U.S. patent application number 12/935930 was filed with the patent office on 2011-02-10 for fuel staging in a burner.
Invention is credited to Vladimir Milosavljevic.
Application Number | 20110033806 12/935930 |
Document ID | / |
Family ID | 39865686 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110033806 |
Kind Code |
A1 |
Milosavljevic; Vladimir |
February 10, 2011 |
Fuel Staging in a Burner
Abstract
A method for a staging operation at start up of a burner for a
gas turbine engine combustor is provided. The method will stabilize
the combustion of a lean-rich partially premised low emission
burner for a gas turbine combustor at all engine load conditions.
The method includes, adding fuel mixed with air to the pilot
combustor, igniting the mixture utilizing an igniter provided at an
upstream end of the pilot combustor for initiating a lean flame
inside the pilot combustor and for providing the flow of the
radicals and heat, imparting a swirl of fuel and air at the outside
at the exit of the pilot combustor at the upstream end of the
combustion room for creating and sustaining the main lean flame and
gradually adding a swirl of air and fuel for establishing a full
load stage to at least one channel.
Inventors: |
Milosavljevic; Vladimir;
(Norrkoping, SE) |
Correspondence
Address: |
SIEMENS CORPORATION;INTELLECTUAL PROPERTY DEPARTMENT
170 WOOD AVENUE SOUTH
ISELIN
NJ
08830
US
|
Family ID: |
39865686 |
Appl. No.: |
12/935930 |
Filed: |
March 26, 2009 |
PCT Filed: |
March 26, 2009 |
PCT NO: |
PCT/EP09/53562 |
371 Date: |
October 1, 2010 |
Current U.S.
Class: |
431/9 |
Current CPC
Class: |
F23D 2900/00015
20130101; F23R 3/346 20130101; F23R 3/343 20130101 |
Class at
Publication: |
431/9 |
International
Class: |
F23N 1/02 20060101
F23N001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 1, 2008 |
EP |
08006661.6 |
Claims
1.-3. (canceled)
4. A method for staging an operation at start up of a burner for a
gas turbine engine combustor, comprising: providing a burner
housing, the burner housing enclosing the burner and a pilot
combustor; adding fuel mixed with air to the pilot combustor;
igniting the mixture utilizing an igniter provided at an upstream
end of the pilot combustor for initiating a lean flame inside the
pilot combustor and for providing a flow of an unquenched
concentration of radicals at non-equilibrium and heat to a
downstream main combustion room; imparting a first swirl of fuel
and air outside a first exit of the pilot combustor at an upstream
end of an combustion room for creating and sustaining a main lean
flame in the main combustion room; generating a second swirl of air
and fuel from a second exit of a first lean premixing channel
downstream of the first exit and from a third exit of a second lean
premixing channel downstream of the second exit, wherein the burner
includes axially opposed upstream and downstream end portions, and
wherein the pilot combustor is located at an upstream end of the
burner, wherein the pilot combustor is provided with fuel and air
for burning fuel for a creation of the flow of radicals and heat
from a pilot combustion zone directed downstream along a center
line of the pilot combustor through a throat at an exit of the
pilot combustor, and wherein the burner is further provided with
the main combustion room defined by a plurality of quarl walls
downstream of the exit of the pilot combustor for housing the main
flame and a main recirculation zone for recirculating unburnt
radicals.
5. The method as claimed in claim 4, further including: adjusting,
after ignition, an equivalence ratio of a pilot combustor flame in
the pilot combustor to be a lean flame below equivalence ratio
1.
6. The method as claimed in claim 5, wherein the equivalence ratio
is 0.8.
7. The method as claimed in claim 4, further including: adjusting,
after ignition, an equivalence ratio of a pilot combustor flame in
the pilot combustor to be a rich flame above equivalence ratio
1.
8. The method as claimed in claim 7, wherein the equivalence ratio
is between 1.4 and 1.6.
9. The method as claimed in claim 5, further including: providing
cooling air for cooling a plurality of walls of the pilot
combustor; preheating through the cooling of the plurality of walls
the cooling air to a temperature above 750.degree. C.; and adding
fuel to the cooling air to an amount that corresponds to
equivalence ratios up to more than 3.
10. The method as claimed in claim 7, further including: providing
cooling air for cooling a plurality of walls of the pilot
combustor; preheating through the cooling of the plurality of walls
the cooling air to a temperature above 750.degree. C.; and adding
fuel to the cooling air to an amount that corresponds to
equivalence ratios up to more than 3.
11. The method as claimed in claim 6, further including: providing
cooling air for cooling a plurality of walls of the pilot
combustor; preheating through the cooling of the plurality of walls
the cooling air to a temperature above 750.degree. C.; and adding
fuel to the cooling air to an amount that corresponds to
equivalence ratios up to more than 3.
