Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR STABILIZING COMBUSTION IN GAS TURBINE ENGINES
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to burners for gas turbines, and more
particularly, to burners adapted to stabilize engine combustion, and still
further, to
burners which use a quad device in combination with and a central bluff body
flame
holder to stabilize the combustion process.
2. Background of the Related Art
Gas turbines 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, fuel and air are provided to a burner
chamber
where they are mixed and ignited by a flame, thereby initiating combustion.
Several
major technical problems are associated with the combustion process in gas
turbine
engines. These problems include, for example, thermal efficiency of the
burner/combustor, proper mixing of the fuel and air, flame stabilization, the
elimination
of pulsations and noise, and the control of polluting emissions, especially
nitrogen oxides
(NQx). Flame stabilization refers to fixing the position and intensity of the
flame within
the burner so as to, among other things, eliminate pulsations and reduce
noise.
Stable combustion in gas turbine engines requires a cyclic process of
combustion producing products, i. e., heat and free radicals, which are
transported back
upstream to the flame initiation point to facilitate the combustion process.
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It is presently known to provide swirled air to the fuel-air mixture or to
impart a swirl to the fuel-air mixture in order to improve flame stabilization
and thereby
stabilize the combustion process. Swirl stabilized combustion flows facilitate
combustion by developing reverse flow about the centerline of the burner,
which returns
heat and free radicals back upstream to the un-burnt fuel-air mixture.
U.S. Patent Nos. 5,131,334; 5,365,865; and 5,415,114 to Monroe et al.
each disclose coal fired burners that include a flame stabilizer for imparting
a swirl to the
fuel-air mixture. The disclosed flame stabilizers include a plurality of
radially spaced-
apart vane elements mounted on a ring member which is positioned over a
central fuel
supply tube. The vanes are shaped and oriented to provide swirled air to the
downstream
end of the fuel supply tube.
U.S. Patent No. 5,477,685 to Samuelson, which is herein incorporated by
reference in its entirety, discloses a swirl stabilized, lean burn injector
for a gas turbine
combustor. In an illustrative embodiment, a fuel-air mixture exits a centrally
positioned
nozzle through a plurality of radially-oriented exit ports. An air swirler and
a quad
device are attached to the downstream end of the Samuelson injector for
facilitating the
re-circulating flow. The fuel-air mixture exiting radially from the nozzle is
met by air
traveling axially through the injector in a helical path due to the air
swirler. A quad is a
device, which is used in industrial boilers and furnaces to strengthen and
modify the
shape of the recirculating hot combustion products.
Conventional burners, which utilize swirl stabilized combustion, such as
those disclosed above, must have a swirl strength that is sufficient to allow
the
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recirculation about the centerline to develop as shown in Fig. 1. As noted
above, in
swirl-stabilized combustion, the combustion is stabilized when the heat and
free radicals
produced by the process are transported back upstream in the recirculation
zone to mix
with and initiate combustion of the un-reacted fuel-air mixture. Stable
combustion is
very dependant upon the recirculation of these hot combustion products back
upstream.
Still further, when the velocity of the recirculated combustion products is
increased, the
flux of hot and chemically active combustion products upstream increases and
the
combustion process tends to become more stable over a wider range of operating
conditions.
The swirl strength strongly influences the size, shape and strength of the
recirculation zone of hot combustion products. The swirl strength is measured
by a
nondimensional number defined as the ratio of axial flux of angular momentum
to the
axial flux of axial momentum. In general, a recirculation zone is not created
when the
swirl number is less than 0.4,. When the swirl number increases, this causes
the total
pressure at the forward stagnation point to decrease. Shown in Fig. 1, the
forward
stagnation point is the point where the upstream flow of combustion products
along the
centerline meets the downstream axial flow of the air from the burner, at this
point all
velocities are zero. Typically, a swirl number greater than approximately 0.6
creates a
low pressure region at the forward stagnation point. This low pressure region
causes the
combustion products to flow from the downstream end of the burner where the
pressure
within the burner is higher, upstream to the forward stagnation point where
the pressure
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is reduced. This is the mechanism that results in the formation of the main
recirculation
zone (see Fig. 1).
Increasing the swirl number (S") tends to decrease the pressure at the
forward stagnation and increases the upstream recirculation velocity near the
centerline.
