Note: Descriptions are shown in the official language in which they were submitted.
. . . 21 87255
COMBUSTOR OSCILLATING PRESSURE STABILIZATION AND METHOD
BACKGROUND OF THE INVENTION
The present invention relates generally to the control of combustion oscillations
in combustion systems, and more particularly to the apparatus and method for reducing
undesirably high dynamic pressure oscillations in a combustion chamber to acceptably
lower levels by using a pulsating pilot flame for selectively positioning or restructuring
the oscillating flame front in the combustion chamber. The United States Government
has rights in this invention pursuant to the employer-employee relationship of the U. S.
Department of Energy and the inventors.
Combustion systems such as used in conjunction with gas turbines and steam-
generators commonly utilize a hydrocarbon fuel with air in substantially stoichiometric
ratios in an associated combustion chamber for the generation of sufficient heat energy
for driving the turbine or for the generation of steam. Some environmental pollution has
been caused by the exhaust emissions resulting from the burning of hydrocarbon fuels
in such applications. Efforts to reduce these environmental polluting emissions include
pre-mixing the fuel and air prior to introducing the mixture into the combustion chamber.
Also, the use of such pre-mixes in a so-called lean pre-mix, i.e., a mixture where the
volume of fuel is present at less than stoichiometric ratio with the air, provides for a
reduction in nitrous oxide emissions. A typical combustion system using a lean pre-mix
is described in U.S. Patent 5,372,008, which issued December 13, 1994, and which is
incorporated herein by reference.
While the use of pre-mixes tlean and near or essentially stoichiometric) of a
hydrocarbon fuel and air has been successful in reducing the emissions of environmental
pollutants so as to alleviate the impact of these emissions on the environment, it has
been found that combustion instability in the form of dynamic pressure oscillations
occurs in combustion systems using such pre-mixes, especially lean pre-mixes. Asindicated by Rayleigh's criteria, "Theory of Sound", Volume ll, No. 8, Dover, New York,
- . ~ 1 87255
1995, the amplitude of the oscillations in the combustion chamber will be the strongest
when the pressure wave is in-phase with the periodic heat release produced by the
combustion of the fuel-air mixture. These dynamic pressure oscillations are frequently
of a sufficiently high magnitude so as to produce undesirable operating conditions
including the reduction of the useful life of the combustion system components due to
structural fatigue, vibrations, and cycling fatigue.
Efforts to remove or reduce unacceptably high-dynamic pressure oscillations in
hydrocarbon-fueled combustion systems have been somewhat successful. For example,
as described in the publication, rPractical Active Control System for CombustionOscillationsn, P. J. Langhorne, J. Propulsion, Volume 6 No. 3, pp 324-333, May-June
1990, the low frequency combustion instability in a flame present in a duct defining a
combustion chamber was successfully controlled by the unsteady addition of extra fuel.
As reported in this publication, in an application where the main fuel was introduced into
the duct as a lean pre-mix with a stoichiometric ratio of fuel to air in the range of about
0.63 to 0.70, the addition of only 3% more fuel reduced by 12 decibels (dB) the 164
dB peak in the pressure spectrum in the duct due to the combustion instability. As
further reported in this publication, the unsteady or phased introduction of the excess
fuel into the duct was a pre-mix with air and was injected sufficiently close to the main
flame to create local rich spots which destabilize the flame and thereby reduce the
oscillations. A feedback control loop using unsteady signals from the combustionchamber, such as light emissions produced from combustion generated radicals near the
combustion region, was used for controlling the introduction of the secondary fuel.
Another previously known technique for reducing pressure oscillations in
combustion chambers using lean pre-mixes of fuel and air is described in the paper
entitled, "Combustion Oscillation Control by Cyclic Fuel Injection", G.A. Richards, M.J.
Yip, E. Robey, L. Caldwell, and D. Rolands, presented at the 1995 ASME Turbo Expo
21 87255
Meeting, Houston, Texas, June 6, 1995. In this paper, the control over pressure
oscillations in the combustion chamber was achieved by the cyclic injection of small
quantities of extra fuel for counteracting the periodic heat release associated with the
high-pressure oscillations. The added fuel produces secondary thermal energy which
decouples the interaction between the thermal and acoustic energies leading to areduction in the dynamic pressure amplitude.