12. The method as claimed in claim 8, further including: providing
cooling air for cooling a plurality of walls of the pilot
combustor; preheating through the cooling of the plurality of walls
the cooling air to a temperature above 750.degree. C.; and adding
fuel to the cooling air to an amount that corresponds to
equivalence ratios up to more than 3.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2009/053562, filed Mar. 26, 2009 and claims
the benefit thereof. The International Application claims the
benefits of European Patent Office application No. 08006661.6 EP
filed Apr. 1, 2008. All of the applications are incorporated by
reference herein in their entirety.
TECHNICAL FIELD
[0002] The present invention refers to a burner preferably for use
in gas turbine engines, and more particularly to fuel staging of a
burner adapted to stabilize engine combustion, and further to fuel
staging in a burner that use a pilot combustor to provide
combustion products to stabilize main lean premixed combustion.
TECHNICAL BACKGROUND
[0003] Gas turbine engines are employed in a variety of
applications including electric power generation, military and
commercial aviation, pipeline transmission and marine
transportation. In a gas turbine engine which operates in LPP mode,
fuel and air are provided to a burner chamber where they are mixed
and ignited by a flame, thereby initiating combustion. The major
problems associated with the combustion process in gas turbine
engines, in addition to thermal efficiency and proper mixing of the
fuel and the air, are associated to flame stabilization, the
elimination of pulsations and noise, and the control of polluting
emissions, especially nitrogen oxides (NOx), CO, UHC, smoke and
particulated emission
[0004] Patent application EP 1 659 339 A1 discloses a method to
start a burner preferably for combustion of syngas including the
operation of diffusion flame pilot burner arranged in a center of
swirler nozzles providing a mixture of fuel and air. U.S. Pat. No.
5,321,948 A1 discloses a fuel stages premixed dry low No.sub.x
combustor comprising at least two concentric cylinders in a
staggered arrangement, between which a channel is formed to provide
a mixture of fuel and air into a combustion zone.
[0005] In industrial gas turbine engines, which operate in LPP
mode, flame temperature is reduced by an addition of more air than
required for the combustion process itself. The excess air that is
not reacted must be heated during combustion, and as a result flame
temperature of the combustion process is reduced (below
stoichiometric point) from approximately 2300K to 1800 K and below.
This reduction in flame temperature is required in order to
significantly reduce NOx emissions. A method shown to be most
successful in reducing NOx emissions is to make combustion process
so lean that the temperature of the flame is reduced below the
temperature at which diatomic Nitrogen and Oxygen (N2 and O2)
dissociate and recombine into NO and NO2. Swirl stabilized
combustion flows are commonly used in industrial gas turbine
engines to stabilize combustion by, as indicated above, developing
reverse flow (Swirl Induced Recirculation Zone) about the
centreline, whereby the reverse flow returns heat and free radicals
back to the incoming un-burnt fuel and air mixture. The heat and
free radicals from the previously reacted fuel and air are required
to initiate (pyrolyze fuel and initiate chain branching process)
and sustain stable combustion of the fresh un-reacted fuel and air
mixture. Stable combustion in gas turbine engines requires a cyclic
process of combustion producing combustion products that are
transported back upstream to initiate the combustion process. A
flame front is stabilised in a Shear-Layer of the Swirl Induced
Recirculation Zone. Within the Shear-Layer "Local Turbulent Flame
Speed of the Air/Fuel Mixture" has to be higher then "Local
Air/Fuel Mixture Velocity" and as a result the Flame
Front/combustion process can be stabilised.
[0006] Lean premixed combustion is inherently less stable than
diffusion flame combustion for the following reasons:
[0007] The amount of air required to reduce the flame temperature
from 2300K to 1700-1800 K is approximately twice the amount of air
required for stoichiometric combustion. This makes the overall
fuel/air ratio (.PHI.) very close (around or below 0.5;
.PHI..gtoreq.0.5) or similar to a fuel/air ratio at which lean
extinction of the premixed flame occurs. Under these conditions the
flame can locally extinguish and re-light in a periodic manner.
[0008] Near the lean extinction limit the flame speed of the lean
partially premixed flames is very sensitive to the equivalence
ratio fluctuations. Fluctuations in flame speed can result in
spatial fluctuations/movements of the flame front (Swirl Induced
Recirculation Zone). A less stable, easy to move flame front of a
pre-mixed flame results in a periodic heat release rate, that, in
turn, results in movement of the flame, unsteady fluid dynamic
processes, and thermo-acoustic instabilities develop.
[0009] Equivalence ratio fluctuations are probably the most common
coupling mechanism to link unsteady heat release to unsteady
pressure oscillations.
[0010] In order to make the combustion sufficiently lean, in order
to be able to significantly reduce NOx emissions, nearly all of the
air used in the engine must go through the injector and has to be
premixed with fuel. Therefore, all the flow in the burners has the
potential to be reactive and requires that the point where
combustion is initiated is fixed.
[0011] When the heat required for reactions to occur is the
stability-limiting factor, very small temporal fluctuations in
fuel/air equivalence ratios (which could either result either from
fluctuation of fuel or air flow through the Burner/Injector) can
cause flame to partially extinguish and re-light.