This increased upstream flow of combustion products increases the flux of hot
gas and
chemically active species to the forward stagnation point where strong
combustion can be
initiated. When the swirl number is low (i.e., 0.4 < Sn < 0.6), the pressure
at the forward
stagnation point is only slightly lower that the pressure at the aft
stagnation point of the
recirculation zone. As a result, the flux of the hot and chemically reactive
combustion
products that are transported upstream is low and the combustion is less
stable, especially
when the combustion is lean.
The swirl number has other effects on the recirculation zone. For
example, increasing S" reduces the low pressure at the forward stagnation
point and pulls
the aft stagnation point upstream, making the recirculation zone shorter.
Additionally,
the circumferential forces that increase as S" increases, result in the
diameter of the
recirculation zone increasing as well.
A quad is a device, which is used in industrial boilers and furnaces to
desensitize the length and diameter of the recirculation zone from the
magnitude of the
swirl number. The quart also allows the diameter of the recirculation zone to
be
expanded to the diameter of the exit of the Quart without having to increase
Sn. Still
further, when a quad is used, the length of the recirculation zone is less
sensitive to the
swirl number and assumes a length of about 2 to about 2.5 times the quart exit
diameter.
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A quarl allows for a high S" to be used without producing a large diameter
recirculation zone. However, in burners which use a quarl, when the swirl
strength is
strong, the flame has a tendency to move upstream, deep inside the burner
causing
damage to the burner components. Additionally, when the combustion is
initially on the
lean side of stoichiometric, making the mixture richer increases the flame
speed. This
increase in flame speed causes the flame to travel further upstream into the
burner. In
addition to damaging the burner hardware, the uncontrolled movement of flame
deep into
the burner can result in high NOx emissions.
Moreover, stability issues can become magnified when changes are made
to the fuel/air ratio. When lean premixed combustion becomes very lean, the
flame speed
becomes very sensitive to changes in the fuel/air ratio. The continually
varying flame
speed often results in a shifting flame position, causing combustion pressure
oscillations
and noise.
Combustions instability can also occur when the flame moves inside the
burner causing the fuel/air ratio to become richer, which causes the flame to
move deeper
into the burner. The richer fuel/air ratio is typically counteracted by
decreasing the swirl
strength. However, this will result in a cyclic process of the flame moving in
and out of
the burner. This common instability problem can result in very high-pressure
pulsations
and an increase in NOx emissions. This instability is typically a low
frequency
instability, generally ranging between 80 to 150 Hz. The amplitude of the
pressure
pulsations can exceed 0.1 bar and are destructive to the gas turbine engine.
Furthermore,
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during the part of the instability cycle where the combustion is rich,
significant amounts
of NOx can be produced.
In view of the foregoing, a need exists for an improved burner, which
improves flame stabilization, reduces pressure pulsations, noise and NOx
emissions.
SUMMARY OF THE INVENTION
The subject application is directed to a burner for a gas
turbine combustor that uses a central bluff body flame holder and a quart to
stabilize the
combustion process. The burner includes, among other elements, a cylindrical
main body
and a flame holder.
The main burner body includes axially opposed upstream and downstream
end portions and has at least one fuel inlet passage and at least one air
inlet passage
formed therein. The fuel and air inlet passages are adapted to supply fuel and
air
respectively to a mixing chamber that is formed in the downstream end portion
of the
main body. The mixing chamber has a plurality of circumferentially spaced-
apart
surfaces formed on an interior thereof for swirling and mixing the fuel and
air supplied to
the mixing chamber.
The flame holder is disposed within the mixing chamber and includes a
base portion and an elongated bluff body. The base portion engages with the
main body
of the burner in a supporting manner and the elongated bluff body extends in
an axially
downstream direction from the base portion through the internal mixing chamber
so as to
position a combustion ignition point downstream of the internal mixing
chamber.
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The burner further includes a quad device disposed adjacent to the
downstream end portion of the main body. The quart device defines an interior
recirculation chamber and a burner exit. The interior recirculation chamber is
adapted for
receiving precombustion gases from the mixing chamber and for recirculating a
portion
of the combustion products gases in an upstream direction so as to aid in
stabilizing
combustion.