A further previously known technique for reducing the pressure oscillations in
hydrocarbon-fueled combustion chambers is reported in the publication, "PeriodicChemical Energy Release for Active Combustion Controln, K. C. Schadow et al, ISABE-
International Symposium on Air Breathing Engines, 11, Volume 1, pp 479-485, Tokyo,
Japan, September, 1993. In this Schadow et al publication a spark-initiated pulsed
combustion actuator mounted in a tube located upstream of a combustion chamber
generates periodic pulses of pilot flame which suppress pressure oscillations in the
combustion chamber. In the combustion actuator a stoichiometric mixture of fuel and
air was spark ignited at a frequency in the range of 50 Hz to over 1 kHz. A frequency
generator in an open loop or a closed feedback using pressure fluxuations from the
combustion chamber is utilized for controlling the frequency of the spark ignition.
SUMMARY OF THE INVENTION
While the previously known active control techniques for reducing or suppressingundesirable pressure oscillations in hydrocarbon-fueled combustion systems such as
typified by those described in the aforementioned articles have been somewhat
successful, the primary objective or aim of the present invention is directed to an even
more effective active control apparatus and method for effecting the control or
stabilization of unsteady combustion oscillating pressures in hydrocarbon-fueledcombustors. Briefly, when unsteady pressure oscillations occur in a combustion
chamber such as due to the transport time of the fuel to the flame front being at some
21 8725~
fraction of the acoustic period, the amplitude of the oscillations in the combustion
chamber wili be the strongest when the pressure wave is in-phase with the periodic heat
release produced by the combustion of the fuel-air mixture. In accordance with the
objective of the present invention, the active control of unsteady combustion induced
oscillations in a combustion chamber fired by a suitable fuel and oxidizer mixture, such
as a hydrocarbon fuel and air mixture, is provided by restructuring and moving the
position of the main flame front and thereby increasing the transport time and displacing
the pressure wave further away from the in-phase relationship with the periodic heat
release. The restructuring and repositioning of the main flame are achieved by utilizing
a pilot flame which is pulsed at a predetermined frequency corresponding to less than
about one-half the frequency of the combustion oscillation frequency with the duration
of each pulse being sufficient to produce adequate secondary thermal energy to
restructure the main flame and thereby decouple the heat release from the acoustic
coupling so as to lead to a reduction in the dynamic pressure amplitude. The pulsating
pilot flame produces a relatively small and intermittently existing flame front in the
combustion zone that is separate from the oscillating main flame front but whichprovides the thermal energy necessary to effectively reposition the location of the
oscillating main flame front out of the region in the combustion zone where the acoustic
coupling can occur with the main flame and thereby effectively altering the oscillation-
causing phase relationship with the heat of combustion.
Generally, the apparatus of the present invention is used for effecting the active
control over unstable combustion in a combustion system conventionally comprising a
combustion chamber having a combustion zone with opposite end regions and means
for introducing a mixture of fuel and a suitable oxidizer into the combustion zone at one
end region thereof. Unstable combustion of this fuel and oxidizer mixture forms an
oscillating flame front within the combustion zone at a location intermediate the end
2 ~ 8 7~ ~ ~
regions thereof with the pressure and amplitude of the resulting dynamic pressure
oscillations being dependent upon the degree of in-phase relationship of the pressure
wave produced by each oscillation with the heat produced by the combustion of the fuel
in the fuel and oxidizer mixture. The apparatus of the present invention effectively
reduces the pressure and amplitude of these pressure oscillations within the combustion
chamber and comprises: a suitable fuel and oxidizer mixing volume or chamber means
communicating with the combustion chamber at the aforementioned one end region
thereof; oxidizer supplying means and fuel supplying means coupled to the chamber
means for introducing thereinto at least one stream of fuel and oxidizer for forming a
combustible mixture thereof in order to produce a pilot flame in the combustion zone at
or near the one end region thereof; and, flow control means for intermittently
interrupting or initiating the flow of the fuel from the fuel supplying means and/or the
oxidizer from the oxidizer supplying means into the chamber means, or the combustible
mixture from the mixing volume or chamber means into the combustion chamber, at a
selected frequency and a selected duration for sequentially interrupting or initiating, i.e.,
establishing, the pilot flame and thereby sufficiently pulsing the pilot flame in said
combustion zone to provide a source of secondary thermal energy contactable with the
oscillating flame in the combustion zone for restructuring the oscillating flame for
changing or moving the phase relationship of the pressure waves with the heat ofcombustion away from an in-phase relationship to effectively reduce the amplitude of
the pressure oscillations. The selected frequency and duration of the energy pulses
provided by the pulsating pilot flame are effective to sufficiently reposition the
oscillating flame front in the combustion zone and thereby change the phase relationship
of the pressure waves with the heat of combustion away from an in-phase relationship
for reducing the amplitude of the pressure oscillations.