[0012] An additional and very important reason for the decrease in
stability in the pre-mixed flame is that the steep gradient of fuel
and air mixing is eliminated from the combustion process. This
makes the premixed flow combustible anywhere where there is a
sufficient temperature for reaction to occur. When the flame can,
more easily, occur in multiple positions, it becomes more unstable.
The only means for stabilizing a premixed flame to a fixed position
are based on the temperature gradient produced where the unburnt
premixed fuel and air mix with the hot products of combustion
(flame cannot occur where the temperature is too low). This leaves
the thermal gradient produced by the generation, radiation,
diffusion and convection of heat as a method to stabilize the
premixed flame. Radiation heating of the fluid does not produce a
sharp gradient; therefore, stability must come from the generation,
diffusion and convection of heat into the pre-reacted zone.
Diffusion only produces a sharp gradient in laminar flow and not
turbulent flows, leaving only convection and energy generation to
produce the sharp gradients desired for flame stabilization which
is actually heat and free radial gradients. Both, heat and free
radial gradients, are generated, diffused and convected by the same
mechanisms through recirculating products of combustion within the
Swirl Induced Recirculation Zone.
[0013] In pre-mixed flows, as well as diffusion flows, rapid
expansion causing separations and swirling recirculating flows, are
both commonly used to produce gradients of heat and free radicals
into the pre-reacted fuel and air.
[0014] Document WO 2005/040682 A2 describes a solution directed to
a burner for gas turbine engines that use a pilot flame to assist
in sustaining and stabilizing the combustion process.
[0015] When an igniter, as in prior art burners, is placed in an
outer recirculation zone, the fuel/air mixture entering this region
must often be made rich in order to make the flame temperature
sufficiently hot to sustain stable combustion in this region. The
flame then often cannot be propagated to the main recirculation
until the main premixed fuel and airflow becomes sufficiently rich,
hot and has a sufficient pool of free radicals, which occurs at
higher fuel flow rates. When the flame cannot propagate from the
outer recirculation zone to the inner main recirculation zone
shortly after ignition, it must propagate at higher pressure after
the engine speed begins to increase. This transfer of the
initiation of the main flame from the outer recirculation zone
pilot only after combustor pressure begins to rise results in more
rapid relaxation of the free radicals to low equilibrium levels,
which is an undesirable characteristic that is counter productive
for ignition of the flame at the forward stagnation point of the
main recirculation zone. Ignition of the main recirculation may not
occur until the pilot sufficiently raises the bulk temperature to a
level where the equilibrium levels of free radicals entrained in
the main recirculation zone and the production of addition free
radicals in the premixed main fuel and air mixture are sufficient
to ignite the main recirculation zone. In the process of getting
the flame to propagate from the outer to the main recirculation
zone, significant amounts of fuel exits the engine without burning
from the un-ignited main premixed fuel and air mixture. A problem
occurs if the flame transitions to the main recirculation zone in
some burner before others in the same engine, because the burners
where the flames are stabilized on the inside burn hotter since all
of the fuel is burnt. This leads to a burner-to-burner temperature
variation which can damage engine components.
SUMMARY OF THE INVENTION
[0016] The aspects of fuel staging according to the present
invention is described herein, as an example, in connection with a
is a lean-rich partially premixed low emissions burner for a gas
turbine combustor that provides stable ignition and combustion
process at all engine load conditions. This burner operates
according to the principle of "supplying" heat and high
concentration of free radicals from a pilot combustor exhaust to a
main flame burning in a lean premixed air/fuel swirl, whereby a
rapid and stable combustion of the main lean premixed flame is
supported. The pilot combustor supplies heat and supplements a high
concentration of free radicals directly to a forward stagnation
point and a shear layer of the main swirl induced recirculation
zone, where the main lean premixed flow is mixed with hot gases
products of combustion provided by the pilot combustor. This allows
a leaner mix and lower temperatures of the main premixed air/fuel
swirl combustion that otherwise would not be self-sustaining in
swirl stabilized recirculating flows during the operating
conditions of the burner.
[0017] According to a first aspect of the invention there is herein
presented a method for fuel staging characterized by the features
of the claims.
[0018] Further aspects of the invention are presented in the
dependent claims.
[0019] The burner utilizes:
A swirl of air/fuel above swirl number (S.sub.N) 0.7 (that is above
critical S.sub.N=0.6), generated-imparted into the flow, by a
radial swirler; active species--non-equilibrium free radicals being
released close to the forward stagnation point, particular type of
the burner geometry with a multi quarl device, and internal staging
of fuel and air within the burner to stabilize combustion process
at all gas turbine operating conditions. In short, the disclosed
burner provides stable ignition and combustion process at all
engine load conditions. Some important features related to the
inventive burner are: the geometric location of the burner
elements; the amount of fuel and air staged within the burner; the
minimum amount of active species--radicals generated and required
at different engine/burner operating conditions; fuel profile;
mixing of fuel and air at different engine operating conditions;
imparted level of swirl; multi (minimum double quarl) quarl
arrangement.