It is presently envisioned that the bluff body of the flame holder is
centered within the mixing chamber and has a tapered upstream section and a
substantially cylindrical tip region. Ideally, the flame holder has an axial
length which is
adapted for achieving an S" of greater than about 0.6. The swirl number being
a ratio of
the tangential momentum to axially momentum that defines how much of the
combustion
air going through the burner is rotating versus how much of the combustion air
exiting
the burner is in an axial flow condition. A mathematical definition of the
swirl number
can be found in U.S. Patent No. 5,365,865 to Monroe, which is herein
incorporated by
reference in its entirety.
In an exemplary embodiment, the at least one air inlet passage is formed in
a substantially radially inward direction and the fuel enters the mixing
chamber of the
main body in a substantially axial direction. Ideally, the air enters in a
tangential and
radially inward direction of the air inlet imparts swirl on the air passing
through the
burner, which is adapted for achieving an S" of greater than about 0.6.
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Those skilled in the art will readily appreciate that the inventive aspects of
this disclosure can be applied to any type of combustor or burner, such as a
solid fuel
burner or furnace.
BRIEF DESCRIPTION OF THE DRAWINGS
So that those having ordinary skill in the art to which the present
application appertains will more readily understand how to make and use the
same,
reference may be had to the drawings wherein:
Fig. 1 is a perspective view in cross-section of a prior art swirl stabilized
burner;
Fig. 2 is a perspective view in cross-section of the swirl stabilized burner
of the subject invention which includes a bluff body flame holder;
Fig. 3 is a cross-sectional view of the burner of Fig. 2 illustrating the
swirl
flow within the burner and the anchoring of the forward stagnation point of a
main
recirculation zone and the flame front by the center bluff body flame holder;
Fig. 4a is a cross-sectional view of a burner constructed in accordance
with a preferred embodiment of the present invention which illustrates the
flame
stabilized on the center bluff body flame holder;
Fig. 4b is a cross-sectional view of a prior art burner without a center bluff
body flame holder illustrating the flame in the flash back position;
Fig. 4c is a cross-sectional view of the burner of Fig. 4b illustrating the
flame positioned in the downstream end portion of the burner, near the exit;
and
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Fig. 4d is a cross-sectional view of the burner of Fig. 4b illustrating the
flame positioned outside the burner exit.
These and other features of the burner of the present application will
become more readily apparent to those having ordinary skill in the art form
the
following detailed description of the preferred embodiments.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings wherein like reference numerals identify
similar structural aspects of the subject invention, there is illustrated in
Fig. 2 a burner for
a gas turbine combustor designated generally as reference numeral 100. Burner
100 uses
a central bluff body flame holder 20 and a quarl device 80 to stabilize the
combustion
process. The burner 100 includes, among other elements, a cylindrical main
body 50, a
flame holder 20 and a quarl device 80. The main body 50 and the flame holder
20 may
be attached to one another in a conventional manner, or held together by an
interference
fit, or mechanically interlocked.
The burner main body 50 includes axially opposed upstream and
downstream end portions, 52 and 54, respectively. A plurality of axially-
oriented fuel
inlet passages 56 and a plurality of radially-oriented air inlet passages 58
are formed in
main body 50. Those skilled in the art would readily appreciate that the
location,
quantity and orientation of the fuel inlet passages 56 and air inlet passages
58 can vary
without departing from the inventive aspects of this disclosure and the
configuration
depicted herein is for illustrative purposes only.
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The fuel and air inlet passages 56 and 58 are adapted to supply fuel and air
respectively to a mixing chamber 60 that is formed in the downstream end
portion 54 of
the main body 50. The mixing chamber 60 has a plurality of circumferentially
spaced-
apart surfaces 62 or vanes formed on an interior thereof for imparting a
swirling motion
to and mixing the fuel and air supplied to the mixing chamber 60.
The flame holder 20 is disposed within the mixing chamber 60 and
includes a base portion 22 and an elongated bluff body 24. The base portion 22
engages
with the main body 50 of the burner 100 in a supporting manner and the
elongated bluff
body 24 extends in an axially downstream direction from the base portion 22
through the
internal mixing chamber 60 so as to position a combustion ignition point or
forward
stagnation point 75 (see Fig. 3) downstream of the internal mixing chamber 60.
The
elongated bluff body 24 has a plurality of axially-extending flutes 27 formed
in its outer
surface to define the scale of turbulence within burner 100.