_ 2 1 8725~ -
As a general rule, when the pressure oscillations in the combustion chamber are
at a frequency in the range of about 20 to 5000 Hz, the intermittent interruption or
initiation of the flow of at least one of the fuel and the oxidizer into the pilot chamber,
or a mixture thereof into the pilot chamber or the combustion chamber, is provided at
a frequency less than about one-half of that of the main-flame combustion oscillations.
Also, establishing the flow of each pilot fuel and oxidizer mixture for a duration in the
range of about 0.1 to 0.5 of the control frequency period, preferably about 1-20
milliseconds (ms), between each interruption of the flow provides for the desired
restructuring of the oscillating flame when the fuel to oxidizer equivalence ratio is in the
range of about 0.5 to about 1.0, as will be described in greater detail below
The method of the present invention for reducing the amplitude of the
oscillations within the combustion chamber is achieved by the steps comprising: forming
a second mixture of fuel and oxidizer, introducing the second mixture into the
combustion zone at a location adjacent to the one end region thereof at least for
forming a pilot flame in the combustion zone at the aforementioned one end region
thereof; and, intermittently interrupting or initiating the flow of the fuel and/or the
oxidizer forming the second mixture, or the second mixture after the formation thereof,
to produce discrete charges of the second mixture for intermittently producing and
interrupting the pilot flame at a rate and for a duration adequate to sufficiently
restructure and reposition the oscillating flame in the combustion zone from the first
location to a second location for changing the phase relationship of the pressure waves
to the heat of combustion with respect to in-phase relationship to substantially reduce
the amplitude of the pressure oscillations in the combustion chamber. The rate of
intermittently interrupting or initiating the flow of at least one of the oxidizer and the
fuel defining the discrete charges of pilot flame mixture and the duration of each flow
correspond to the control frequency and the duration described above.
21 ~7255
Other and further objects of the present invention will become obvious upon an
understanding of the illustrative embodiments and method about to be described or will
be indicated in the appended claims, and various advantages not referred to herein will
occur to one skilled in the art upon employment of the invention in practice.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a combustion system illustrating unsteady
combustion-induced oscillations of the main flame front in the combustion chamber as
indicated by the double-ended arrows;
Figure 2 is a graph illustrating the dynamic pressure oscillation occurring in acombustion chamber operating with unsteady combustion-induced oscillations as shown
in Figure 1;
Figure 3 is a schematic diagram of a combustion system similar to Figure 1 but
provided with means in accordance with the present invention for intermittently pulsing
the pilot fuel into the pilot chamber for admixture with an introduced stream of oxidizer
to produce a series of discrete combustible pilot mixtures which when introduced into
the combustion chamber form intermittently existing pilot flame fronts which restructure
and displace the main flame front and thereby provide a decoupling action between the
pressure oscillations and the heat of combustion for reducing the amplitude of the
pressure oscillations;
Figure 4 is a graph illustrating the reduction of the high-amplitude pressure
oscillations in the combustion system of Figure 3 when provided with the combustor
oscillating pressure stabilization mechanism of the present invention;
Figure 5 is a detailed view of one embodiment of the present invention wherein
a combustion chamber is provided with a pilot fuel-oxidizer mixing chamber with a
control over the fuel supplied to the pilot chamber for effecting the intermittent
introduction thereof into the pilot chamber so as to produce a pulsating pilot flame in
_ 21 872~5
the combustion chamber for restructuring the oscillating flame front within the
combustion chamber and thereby reducing the amplitude of the combustion oscillations;
Figure 6 is a schematic diagram of a combustion system provided with an open
loop control arrangement for pulsing the fuel to the pilot chamber for selected durations
for reducing the amplitude pressure oscillations in accordance with the present
invention; and
Figure 7 is a schematic view of a further embodiment of the present invention
wherein the oxidizer rather than the fuel supplied to the pilot chamber as in the Figure
5 embodiment is intermittently interrupted or initiated to pulse the pilot flame for
displacing the oscillating flame front and thereby effecting the reduction in the pressure
amplitude of the oscillations.