[0020] To achieve as low as possible emission levels, a target in
this design/invention is to have uniform mixing profiles at the
exit of lean premixing channels. Two distinct combustion zones
exist within the burner covered by this disclosure, where fuel is
burnt simultaneously at all times. Both combustion zones are swirl
stabilized and fuel and air are premixed prior to the combustion
process. A main combustion process, during which more than 90% of
fuel is burned, is lean. A supporting combustion process, which
occurs within the small pilot combustor, wherein up to 1% of the
total fuel flow is consumed, could be lean, stoichiometric and rich
(equivalence ratio, .PHI.=1.4 and higher).
[0021] An important difference between the disclosed burner and a
burner as presented in the prior art document is that a bluff body
is not needed in the pilot combustor as, the present invention uses
un un-quenched flow of radicals directed downstream from a
combustion zone of the pilot combustor along a centre line of the
pilot combustor, said flow of radicals being released through the
full opening area of a throat of the pilot combustor at an exit of
the pilot combustor.
[0022] The main reason why the supporting combustion process in the
small pilot combustor could be lean, stoichiometric or rich and
still provide stable ignition and combustion process at all engine
load conditions is related to combustion efficiency. The combustion
process, which occurs within the small combustor-pilot, has low
efficiency due to the high surface area which results in flame
quenching on the walls of the pilot combustor. Inefficient
combustion process, either being lean, stoichiometric or rich,
could generate a large pool of active species--radicals which is
necessary to enhance stability of the main lean flame and is
beneficial for a successful operation of the present burner
design/invention (Note: the flame occurring in the premixed lean
air/fuel mixture is herein called the lean flame).
[0023] It would be very difficult to sustain (but not to ignite,
because the small pilot combustor can act as a torch igniter)
combustion in the shear layer of the main recirculation zone below
LBO (Lean Blow Off) limits of the main lean flame (approx.
T>1350 K and .PHI..gtoreq.0.25). For engine operation below LBO
limits of the main lean flame, in this burner design, additional
"staging" of the small combustor-pilot is used/provided. The air
which is used to cool the small pilot combustor internal walls
(performed by a combination of impingement and convecting cooling)
and which represents approximately 5-8% of the total air flow
through the burner, is premixed with fuel prior the swirler.
Relatively large amount of fuel can be added to the small pilot
combustor cooling air which corresponds to very rich equivalence
ratios (.PHI.>3). Swirled cooling air and fuel and hot products
of combustion from the small pilot combustor, can very effectively
sustain combustion of the main lean flame below, at and above LBO
limits. The combustion process is very stable and efficient because
hot combustion products and very hot cooling air (above 750.degree.
C.), premixed with fuel, provide heat and active species (radicals)
to the forward stagnation point of the main flame recirculation
zone. During this combustion process the small pilot combustor,
combined with very hot cooling air (above 750.degree. C.) premixed
with fuel act as a flameless burner, where reactants (oxygen &
fuel) are premixed with products of combustion and a distributed
flame is established at the forward stagnation point of the swirl
induced recirculation zone.
[0024] To enable a proper function and stable operation of the
burner disclosed in the present application, it is required that
the imparted level of swirl and the swirl number is above the
critical one (not lower then 0.6 and not higher then 0.8) at which
vortex breakdown--recirculation zone will form and will be firmly
positioned within the multi quarl arrangement. The forward
stagnation point P should be located within the quart and at the
exit of the pilot combustor. The main reasons, for this
requirement, are:
[0025] If the imparted level of swirl is low and the resulting
swirl number is below 0.6, for most burner geometries, a weak,
recirculation zone will form and unstable combustion can occur.
[0026] A strong recirculation zone is required to enable transport
of heat and free radicals from the previously combusted fuel and
air, back upstream towards the flame front. A well established and
a strong recirculation zone is required to provide a shear layer
region where turbulent flame speed can "match" or be proportional
to the local fuel/air mixture, and a stable flame can establish.
This flame front established in the shear layer of the main
recirculation zone has to be steady and no periodic movements or
procession of the flame front should occur. The imparted swirl
number can be high, but should not be higher then 0.8, because at
and above this swirl number more then 80% of the total amount of
the flow will be recirculated back. A further increase in swirl
number will not contribute more to the increase in the amount of
the recirculated mass of the combustion products, and the flame in
the shear layer of the recirculation zone will be subjected to high
turbulence and strain which can result in quenching and partial
extinction and reignition of the flame. Any type of the swirl
generator, radial, axial and axial-radial can be used in the
burner, covered by this disclosure. In this disclosure a radial
swirler configuration is shown.