Quail device 80 disposed adjacent to the downstream end portion 54 of the
burner main body 50. The quail device 80 defines an interior recirculation
chamber 82
and a burner exit 84. The interior recirculation chamber 82 which is defined
by interior
surface 82a is adapted for receiving precombustion gases from the mixing
chamber 60
and for recirculating a portion of the combustion product gases in an upstream
direction,
so as to aid in stabilizing combustion. In the embodiment disclosed herein,
interior
recirculation chamber 82 is shaped in a classical venturi shape. However,
other shapes
that accomplish the pressure gradient separation of the mixing chamber and the
recirculation chamber, are contemplated by the invention herein.
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The bluff body portion 24 of the flame holder 20 is centered within the
mixing chamber 60 and has a tapered upstream section 26 and a downstream neck
region
28 that has a radially enlarged head. The shape of the neck region can be
adapted to
further improve the recirculation of the combustion products and flame
stability. The
length of the flame holder 20 is chosen so as to anchoring a main
recirculation witha a
swirl number of greater than about 0.6, but not larger than about 2Ø As
stated
previously, the swirl number is defined as the ratio of the amount of rotating
combustion
air going through the burner versus the amount of the combustion air exiting
the burner is
in an axial flow condition.
Burner 100 is adapted for making the cyclic combustion process more
stable and significantly reduces the tendency for a gas turbine engine using a
lean
premixed combustion to flame out or to produce pressure pulsations resulting
from
unstable combustion. The center body flame holder 20 and the quart 80 have two
key
effects: 1) the point where combustion is initiated is fixed in space, and 2)
higher swirl
velocities can be obtained without combustion to flash back into the mixing
chamber 50
of the burner 100. The anchoring of the flame using the bluff body flame
holder 20 on
the central axis allows for natural fluctuations of the fuel/air ratio and
variations in the
swirling velocity to occur without a change in the flame position. The ability
to increase
swirl strength without causing a flash back and the fixing the combustion
initiation point,
both make the combustion process more stable. Therefore, the use of a quad 80
and the
bluff body flame holder 20, fundamentally changes the stability of swirl-
stabilized
combustion, as compared to prior art burners
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The flame holder 20 physically prevents the flame from flashing back up
the centerline of the burner 100 into the mixing chamber 50. By preventing
flash back up
the centerline into the mixing chamber 50 the fuel-air mixture can have a
higher
tangential swirl component. Increasing the swirl strength without flash back
makes the
quarl more efficient in strengthening the recirculation of hot gas upstream,
making the
entire combustion process more stable. Increasing the amount of heat re-
circulated
upstream allows stable combustion of leaner fuel-air mixtures. This provides
for greater
flexibility and robustness in engine operation, while maintaining low engine
emissions.
Quarl device 80 is used to make the recirculation zone smaller than what
would be produced as a result of the influence of the swirl number alone. The
quart
device 80 allows for a high swirl number while maintaining a small diameter
and short
length of the recirculation zone. The high swirl number results in a large
difference in
pressure between the forward and aft stagnation points. This high pressure
gradient
results in high velocity and high flux of hot chemically active combustion
products to
flow about the centerline to the forward stagnation region where combustion is
initiated.
A high flux of hot chemically active combustion products at the location where
combustion is to be initiated allows for stable combustion of lean fuel and
air mixtures.
Stable combustion of lean fuel and air mixtures is important to produce low
nitrous
oxide, NO and N02, emissions in gas turbine engines.
Keeping the recirculation zone small helps to preserve the chemical
activity of the hot combustion gases, allowing for more rapid and stable
initiation of
combustion, especially at low combustion temperatures, such as those that
often occur
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below 1700 K in low NOx (NO and N02) engines. This low residence time in the
recirculation for the chemically reactive products of combustion becomes more
important
as combustion pressures rise and combustion temperatures decrease. At high
pressure
chemically reactive species, also known as free radicals that are useful to
initiate rapid
combustion, relax to equilibrium level quickly under the effect of high
pressure. The life
of free radicals that are above equilibrium levels become shorter as pressure
is increased.
Effective use of these high non-equilibrium levels of free radical becomes
more
important when the combustion temperature is low, such as in low NOx engines,
because
the equilibrium levels of free radicals are low at low temperatures.