Preferred embodiments of the invention have been chosen for the purpose of
illustration and description. The preferred embodiments illustrated are not intended to
be exhaustive nor to limit the invention to the precise forms shown. The preferred
embodiments are chosen and described in order to best explain the principles of the
invention and their application and practical use to thereby enable others skilled in the
art to best utilize the invention in various embodiments and modifications as are best
adapted to the particular use contemplated. Also, while the combustion chambers
illustrated in these drawings are somewhat limited in detail, it will appear clear that the
particular construction and operational details of the combustion chamber are not critical
since the present invention can be utilized in any combustion chamber of essentially any
configuration in which the high-pressure oscillations are produced during the combustion
process and wherein a pulsating pilot flame as provided by the present invention can be
utilized for the restructuring the position of the main flame front. Further, while the
combustion charnbers as shown in the drawings are each provided with a pilot chamber
located coaxially with the main axis of the combustion chamber, it will appear clear that
_ 21 872~
the pilot chamber can be repositioned at other locations on the combustion chamber so
as to introduce a pulsating pilot flame at a location different from that of the oscillating
flame front so as to effectively restructure the primary or oscillating flame front and
thereby reduce the amplitude of the oscillations in accordance with the teachings of the
present invention.
The phrases "pilot chamber" and "chamber means" as used herein are intended
to be descriptive of any walle~ structure containing a cavity or volume communicating
with the fuel and oxidizer supplies and the interior of the combustion chamber and
which is of a sufficient capacity to effect adequate mixing of the discrete charges of the
pilot fuel and oxidizer. The particular shape and/or position of the pilot chamber or
chamber means is not critical to the operation of the present invention as long as it
provides for the mixing of the pilot fuel-oxidizer charges and the introduction of these
charges into combustion chamber at the appropriate location therein for producing
therein the pulsed pilot flame for the restructuring of the main flame. Also, the term
"pilot" as used herein denotes a supply rate of fuel and oxidizer mixture that corresponds
to about 2 to about 20 percent of the supply rate of the fuel and oxidizer mixture used
for the main flame. Further, while the description below is primarily directed to the
embodiments where the flow of the fuel or the oxidizer is selectively interrupted for
esta~ hing the pulsed pilot flame in the combustion chamber, it will appear clear that
the flows of both the fuel and the oxidizer can be simultaneously controlled at selected
frequencies and durations to produce the discrete charges of the pilot flame mixture, or
that the flow of mixtures of the pilot fuel and oxidizer to the combustion chamber can
be selectively interrupted to establish the pulsed pilot flame.