[0027] The burner utilizes aerodynamics stabilization of the flame
and confines the flame stabilization zone--the recirculation
zone--in the multiple quarl arrangement. The multiple quarl
arrangement is an important feature of the design of the provided
burner for the following reasons. The quarl (or also called
diffuser):
provides a flame front (main recirculation zone) anchoring the
flame in a defined position in space, without a need to anchore the
flame to a solid surface/bluff body, and in that way a high thermal
loading and issues related to the burner mechanical integrity are
avoided; geometry (quarl half angle .alpha. and length L) is
important to control size and shape of the recirculation zone in
conjunction with the swirl number. The length of the recirculation
zone is roughly proportional to 2 to 2.5 of the quarl length;
optimal length L is of the order of L/D=1 (D is the quarl throat
diameter). The minimum length of the quarl should not be smaller
then L/D=0.5 and not longer then L/D=2; optimal quarl half angle
.alpha. should not be smaller then 20 and larger then 25 degrees,
allows for a lower swirl before decrease in stability, when
compared to a less confined flame front; and has the important task
to control the size and shape of the recirculation zone as the
expansion of the hot gases as a result of combustion reduces
transport time of free radicals in the recirculation zone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a simplified cross section schematically showing
the burner according to the aspects of the invention enclosed in a
housing without any details showing how the burner is configured
inside said housing.
[0029] FIG. 2 is a cross section through the burner schematically
showing a section above a symmetry axis, whereby a rotation around
the symmetry axis forms a rotational body displaying a layout of
the burner.
[0030] FIG. 3 shows a diagram of stability limits of the flame as a
function of the swirl number, imparted level of swirl and
equivalence ratio.
[0031] FIG. 4a: shows a diagram of combustor near field
aerodynamics.
[0032] FIG. 4b: shows a diagram of combustor near field
aerodynamics.
[0033] FIG. 5 shows a diagram of turbulence intensity.
[0034] FIG. 6 shows a diagram of relaxation time as a function of
combustion pressure.
EMBODIMENTS OF THE INVENTION
[0035] In the following a number of embodiments will be described
in more detail with references to the enclosed drawings.
[0036] In FIG. 1 the burner is depicted with the burner 1 having a
housing 2 enclosing the burner components.
[0037] FIG. 2 shows for the sake of clarity a cross sectional view
of the burner above a rotational symmetry axis. The main parts of
the burner are the radial swirler 3, the multi quarl 4a, 4b, 4c and
the pilot combustor 5.
[0038] As stated, the burner 1 operates according to the principle
of "supplying" heat and high concentration of free radicals from
the a pilot combustor 5 exhaust 6 to a main flame 7 burning in a
lean premixed air/fuel swirl emerging from a first exit 8 of a
first lean premixing channel 10 and from a second exit 9 of a
second lean premixing channel 11, whereby a rapid and stable
combustion of the main lean premixed flame 7 is supported. Said
first lean premixing channel 10 is formed by and between the walls
4a and 4b of the multi quarl. The second lean premixing channel 11
is formed by and between the walls 4b and 4c of the multi quarl.
The outermost rotational symmetric wall 4c of the multi quarl is
provided with an extension 4c1 to provide for the optimal length of
the multi quarl arrangement. The first 10 and second 11 lean
premixing channels are provided with swirler wings forming the
swirler 3 to impart rotation to the air/fuel mixture passing
through the channels.
[0039] Air 12 is provided to the first 10 and second 11 channels at
the inlet 13 of said first and second channels. According to the
embodiment shown the swirler 3 is located close to the inlet 13 of
the first and second channels. Further, fuel 14 is introduced to
the air/fuel swirl through a tube 15 provided with small diffusor
holes 15b located at the air 12 inlet 13 between the swirler 3
wings, whereby the fuel is distributed into the air flow through
said holes as a spray and effectively mixed with the air flow.
Additional fuel can be added through a second tube 16 emerging into
the first channel 10.
[0040] When the lean premixed air/fuel flow is burnt the main flame
7 is generated. The flame 7 is formed as a conical rotational
symmetric shear layer 18 around a main recirculation zone 20 (below
sometimes abbreviated RZ). The flame 7 is enclosed inside the
extension 4c1 of the outermost quarl, in this example quarl 4c.
[0041] The pilot combustor 5 supplies heat and supplements a high
concentration of free radicals directly to a forward stagnation
point P and the shear layer 18 of the main swirl induced
recirculation zone 20, where the main lean premixed flow is mixed
with hot gases products of combustion provided by the pilot
combustor 5.
[0042] The pilot combustor 5 is provided with walls 21 enclosing a
combustion room for a pilot combustion zone 22. Air is supplied to
the combustion room through fuel channel 23 and air channel 24.
