U.S. patent application number 15/753117 was filed with the patent office on 2018-07-19 for mitigating instability by actuating the swirler in a combustor.
The applicant listed for this patent is Indian Institute of Science. Invention is credited to Swetaprovo CHAUDHURI, R. GOPAKUMAR, S. MAHESH, Sudeepta MONDAL, R. PAUL.
Application Number | 20180202660 15/753117 |
Document ID | / |
Family ID | 58487211 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180202660 |
Kind Code |
A1 |
CHAUDHURI; Swetaprovo ; et
al. |
July 19, 2018 |
MITIGATING INSTABILITY BY ACTUATING THE SWIRLER IN A COMBUSTOR
Abstract
The present disclosure relates to a method and apparatus to
mitigate thermo-acoustic instabilities in combustor of gas turbine
engines using a lean premixed flame; and provides a dynamic control
strategy for mitigating thermo-acoustic instability in a swirl
stabilized, lean premixed combustor by rotating the otherwise
static swirler meant for stabilizing the lean premixed flame. The
swirler is subjected to a controlled rotation for imparting
increased turbulence intensity and higher tangential momentum to
the premixed reactants towards mitigating thermo-acoustic
instability. The rotating swirler induces vortex breakdown and
increased turbulence intensity to decimate periodic interactions
found during a particular phase of the instability cycle on account
of periodic collision of diverging flame base with the flame
segment above the dump plane in the combustor. This prevents
flame-flame interactions and emergence of strongly positive
Rayleigh indices which contribute as sources of acoustic energy to
drive the self-excited instability.
Inventors: |
CHAUDHURI; Swetaprovo;
(Bangalore, Karnataka, IN) ; GOPAKUMAR; R.;
(Bangalore, Karnataka, IN) ; MONDAL; Sudeepta;
(Bangalore, Karnataka, IN) ; PAUL; R.; (Bangalore,
Karnataka, IN) ; MAHESH; S.; (Bangalore, Karnataka,
IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Indian Institute of Science |
Bangalore, Karnataka |
|
IN |
|
|
Family ID: |
58487211 |
Appl. No.: |
15/753117 |
Filed: |
October 5, 2016 |
PCT Filed: |
October 5, 2016 |
PCT NO: |
PCT/IB2016/055940 |
371 Date: |
February 15, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F23R 3/14 20130101; Y02T
50/675 20130101; F23R 2900/00014 20130101; Y02T 50/60 20130101;
F23R 3/286 20130101 |
International
Class: |
F23R 3/14 20060101
F23R003/14; F23R 3/28 20060101 F23R003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2015 |
IN |
5378/CHE/2015 |
Claims
1. A combustor configured to receive fuel and air, and burn the
fuel-air mixture, the combustor comprising: (a) at least one
swirler in feeding path of the fuel-air mixture and configured to
create turbulence by swirling of the air; and (b) means to rotate
the at least one swirler at different rotational speeds; wherein
rotation of the at least one swirler at different rotational speeds
results in mitigating thermo-acoustic instabilities.
2. The combustor of claim 1, wherein the combustor is configured to
receive air pre-mixed with fuel and burn the fuel-air mixture; and
the at least one swirler is in feeding path of the air pre-mixed
with fuel.
3. The combustor of claim 1, wherein the at least one swirler is a
vane swirler.
4. The combustor of claim 1, wherein the combustor further
comprises means to detect thermo-acoustic instabilities.
5. The combustor of claim 4, wherein the means to detect
thermo-acoustic instabilities includes one or more pressure sensors
for sensing pressure in the combustor and a processor to process
sensed pressure signals to detect occurrences of thermo-acoustic
instabilities.
6. The combustor of claim 5, wherein the combustor further
comprises controller to actuate the means to rotate the at least
one swirler at a desired rotational speed based on detection of
occurrence of thermo-acoustic instability.
7. The combustor of claim 1, wherein the means to rotate the at
least one swirler at different rotational speeds is at least one
motor operatively coupled to the at least one swirler through at
least one shaft.
8. The combustor of claim 7, wherein the at least one motor is a
variable speed motor to facilitate rotation of the at least one
swirler at different rotational speeds.
9. A method for mitigating thermo-acoustic instabilities in a
combustor, the method comprising steps of: (a) providing at least
one swirler in feeding path of fuel-air mixture to stabilize the
premixed flame in the combustor. (b) providing means to rotate the
at least one swirler at different rotational speeds; (c) providing
means to detect the occurrence of thermo-acoustic instability; and
(d) rotating the at least one swirler, on detection of
thermo-acoustic instability, at a desired rotational speed; (e)
wherein rotation of the at least one swirler at different
rotational speeds results in a change in swirl number and change in
turbulence intensity by further swirling the fuel-air mixture
thereby mitigating thermo-acoustic instabilities.