DETAILED DESCRIPTION OF THE INVENTION
As briefly described above, it has been found that the oscillations frequently
present in combustion chambers fired by liquid, gaseous, and solid hydrocarbon or other
`~ 2 1 87255
fuels in the presence of a suitable oxidizer are often of such high pressure and amplitude
so as to deleteriously reduce the efficiency of the combustion system as well assignificantly shorten the expected life of various components associated with the
combustion chamber due oscillation-induced vibrations and cyclic failures. Also, as
pointed out above, the efforts to reduce environment-impacting emissions from
hydrocarbon-fueled combustion chambers by the use of a stoichiometrically lean pre-mix
is frequently responsible for the formation of the undesirable unsteady combustion-
induced oscillations. For example, as illustrated in Figures 1 and 3, a conventional
combustion system 8 inco-,uor~ling a combustion chamber 10 of a suitable configuration
and type such as used in a gas turbine application or in a steam-generating boiler is
shown provided with a pilot chamber 12, an oxidizer supply at ambient pressure or at
elevated pressures such as provided by a conventional compressor 14, and associated
conduits or piping 15 for conveying the oxidizer from the oxidizer supply to thecombustion chamber and the pilot chamber 12 as separate, appropriately portioned,
streams. A fuel supply 16 including the necessary piping 17 and 18 is respectively
provided for conveying separate streams of fuel from the fuel supply 16 to the
combustion chamber 10 and to the pilot chamber 12. This fuel can be a hydrocarbon
fuel or any other suitable fuel in any form such as gaseous, liquid, solid, or any suitable
combination thereof. The oxidizer can be of any suitable combustion supporting
medium for the particular fuel being used. For example, with a hydrocarbon fuel, the
oxidizer can be readily provided by air, oxygen, oxygen-enriched air, or other oxygen
containing gas or gases.
The separated stream of oxidizer from the oxidizer supply 14 and the stream of
fuel in conduit or piping 17 from the fuel supply 16 used for producing the primary or
main flame front in the combustion chamber 10 as generally shown at 20 are pre-mixed
in a conventional pre-mixing chamber such as the annular chamber 21 generally shown
`-- 21 872~5
1 1
coaxially disposed about the pilot chamber 12. This pre-mix of fuel and oxidizer is
shown introduced into the combustion zone 22 at the upstream end region of the
combustion chamber 10 where the fuel-oxidizer mixture is burned to produce the main
flame front 20 for providing the heat energy to be used for desired work application.
The hot products of combustion are discharged from the combustion chamber 10
through appropriate discharge means (not shown) that are located in the down stream
region of the combustion chamber 10.
In the operation of a combustion system provided with a pilot chamber 12 as in
Figure 1, the balance of the oxidizer and fuel from the oxidizer supply 14 and in the
conduit or piping 18 from the fuel supply 16 are mixed in the pilot chamber 12,
continually discharged therefrom into the combustion chamber 10, and initially fired by
any suitable means such as a glow plug or a spark ignition device (not shown) toproduce a steady state pilot flame 23 which extends downstream into the combustion
zone 22 to establish and maintain the main flame front 20. Normally, and especially
with lean pre-mixes, the combustion of the pre-mixed principal fuel-oxidizer mixture does
not provide a steady state burn, but instead produces an unsteady burn forming
intermittent pressure waves and periodic heat releases which cause the flame front 20
to oscillate back and forth in the combustion zone 22 as generally indicated by the
double-ended arrows in the combustion zone 22. As pointed out above, it has beendetermined that when a pressure wave is in or essentially in an in-phase relationship
with the heat release produced by the combustion of the oxidizer-fuel mixture, the
oscillation pressure wave will be the strongest. In Figure 2, the curve 24 illustrates an
oscillating pressure wave occurring in a combustion chamber as in Figure 1 undergoing
unsteady combustion oscillation. The oscillation of the flame front 20 occurs when the
transport time t1 is some fraction of the combustion acoustic period with the amplitude
2 ~ 5:~
of the oscillation as shown by curve 24 increasing as the in-phase relationship of the
pressure wave and the heat release approach one another.