Around the walls 21 of the pilot combustor 5 there is a distributor
plate 25 provided with holes over the surface of the plate. Said
distributor plate 25 is separated a certain distance from said
walls 21 forming a cooling space layer 25a. Cooling air 26 is taken
in through a cooling inlet 27 and meets the outside of said
distributor plate 25, whereupon the cooling air 26 is distributed
across the walls 21 of the pilot combustor to effectively cool said
walls 21. The cooling air 26 is after said cooling let out through
a second swirler 28 arranged around a pilot quarl 29 of the pilot
combustor 5. Further fuel can be added to the combustion in the
main lean flame 7 by supplying fuel in a duct 30 arranged around
and outside the cooling space layer 25a. Said further fuel is then
let out and into the second swirler 28, where the now hot cooling
air 26 and the fuel added through duct 30 is effectively
premixed.
[0043] A relatively large amount of fuel can be added to the small
pilot combustor 5 cooling air which corresponds to very rich
equivalence ratios (.PHI.>3). Swirled cooling air and fuel and
hot products of combustion from the small pilot combustor, can very
effectively sustain combustion of the main lean flame 7 below, at
and above LBO limits. The combustion process is very stable and
efficient because hot combustion products and very hot cooling air
(above 750.degree. C.), premixed with fuel, provide heat and active
species (radicals) to the forward stagnation point P of the main
flame recirculation zone 20. During this combustion process the
small pilot combustor 5, combined with very hot cooling air (above
750.degree. C.) premixed with fuel act as a flameless burner, where
reactants (oxygen & fuel) are premixed with products of
combustion and a distributed flame is established at the forward
stagnation point P of the swirl induced recirculation zone 20.
[0044] To enable a proper function and stable operation of the
burner 1 disclosed in the present application, it is required that
the imparted level of swirl and the swirl number is above the
critical one (not lower then 0.6 and not higher then 0.8, see also
FIG. 3) at which vortex breakdown--recirculation zone 20--will form
and will be firmly positioned within the multi quarl 4a, 4b, 4c
arrangement. The forward stagnation point P should be located
within the quarl 4a, 4b, 4c and at the exit 6 of the pilot
combustor 5. Some main reasons, for this requirement, were
mentioned in the summary above. A further reason is:
[0045] If the swirl number is larger than 0.8, the swirling flow
will extend to the exit of the combustor, which can result in an
overheating of subsequent guide vanes of a turbine.
[0046] Below is presented a summary of the imparted level of swirl
and swirl number requirements. See also FIGS. 4a and 4b.
[0047] The imparted level of swirl (the ratio between tangential
and axial momentum) has to be higher then the critical one
(0.4-0.6), so that a stable central recirculation zone 20 can form.
The critical swirl number, S.sub.N, is also a function of the
burner geometry, which is the reason for why it varies between 0.4
and 0.6. If the imparted swirl number is .ltoreq.0.4 or in the
range of 0.4 to 0.6, the main recirculation zone 20, may not form
at all or may form and extinguish periodically at low frequencies
(below 150 Hz) and the resulting aerodynamics could be very
unstable which will result in a transient combustion process.
[0048] In the shear layer 18 of the stable and steady recirculation
zone 20, with strong velocity gradient and turbulence levels, flame
stabilization can occur if:
[0049] turbulent flame speed (ST)>local velocity of the fuel air
mixture (UF/A).
[0050] Recirculating products which are: source of heat and active
species (symbolized by means of arrows 1a and 1b), located within
the recirculation zone 20, have to be stationary in space and time
downstream from the mixing section of the burner 1 to enable
pyrolysis of the incoming mixture of fuel and air. If a steady
combustion process is not prevailing, thermo-acoustics
instabilities will occur.
[0051] Swirl stabilized flames are up to five times shorter and
have significantly leaner blow-off limits then jet flames.
[0052] A premixed or turbulent diffusion combustion swirl provides
an effective way of premixing fuel and air.
[0053] The entrainment of the fuel/air mixture into the shear layer
of the recirculation zone 20 is proportional to the strength of the
recirculation zone, the swirl number and the characteristics
recirculation zone velocity URZ.
[0054] The characteristics recirculation zone velocity, URZ, can be
expressed as:
URZ=UF/A f (MR, dF/A,cent/dF/A, S.sub.N),
wherein:
[0055] MR=rcent (UF/A,cent)2/rF/A (UF/A)2
[0056] Experiments (Driscoll1990, Whitelaw1991) have shown that
[0057] RZ strength=(MR) exp-1/2 (dF/A/dF/A,cent) (URZ/UF/A)
(b/dF/A),
[0058] and
[0059] MR should be<1.
[0060] (dF/A/dF/A,cent), only important for turbulent diffusion
flames.
[0061] recirculation zones size/length is "fixed" and proportional
to 2-2.5 dF/A.
[0062] Not more than approximately 80% of the mass recirculates
back above S.sub.N=0.8 independently of how high S.sub.N is further
increased
[0063] Addition of Quarl-diverging walls downstream of the throat
of the burner-enhances recirculation (Batchelor 67, Hallet 87,
Lauckel 70, Whitelow 90); and Lauckel 70 has found that optimal
geometrical parameters were: .alpha.=20.degree.-25.degree.;
L/dF/A,min=1 and higher.