Referring now to Fig. 3, there is illustrated a cross-section of the swirl
stabilized burner 100 that depicts the recirculation of combustion products
upstream so as
to sustain the combustion process. The upstream and downstream ends of burner
100 are
identified by reference character "U" and "D", respectively. As shown in this
figure, the
flow of combustion products is separated into distinct zones, namely a main
recirculation
zone 90 and an outer recirculation zone 92.
As stated earlier, the process of swirling the fuel-air mixture so as to cause
the combustion products to travel upstream is commonly used to stabilize
combustion. In
the disclosed burner 100, the bluff body flame holder 20 anchors the main re-
circulation
zone 90 into a fixed position. The flame front or combustion initiation point
94 of the
premixed flow occurs along the outer surface of the main re-circulation zone
90 where
the heat and free radicals mix and initiate combustion of the unreacted
premixed fuel and
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air. The flame starts at the end of the flame holder 20 and expands in a conic
pattern
downstream.
Burner 100 maintains the flame position fixed to the tip 24 of the flame
holder 20, even when significant changes occur in the fuel/air ratio. When
lean premixed
combustion becomes very lean the flame speed becomes very sensitive to the
fuel/air
ratio. This change in flame speed often results in shifts in flame position,
which may
result in combustion pressure oscillations. By anchoring the flame with the
center bluff
body flame holder 20 and preventing the flame from moving, pressure
oscillations can
also be prevented.
Fig. 4a provide a cross-sectional view of burner 100 that illustrate conical
flame 98 anchored to flame holder 20. Figs. 4b-4d depict a burner 200 that
does not
include a center body flame holder. In burner 200, when the swirl strength is
strong, or
the premixed fuel/air ratio is rich, the flame 298 has a tendency to move deep
inside the
burner, as shown in Fig. 4b. When the combustion is on the lean side of
stoichiometric,
making the mixture richer increases the flame speed. Increased flame speed
makes it
possible for the flame to travel further upstream. Increasing the swirl
strength will also
produce the same tendency to move the flame further upstream. Generally, it is
not
desirable to have the flame 298 move into the burner mixing region 260, as
shown in Fig.
4b. The uncontrolled movement of flame deep into burner 200 can result in
damage to
hardware and result in high NOx emissions. The addition of the center bluff
body flame
holder 20 to the quarl modified burner anchors the forward stagnation point 96
of the
main recirculation zone 90 to the end of the flame holder 20 preventing the
main
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recirculation zone 90 and flame from traveling into the mixing chamber 60. The
center
bluff body flame holder anchors the forward stagnation point 96 (see Fig. 3)
to the end of
the flame holder 20 for swirl strengths that would have otherwise driven the
forward
stagnation 96 point deep inside the burner 100, or toward the exit 84, or even
outside, the
burner 100. The center bluff body flame holder 20 anchors the forward
stagnation point
96 and flame 198 to a single fixed location, instead of moving continuously as
the swirl
strength varies.
There is an optimum location for the center bluff body flame holder 20
where the swirl number can be equally increased or decreased and the forward
stagnation
point 96 and flame 198 stays attached to the flame holder 20. If the swirl
strength is
continually decreased, the flame 198 will stay attached to the flame holder
until finally
the flame jumps off the flame holder and stabilizes a significant distant
downstream, or
outside the burner's exit 84. Stating from the same optimum swirl number and
center
bluff body flame holder position, increasing the swirl strength will not
effect the flame
position, until at some critical swirl strength, the flame position will jump
upstream
engulfing the end of the flame holder 20 inside the main re-circulation zone.
As long as
the operating conditions stay within reasonable range of swirl strength and
fuel/air ratio,
the flame position will stay fixed even as engine conditions change. These
ranges have
been shown to be very wide, which is a positive attribute of burner 100.