In accordance with the present invention the stabilization of the oscillations in
the combustion zone 22 of the combustion chamber 10 is achieved by restructuring or
moving the main flame front 20 to increase the transport time t1 and thereby decouple
the pressure and heat release parameters. When this in-phase relationship is changed
or altered i.e., decoupled, the oscillation amplitude is reduced with the reduction and
oscillating amplitude being greater with increasing separation of the in-phase relationship
of the pressure wave to the periodic heat release. As shown in Figures 3 and 4, the
combustion system 8 of Figure 1 is modified by using a pilot fuel control system 26 for
the intermittent or pulsing delivery of the pilot fuel from the fuel supply 16 to the pilot
chamber 12. The control system 26 generally utilizes a quick acting on/off valve 27 in
the pilot fuel conduit 18 with this valve 27 being operated by a suitable on-off control
for pulsing the injection or delivery of the pilot fuel into the pilot chamber 12. This
periodic injection or interruption of the injection of the pilot fuel into the pilot oxidizer
stream for mixture therewith forms discrete charges or pulses of the mixture such as
shown at 23 which are sequentially discharged from the pilot chamber into the
combustion zone 22. Each injection of each pilot fuel-oxidizer charge serves to vary the
equivalence ratio for chemical fuel concentration in the combustion zone 22 which
restructures and moves the flame front 20 further away or downstream from the pilot
chamber end of the combustion chamber 10 and produces a larger transport time tl as
shown in Figure 3. The extent of the flame restructuring and repositioning is directly
dependent upon the combination of the frequency of injection of the discrete pilot fuel-
oxidizer charges, the duration of each injection, and the equivalence ratio of the pilot
fuel-oxidizer mixture.
21 ~7255
The pilot fuel is delivered at a frequency independent of and considerably less
than the frequency of the main-flame oscillations occurring in the combustion zone 22.
In the present invention, with the main flame oscillating in the range of about 20-5000
Hz, the pilot fuel is pulsed or intermittently delivered to the pilot chamber 12 at an
established frequency corresponding to less than about one-half the flame oscillating
frequency and at a frequency in the range of about 1 Hz to about 2500 Hz. Preferably,
in a combustion system is fired with a hydrocarbon fuel in the presence of air where
combustion pressure oscillations may occur in the combustion chamber at a frequency
in the range of about 150 - 1200 Hz, the pulsing of the pilot fuel (or oxidizer) at a
control frequency in the range of about 10-50 Hz, preferably 10-20, adequately
restructures and repositions the main flame for effectively reducing the amplitude and
frequency of combustion oscillations to a satisfactory level such as shown by the curve
32 in Figure 4. The magnitude of the flame oscillation shown in Figure 4 as provided
by the practice of the present invention are insufficient to produce undesirable operating
problems associated with the structural damage of the combustion system components
and/or the lowering of the combustion efficiency of the system. It is similarly expected
that the pulsation of the oxidizer for the pilot fuel, separate streams of the pilot fuel and
the oxidizer, or a premix of the pilot fuel and oxidizer, at a frequency in the range of
about 1 Hz to about 2500 Hz wili satisfactorily reduce the pressure amplitude of the
oscillations in a combustion chamber experiencing unsteady combustion-induced
pressure oscillations at frequencies in the range of about 20-5000 Hz.
The fuel to oxidizer ratio in the main fuel-oxidizer mixture introduced into thecombustion chamber can in the range of less than to greater than stoichiometric (a fuel
to oxidizer equivalence ratio of less than one to greater than one). The fuel to oxidizer
equivalence ratio in the mixture thereof used for establishing the discrete pilot flame
pulses can also be in the range of less than one to greater than one to satisfactorily
14
practice the present invention. However, since the secondary thermal energy in each
pilot flame as produced by the combustion of each discrete charge or mixture of the
pilot fuel and oxidizer is directly dependent upon the volume of fuel in each of these
charges, the equivalence ratio of the fuel to the oxidizer in the pilot flame charges is
preferably greater than one in order to assure that sufficient secondary thermal energy
is present to effect the desired restructuring of the oscillating flame while maintaining
an adequate timed spacing between the introduction of the discrete charges of the pilot
fuel and oxidizer into the combustion chamber.
In accordance with the present invention about 2 to 20 percent of the total fuelintroduced into the combustion chamber 10 is periodically injected into the pilot
chamber 12 with the duration of each injection being sufficient to establish the series
of spaced apart fuel-rich pilot flame fronts such as shown at 23 in Figure 3 which
together effect the restructuring of the oscillating main flame front 20 and thestabilization of the oscillating pressure in the combustion chamber 10. For example, in
a typical combustion system wherein the primary fuel is a lean pre-mix or equivalence
ratio with the ratio of fuel to oxidizer at a fraction, say 0.75, of the ratio required for
stoichiometric burning, the pilot fuel can be injected at an equivalence ratio of about
2.24 to provide fuel-rich pilot flame fronts while still maintaining an overall or combined
fuel-oxidizer ratio as a lean pre-mix with an equivalence ratio of about 0.83.