[0064] This suggests that dquar/dF/A=2-3, but stability of the
flame suggests that leaner lean blow-off limits were achieved for
values close to 2 (Whitelaw 90).
[0065] Experiments and practical experience suggest also that UF/A
should be above 30-50 m/s for premixed flames due to risks of
flashback (Proctor 85).
[0066] If a back-facing step is placed at the quart exit, then
external RZ if formed the length of the external RZ, LERZ is
usually 2/3 hERZ.
[0067] Active species--radicals
[0068] In the swirl stabilized combustion, the process is initiated
and stabilized by means of transporting heat and free radicals 31
from the previously combusted fuel and air, back upstream towards
the flame front 7. If the combustion process is very lean, as is
the case in lean-partially premixed combustion systems, and as a
result the combustion temperature is low, the equilibrium levels of
free radicals is also very low. Also, at high engine pressures the
free radicals produced by the combustion process, quickly relax,
see FIG. 6, to the equilibrium level that corresponds to the
temperature of the combustion products. This is due to the fact
that the rate of this relaxation of the free radicals to
equilibrium increases exponentially with increase in pressure,
while on the other hand the equilibrium level of free radicals
decreases exponentially with temperature decrease. The higher the
level of free radicals available for initiation of combustion the
more rapid and stable the combustion process will tend to be. At
higher pressures, at which burners in modern gas turbine engines
operate in lean partially premixed mode, the relaxation time of the
free radicals can be short compared to the "transport" time
required for the free radicals (symbolized by arrows 31) to be
convected downstream, from the point where they were produced in
the shear layer 18 of the main recirculation zone 20, back
upstream, towards the flame front 7 and the forward stagnation
point P of the main recirculation zone 20. As a consequence, by the
time that the reversely circulating flow of radicals 31 within the
main recirculation zone 20 have conveyed free radicals 31 back
towards the flame front 7, and when they begin to mix with the
incoming "fresh" premixed lean fuel and air mixture from the first
10 and second 11 channels at the forward stagnation point P to
initiate/sustain combustion process, the free radicals 31 could
have reached low equilibrium levels.
[0069] This invention utilizes high non-equilibrium levels of free
radicals 32 to stabilize the main lean combustion 7. In this
invention, the scale of the small pilot combustor 5 is kept small
and most of the combustion of fuel occurs in the lean premixed main
combustor (at 7 and 18), and not in the small pilot combustor 5.
The small pilot combustor 5, can be kept small, because the free
radicals 32 are released near the forward stagnation point P of the
main recirculation zone 20. This is generally the most efficient
location to supply additional heat and free radicals to swirl
stabilized combustion (7). As the exit 6 of the small pilot
combustor 5 is located at the forward stagnation point P of the
main-lean re-circulating flow 20, the time scale between quench and
utilization of free radicals 32 is very short not allowing free
radicals 32 to relax to low equilibrium levels. The forward
stagnation point P of the main-lean re-circulating zone 20 is
maintained and aerodynamically stabilized in the quarl section
(4a), at the exit 6 of the small pilot combustor 5. To assure that
the distance and time from lean, stochiometric or rich combustion
(zone 22), within the small pilot combustor 5, is as short and
direct as possible, the exit of the small pilot combustor 5 is
positioned on the centerline and at the small pilot combustor 5
throat 33. On the centerline, at the small pilot combustor 5 throat
33, and within the quarl section 4a, free radicals 32 are mixed
with the products of the lean combustion 31, highly preheated
mixture of fuel and air, from duct 30 and space 25a, and
subsequently with premixed fuel 14 and air 12 in the shear layer 18
of the lean main recirculation zone 20. This is very advantageous
for high-pressure gas turbine engines, which inherently exhibit the
most severe thermo acoustic instabilities. Also, because the free
radicals and heat produced by the small pilot combustor 5 are used
efficiently, its size can be small and the quenching process is not
required. The possibility to keep the size of the pilot combustor
5, small has also beneficial effect on emissions.
[0070] Burner Geometry with Multi Quarl Arrangements
[0071] The burner utilizes aerodynamics stabilization of the flame
and confines the flame stabilization zone--recirculation zone (5),
in the multiple quarl arrangement (4a, 4b and 4c). The multiple
quarl arrangement is an important feature of the disclosed burner
design for the reasons listed below. The quarl (or sometimes called
the diffuser):
[0072] provides a flame front 7 (the main recirculation zone 20 is
anchored without a need to anchore the flame to a solid
surface/bluff body and in that way a high thermal loading and
issues related to the burner mechanical integrity are avoided,
[0073] geometry (quarl half angle .alpha. and length L) is
important to control the size and shape of the recirculation zone
20 in conjunction with the swirl number. The length of the
recirculation zone 20 is roughly proportional to 2 to 2.5 of the
quarl length L,
[0074] optimal length is of the order of L/D=1 (D, is quarl throat
diameter). The minimum length of the quarl should not be smaller
then 0.5 and not longer then 2 (Ref1:The influence of Burner
Geometry and Flow Rates on the Stability and Symmetry of
Swirl-Stabilized Nonpremixed Flames; V. Milosavljevic et al;
Combustion and Flame 80, pages 196-208, 1990),
[0075] optimal quarl half angle .alpha. (Ref1), should not be
smaller then 20 and larger then 25 degrees,
[0076] allows for a lower swirl number before decrease in
stability, when compared to less confined flame front,
[0077] is important to control size and shape of recirculation zone
due to expansion as a result of combustion and reduces transport
time of free radicals in recirculation zone.