The movement of flame position is a significant problem for combustion
systems that operate very lean. The flame 198 shown in Figs. 4c and 4d. is
produced at
the leanest fuel/air ratios and/or the lowest swirl strength. When the
combustion becomes
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very lean, swirl stabilised combustion can become very unstable. However, the
method
shown to be most successful in reducing NOx emissions is to make the
combustion so
lean that the temperature of the flame is reduced below the temperature at
which diatomic
Nitrogen and Oxygen (N2 and O2) disassociate and recombine into NO and NOa_
When
nearly twice the amount of air is mixed with the fuel before the fuel and air
mixture
combust, the excess air acts as inert matter that is heated by the combustion
process. The
amount of energy released by the combustion process is determined only by the
amount
of fuel burnt, as long as sufficient or greater amount of air is supplied to
the combustion
process. The air in excess of the amount necessary for combustion does not
effect the
amount of energy released by the combustion process, but because the combined
mass of
fuel and air is increased while the energy released is constant, the flame
temperature and
the temperature of the combustion products is reduced. This reduction of flame
temperature reduces the formation of NOx (NO and N02). This is the principle
upon
which virtually all-low NOx emissions gas turbine engines are currently based.
As stated above, the addition of the center bluff body flame holder 20 to
the burner 100 allows the swirl strength to be increased without the flame
flashing back
into the mixing chamber 50. The ability to increase the swirl strength
increases the
reverse flow of hot combustion products back upstream. The increased flow of
hot
combustion products provides more heat and free radicals, which makes the
combustion
more robust and less susceptible to instabilities.
If the flame front starts external to the burner, the burner will have a
maximum airflow rate for a fixed pressure drop across the burner. If some
perturbation
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allow the flame to jump inside the burner, the mass flow rate of air through
the burner
will decrease, because the heat from the combustion process will cause the air
to expand
increasing the volumetric flow rate through the exit of the burner. This
increase is
volumetric flow rate for a fixed pressure drop results in a decrease in the
mass flow rate
of air through the burner. For most gas turbine engines, six to 100 burners
would be used
depending upon the power rating of the engine. If the flame jumps into some
burners, but
not all burners, the burners that have flame inside will burn richer. This is
because the
same fuel will be supplied to all burners equally through a common fuel
manifold. The
burners with the flame inside are richer because the mass flow rate of air
decreases due to
the increased volumetric flow rate as a result of the combustion inside the
burner exit.
The result of the richer combustion when the combustion is initially lean is
to increase
the flame speed. The increase in flame speed allows the flame to move deeper
into the
burner. This will increase the volumetric airflow rate and decrease
additionally the mass
flow rate of air making the combustion even richer. Once inside it is possible
for the
flame to stay inside a few of the burners, while staying external to the rest
of the burners.
When this occurs high NOx results and hot spots occur entering the turbine
inlet
corresponding to the richer burners.
The combustion processes inside the burner will also affect the swirl
characteristics, which can lead to the reversal of the previous process
pulling the flame
inside the burner. When the flame pulls inside the burner the mass flow rate
of air
decreases. The density of air passing through the swirler does not change
resulting in
lower velocity and a decrease in swirl strength. The decrease in swirl
strength will tend
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to cause the forward stagnation point of the main re-circulation to move
downstream.
The combustion process itself will also tend to decrease the swirl strength,
because the
combustion process expands the flow uniformly in all directions.
Instability can occur when the flame moves inside the burner causing the
fuel/air ratio to
become richer, which causes the flame to move deeper into the burner.
Counteracting the
richer fuel/air ratio that produces higher flame speed is the decay of swirl
strength. This
will result in a cyclic process of the flame moving in and out of the burner.
This common
instability can result in very high-pressure pulsations and an increase in NOx
emissions.
This instability is a common low frequency instability generally of 80 to 150
Hz. The
amplitude of the pressure pulsations can exceed 0.1 bar pressure oscillations
and be
destructive to the gas turbine engine. During the part of the cycle where the
combustion
is rich, significant amounts of NOx can be produced. The invention of the
center bluff
body flame holder applied to the quart based burner makes the position of the
flame
insensitive to changes in the swirl strength and fuel/air ratios allowing the
flame to
stabilize in a fixed location at the end of the flame holder. This eliminates
the pressure
oscillations and the elevated NOx emissions that would have resulted from the
movement
of the flame.
While the invention has been described with respect to preferred
embodiments, those skilled in the art will readily appreciate that various
changes and/or
modifications can be made to the invention without departing from the spirit
or scope of
the invention as defined by the appended claims.
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The swirl number being a ratio of the tangential momentum to axially
momentum that defines how much of the combustion air going through the burner
is
rotating versus how much of the combustion air exiting the burner is in an
axial flow
condition.