The duration of each pulses of the pilot flame and the equivalence ratio of the
pilot fuel-oxidizer mixtures have a direct bearing on the restructuring of the main flame
front 20 and is of considerable importance to the present invention in that each pulse
of the pilot fuel and oxidizer mixture into the combustion chamber means must besufficient of a sufficient duration to assure that the discrete pilot flames provide
adequate thermal energy to interact with and effectively restructure the oscillating flame
in the combustion zone for repositioning the oscillating flame. With the pilot flame front
2 1 87255
being pulsed at a rate in the range of about 1-2500 Hz, preferably 10-50 Hz when used
for control of combustion oscillations in the range of about 150-1200 Hz, using pilot
fuel-oxidizer charges with equivalence ratios in the range of about 0.5 to 2.5, preferably
in the range of about 0.6 to 2.0, and with each pulse of the pilot fuel injection timed to
last for a duration of about 0.1 ms to a time corresponding to about one-half the control
frequency period, preferably in the range of about 1.0 ms to about 20 ms, the discrete
pilot flame pulses each exist for a duration sufficient to restructure and move the main
flame front 20 for effectively decoupling the heat pulses from the pressure waves. With
the aforementioned range of control frequencies, the duration of the injection of the
pilot fuel increasing from the lower end region of the above range with the equivalence
ratio of the pilot fuel-oxidizer mixture decreasing from the upper end region of the above
range.
With reference to Figure 5, the present invention is shown in combination with
a combustion chamber 34 provided with a coaxially mounted pre-mix chamber 36 formixing the combustion oxidizer supplied at atmospheric pressure or at an elevated
pressure from any suitable source such as a compressor (not shown) with the main fuel
delivered into the mixing chamber 36 through line 38 and spray ring 39. The
combustion chamber 34 is also provided with a pilot chamber 40 shown concentrically
positioned in the mixing chamber 36 and communicating with one end region of thecombustion chamber 34 as in conventional combustion systems employing pilot
chambers. The exhaust gases resulting from the combustion of the fuel-oxidizer
mixtures are normally discharged from the combustion chamber through an opening at
an end region thereof opposite the fuel-oxidizer mixing chamber 36. Pilot fuel, which
is usually conveyed in an amount of about 2 to 20 percent of the total fuel in aconventional lean mix operation, is introduced through line 42 and pre-mixed with a
portion of the combustion oxidizer to establish a pilot flame generally shown at 44. If
_ 2187255
16
this pilot fuel is delivered into the pilot chamber 40 as a continuous or non-interrupted
stream as in a conventional system or even pulse delivered at frequencies greater than
about one-half the frequency of the main flame combustion oscillations, the combustion
of the main fuel and oxidizer mixture together with the pilot flame mixture will be
5expected to form an unsteady oscillating flame front such as described above in Figures
1 and 2 with the amplitude and pressure of the oscillations being substantially greater
than obtainable when using a lower control frequency and dependent upon the expected
in-phase relationship of the pressure wave with the heat release produced by thecombustion of both of the pilot fuel mixture and the main fuel mixture.
10The selective interruption or pulsing of the injection or introduction of the pilot
fuel into the pilot chamber at the selected control frequency and duration is achieved
by employing a suitable fuel flow control device to effectively move the main flame
front 46 further downstream from the pre-mix chamber 36 to a new position as shown
at 46a that is out of the region where the acoustic coupling occurs. This control over
15the periodic delivery or injection of the pilot fuel is shown being provided by an
electronic solenoid such as generally shown at 48 coupled to a fuel injector or valve 50
in fuel line 42. Any suitable on-off valve such as a rotary, ball, or any type valve
capable of rapid on/off operation is satisfactory for use in the present invention. Also,
if desired a fluidics device may be used to pulse the fuel into the pilot chamber.