[0078] Burner Scaling
[0079] The quarl (or diffuser) and the imparted swirl provides a
possibility of a simple scaling of the disclosed burner geometry
for different burner powers.
[0080] To scale burner size down (example):
[0081] The channel 11 should be removed and the shell forming quarl
section 4c should thus substitute the shell previously forming
quarl section 4b, which is taken away; the geometry of the quarl
section 4c should be the same as the geometry of the previously
existing quarl section 4b,
[0082] The Swirl number in channel 10 should stay the same,
[0083] All other Burner parts should be the same; fuel staging
within the burner should stay the same or similar.
[0084] To scale burner size up:
[0085] Channels 10 and 11 should stay as they are,
[0086] Quarl section 4c should be designed in the same as quarl
section 4b (formed as a thin splitter plate),
[0087] A new third channel (herein fictively called 11b and not
disclosed) should be arranged outside and surrounding the second
channel 11 and a new quarl section 4d (not shown in the drawings)
outside and surrounding the second channel 11, thus forming an
outer wall of the third channel; the shape of the new quarl section
4d should be of a shape similar to the shape of former outmost
quarl section 4c.
[0088] The Swirl number in the channels should be
S.sub.N,10>S.sub.N,11>S.sub.N,11b, but they should all be
above S.sub.N=0.6 and not higher then 0.8
[0089] All other burner parts should be the same
[0090] Burner operation and fuel staging within the burner should
stay the same or similar.
[0091] Fuel Staging and Burner Operation
[0092] The present invention also allows for the ignition of the
main combustion 7 to occur at the forward stagnation point P of the
main recirculation zone 20. Most gas turbine engines must use an
outer recirculation zone, see FIG. 4b, as the location where the
spark, or torch igniter, ignites the engine. Ignition can only
occur if stable combustion can also occur; otherwise the flame will
just blow out immediately after ignition. The inner or main
recirculation zone 20, as in the present invention, is generally
more successful at stabilizing the flame, because the recirculated
gas 31 is transported back and the heat from the combustion
products of the recirculated gas 31 is focused to a small region at
the forward stagnation point P of the main recirculation zone 20.
The combustion--flame front 7, also expands outwards in a conical
shape from this forward stagnation point P, as illustrated in FIG.
2. This conical expansion downstream allows the heat and free
radicals 32 generated upstream to support the combustion downstream
allowing the flame front 7 to widen as it moves downstream. The
multi quarl (4a, 4b, 4c), illustrated in FIG. 2, compared to swirl
stabilized combustion without the quarl, show how the quarl shapes
the flame to be more conical and less hemispheric in nature. A more
conical flame front allows for a point source of heat to initiate
combustion of the whole flow field effectively.
[0093] In the present invention the combustion process within the
burner 1 is staged. In the first stage, the ignition stage, lean
flame 35 is initiated in the small pilot combustor 5 by adding fuel
23 mixed with air 24 and igniting the mixture utilizing ignitor 34.
After ignition the equivalence ratio of the flame 35 in the small
pilot combustor 5 is adjusted at either lean (below equivalence
ratio 1, and at approximately equivalence ratio of 0.8) or rich
conditions (above equivalence ratio 1, and at approximately
equivalence ratio between 1.4 and 1.6). The reason why the
equivalence ratio within the small pilot combustor 5 is at rich
conditions in the range between 1.4 and 1.6 is emission levels. It
is possible to operate and maintain the flame 35 in the small
combustor pilot 5 at stoichiometric conditions (equivalence ratio
of 1), but this option is not recommended because it can result in
high emission levels, and higher thermal loading of the walls 21.
The benefit of operating and maintaining the flame 35 in the small
pilot combustor at either lean or rich conditions is that generated
emissions and thermal loading of the walls 21 are low.
[0094] In the next stage, a second-low load stage, fuel is added
through duct 30 to the cooling air 27 and imparted a swirling
motion in swirler 28. In this way combustion of the main lean flame
7, below, at and above LBO limits, is very effectively sustained.
The amount of the fuel which can be added to the hot cooling air
(preheated at temperatures well above 750 C), can correspond to
equivalence ratios >3.
[0095] In the next stage of the burner operation, a third part and
full load stage fuel 15a is gradually added to the air 12, which is
the main air flow to the main flame 7.
* * * * *