustion chamber.
Figure 2 shows a gas turbine combustion system in a
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perspective view in accordance with the present invention.
Figure 3 shows a side elevational view in cross-section of
a gas turbine combustor system in accordance with the present
invention.
Figure 4 shows a cut away version in cross section of the
combustion chamber and venturi and portions of the premix
chamber as utilized in the present invention.
Figure 5 shows a cross-sectional view, partially cut away
of the cooling air passageway at the upstream end of the venturi
in the annular bellyband chamber for receiving cooling air for
introducing the air into the premix chamber.
Figure 6 is a cut away and enlarged view of the aft end of
the combustion chamber wall in cross-section.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1, an existing gas turbine combustor
well known in the prior art 110 is shown. The combustor 110
includes a venturi 111, a premixing chamber 112 for premixing
air and fuel, a combustor chamber 113 and a combustion cap 115.
As shown in this prior art combustor, cooling air represented
by arrows flows under pressure along the external wall of the
venturi 111. The cooling air enters the system through
multiple locations along the liner 110. A portion of the air
enters through holes 120 while the remainder runs along the
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outer shell. The cooling air, which is forced under pressure,
with the turbine compressor as the source, enters the system
through a plurality of holes 121. As seen in Figure 1 the
cooling air impinges and cools the convergent/divergent walls
127 of the venturi 111, which are Conically shaped and travel
downstream through the cylindrical passage 114 cooling the
walls of combustion cylinder chamber 113. The cooling air
exits along the combustion chamber wall through annular
discharge opening 125. This air is then dumped to the
downstream combustion process. A portion of the cooling air
also enters the premixing zone through holes 126. The
remaining cooling air proceeds to the front end of the liner
where it enters through holes 123 and the combustion cap 115.
The portion of the cooling air that does not enter through
holes 123 enters and mixes the gas and fuel through area 124.
U.S. Patent 5,117,636 discusses the prior art configuration of
the venturi shown in Figure 1. Problems are discussed
regarding the cooling air exiting adjacent the venturi 111
through passage exit 125 which interferes with the combustion
process and mixture based on what the '636 Patent states as a
separation zone.
The present invention completely alleviates any of the
problems raised in the '636 Patent.
Referring now to Figures 2 and 3, the present invention is
shown as gas turbine combustor 10 including a venturi 11.
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The venturi 11 includes a cylindrical portion which forms
the combustor chamber 13 and unitarily formed venturi walls
which converge and diverge in the downstream direction forming
an annular or circular restricted throat 11a. The purpose of
the venturi and the restricted throat 11a is to prevent flash
back of the flame from combustion chamber 13.
Chamber 12 is the premix chamber where air and fuel are
mixed and forced under pressure downstream through the venturi
throat lla into the combustor chamber 13.
A concentric, partial cylindrical wall 11b surrounds the
venturi 12 including the converging and diverging venturi walls
to form an air passageway 14 between the venturi 11 and the
concentric wall 11b that allows the cooling air to pass along
the outer surface of the venturi 11 for cooling.
The outside of the combustor 10 is surrounded by a housing
(not shown) and contains air under pressure that moves upstream
towards the premix. zone 12, the air being received from the
compressor of the turbine. This is very high pressure air. The
cooling air passageway 14 has air inlet apertures 27 which
permit the high pressure air surrounding the combustor to enter
through the apertures 27 and to be received in the first portion
45 of passageway 14 that surrounds the venturi 11. The cooling
air passes along the venturi 11 passing the venturi converging
and diverging walls and venturi throat 11a. Preheated cooling
air exits through outlet apertures 28 which exit into an annular
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bellyband chamber 16. The combustor utilizes the cooling air
that has been heated and allowed to enter into premix chamber 12
through apertures 29 and 22. Details are shown in Figures 5 and
6. Note that this is heated air that has been used for cooling
that is now being introduced in the premix chamber, upstream of
the convergent wall of the venturi and upstream of venturi
throat 11a. Using preheated air drives the f/a ratio to a lean
limit to reduce NOx while maintaining a stable flame.
Referring now to Figure 4, the cooling air passage 14
includes a plurality of spacers 14a that separate venturi 11
from wall 11b. The bellyband wall 16 defines a radially outer
boundary of the second portion 46 of the passageway 14 and
provides a substantially annular chamber that allows the
outside pressure air and the exiting cooling air to be received
into the premix chamber 12. At the downstream end of the
combustion chamber 13, defined by the annular aft end of
venturi 11, there is disposed an annular air blocking ring 40
which prevents any cooling air from leaking downstream into the
combustion chamber. This alleviates any combustion problems
caused by the cooling air as delineated in the prior art
discussed above.
Referring now to Figure 5 the air passageway 14 is shown
along the venturi section having the convergent and divergent
walls and the throat 11a with cooling air passing through and
exiting through apertures 28 that go into the air chamber
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formed by bellyband wall 16. Additional air under a higher
pressure enters through apertures 32 and forces air including
the now heated cooling air in passageway 14 to be forced
through apertures 22 and 29 into the premix chamber 12.
Figure 6 shows the aft end portion of the combustion
chamber 13 and the end of venturi 11 that includes the blocking
ring 40 that is annular and disposed and attached in a sealing
manner around the entire aft portion of the venturi 11. The
cooling air that enters into passageway 14 cannot escape or be
allowed to pass into any portions of the combustion chamber 13.
Note that some air is permitted into the after chamber wall
beyond combustion chamber 13 through apertures 30 to 31 which
are disposed around the outside of the Combustor 10 and for
cooling the aft end of the Combustor.
The invention also includes the method of improved cooling
of a combustion chamber and venturi which allows the air used
for cooling to increase the efficiency of the combustion
process itself to reduce NOx emissions. With regards to the
air flow, the cooling air enters the venturi outer passageway
14 through multiple apertures 27. A predetermined amount of
air is directed into the passageway 14 by a element 17. The
cooling air is forced upstream by blocking ring 40 which
expands to contact the Combustor 10 under thermal loading
conditions. The cooling air travels upstream through the
Convergent/divergent sections of the first portion 45 of
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passageway 14 where it exits into the second portion of
passageway 14 through apertures 28 in the venturi 11 and the
combustor 10. The cooling air then fills a chamber created by
a full ring bellyband 16. Due to the pressure drop and
increase in temperature that has occurred throughout the
cooling path, supply air which is at an increased pressure is
introduced into the bellyband chamber 16 through multiple holes
32. The cooling air passes around multiple elements 18 which
are located throughout the bellyband chamber 16 for support of
the bellyband under pressure. The cooling air is then
introduced to the premix chamber through holes 22 and slots 29
in the combustor 10. Undesired leakage does not occur between
the cooling passageway 14 and the premixing chamber 12 because
of the forward support 19 which is fixed to the combustor 10
and venturi 11. The remainder of the cooling air not
introduced to passageway 14 through apertures 27 passes over
the element 17 and travels upstream to be introduced into the
combustor 10 or cap 15. This air is introduced through
multiple locations forward of the bellyband cavity 16.
It is through this process, rerouting air that was used for
cooling and supplying it for combustion, that lowers the fuel to
air ratio such that NOx is reduced without creating an unstable
flame.
While the invention is been described and is known as
presently the preferred embodiment, it is to be understood that
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the invention is not to be limited to the disclosed embodiment
but, on the contrary, it is intended to cover various
modifications and equivalent arrangements within the scope of
the following claims.