20The primary principal associated with the successful control of the combustioninstability is the periodic variation of the equivalence ratio of the pilot fuel and oxidizer
mixture. In some applications a certain level of oscillation within the combustion
chamber may be tolerated or even desirable. Thus, the present invention can be utilized
to reduce and maintain the oscillations at a desired level in a controlled manner.
25With reference to Figure 6, the control over the periodic operation of the pilot
fuel flow control device and the duration of the periodic operation of the pilot fuel
- 21 87255
;
injection may be readily provided by employing an open loop control system as shown
at 51 in combination with the combustion chamber 34 of Figure 5. The open loop
control system 51 is shown comprising the solenoid 48 and injector 50 on the pilot fuel
line 42. Operating signals generated at a frequency and for a duration in the required
ranges described above can be provided in any suitable manner such as by using aconventional variable frequency signal generator shown at 52. These signals are shown
conveyed via line 54 to a solenoid control or drive circuit 56 used to effect the periodic
operation of the solenoid 48 and the associated fuel injector 50. The operation of the
signal generator 52 for pulsing the pilot fuel at the desired frequency and duration can
be achieved manually by a human operator upon noting the presence of the high
pressure oscillations in the combustion chamber through a suitable readout or by using
a simple oscillation detector 58 such as a pressure responsive diaphragm coupled to the
combustion chamber 34 and connected by line 60, which may include suitable
amplifiers, if needed, to an oscillation indicator 62 such as a conventional oscilloscope.
The operator may use the readout from the oscillation indicator 62 for manually
controlling the operation of the variable frequency signal generator 52. Alternatively,
and preferably, the control of the frequency of the pilot fuel pulse can be automatically
achieved by using the signal from the oscillating indicator 62 in a computer generally
shown at 64 for providing through line 66 a logarithm for activating the signal generator
52 at the desired frequency and duration for the injection of the pilot fuel at a
programmed level.
A further embodiment of the present invention is shown in Figure 7. In this
embodiment the selective periodic interruption and formation of the pilot flame pulses
44 is provided by pulsing the flow of the pilot oxidizer rather than the pilot fuel. In this
embodiment a simple on/off valve 68 is shown in the pilot oxidizer delivery line 70 to
provide through the use of a control valve 72 which functions similarly to the pilot fuel
2 1 ~37255
control 26 in Figure 3, the pulsed pilot flame fronts at the frequency and duration
sufficient to achieve essentially the same reduction in the amplitude of the flame front
oscillations in the combustion chamber 34 as achieved by pulsing the pilot fuel. Also,
in this embodiment the pilot fuel can be periodically interrupted along with the pilot
oxidizer or continually delivered to the pilot chamber to provide the desired equivalence
ratio of the oxidizer and fuel in the pilot chamber.
It is to be understood that a successful embodiment of the invention can also bepracticed by using an auxiliary fuel and oxidizer delivery tube that delivers pulsating
charges of the fuel and/or the oxidizer, or premixes thereof directly into to the upstream
end region of the combustion chamber so as to produce a pulsating pilot flame without
the use of a pilot chamber. Generally, it is simply a matter of convenience that the
preferred embodiments of the present invention include the use of a pilot chamber
whereby the pulsed pilot fuel or oxidizer can be delivered as in conventional combustion
systems. Thus, while the description above has been directed to combustion systems
in which the pilot chamber is used in conjunction with the combustion chamber, the
principle of the present invention can be utilized with combustion chambers not fitted
with a pilot chamber simply by diverting a portion of the fuel and oxidizer and pulsing
flame fronts produced by the combustion of this 'pilot' fuel and oxidizer mixture into
the upstream region of the combustion chamber where the pulsating pilot flame fronts
would normally be present. Also, it is expected that essentially all combustion systems
as presently used can be readily adapted to incorporate the combustion oscillating
control of the present invention. It will be seen that the combustor oscillating pressure
stabilizer and method of the present invention provides for the effective and inexpensive
active control capable of reducing dynamic pressure oscillations in combustion systems
to an acceptable level and that essentially all of the equipment utilized in the operation
and practice of the present invention can be mounted sufficiently far away from the
2 1 ~ 72~5
19
high temperature combustion chamber so as to avoid heat damage thereto or the use
of expensive high-temperature equipment.