10. The method of claim 9, wherein the air is pre-mixed with fuel.
Description
FIELD OF THE INVENTION
[0001] The present disclosure relates generally to the field of gas
turbines. In particular it relates to a method of reduction of
thermo-acoustic instabilities in a lean premixed combustor of a gas
turbine.
BACKGROUND
[0002] Background description includes information that may be
useful in understanding the present invention. It is not an
admission that any of the information provided herein is prior art
or relevant to the presently claimed invention, or that any
publication specifically or implicitly referenced is prior art.
[0003] Enhanced periodic pressure fluctuations that result from a
positive feedback between the unsteady heat release rate and the
acoustic pressure fluctuations in a confined combustion chamber is
referred to as thermo-acoustic instability. In case of combustion
chambers of gas turbines, high oscillation amplitudes can occur
which lead to undesirable effects, such as, for example, a high
mechanical/thermal load on the combustion chamber, flashback or the
flame being extinguished. For this reason they have been a matter
of prime concern among combustion researchers over decades.
[0004] Specifically, lean premixed gas turbine combustor operation
is frequently plagued by intense thermo-acoustic oscillation at
nearly discrete, narrow band frequencies leading to structural
damage and possible failure of the engine parts. This is the major
obstacle in using lean premixed combustion technology in gas
turbine combustors despite its other remarkable benefits in terms
of reduced pollutant emission, negligible soot production to name a
few.
[0005] Systematic mitigation of thermo-acoustic instability is thus
an important challenge. Thermo-acoustic instabilities in gas
turbine engines can be suppressed in theory by disrupting the
feedback loop between the unsteady heat release rate and acoustic
pressure oscillations, but is yet to be satisfactorily addressed at
either the design or working stages of an engine.
[0006] In practical combustors, two conceptually different
approaches, namely passive and active control, are utilized for its
mitigation. Passive control strategy involves permanent
modification in the fuel injection system, alteration in combustor
geometry and/or addition of acoustic dampeners (Helmholtz
resonators, quarter wave tubes, liners etc.) for suppressing these
oscillations. However, instability mitigation using mechanisms like
Helmholtz resonators are difficult to realize in realistic
combustor systems mainly due to space and power constraints.
Liquid-fuelled rockets and gas turbine combustors are often
equipped with acoustic dampers like Helmholtz resonators,
perforated liners, quarter and half-wave tubes and baffles; but the
addition of such components affects the combustor performance,
weight and heat loads.
[0007] On the other hand, active control methodology employs an
externally excited auxiliary system for interrupting the heat
release rate-acoustic pressure fluctuation feedback loop within the
combustor. This control strategy typically involves real-time
monitoring of chamber pressure oscillations by a sensor coupled
with feedback control and actuator system for suppressing the
thermo-acoustic instability.
[0008] Compared to the passive strategy, active control approach
offers robust and better mitigating capability of thermo-acoustic
instability over a wider operating range. Hence, development of new
active control strategies is pursued by various combustion research
groups.
[0009] Different active control strategies such as acoustic
excitation of reactants, micro-jet injection, variable angle
swirler are known to have been utilized for mitigating
thermo-acoustic instability in lab scale combustors by various
researchers. Paschereit et al. as disclosed in their paper
"Coherent structures in swirling flows and their role in acoustic
combustion control" (published in Physics of Fluids (1994-present),
1999. 11(9): p. 2667-2678) utilized acoustic excitation of air
supply in a swirl stabilized flame combustor to alter the shear
layer dynamics and suppress the unsteady pressure oscillations.
Subsequently, Uhm and Acharya in their paper "Control of combustion
instability with a high-momentum air jet." (published in Combustion
and flame2004.139(1): p. 106-125) modulated the high momentum air
jet in a swirl stabilized spray combustor using proportional drive
valve for mitigating thermo-acoustic instability. Altay et al. in
their paper "Mitigation of thermo-acoustic instability utilizing
steady air injection near the flame anchoring zone" (published in
Combustion and Flame, 2010. 157(4): p. 686-700) injected micro-jets
close to the dump plane of a lean premixed swirl combustor for
suppressing thermo-acoustic instability. Kim et al. in their paper
"Plasma assisted combustor dynamics control" (published in
Proceedings of the Combustion Institute, 2015. 35(3): p. 3479-3486)
explored possibility of thermo-acoustic mitigation of swirl
stabilized combustor using Nano-Second Pulsed Plasma Discharge
(NSPD).
[0010] Durox et al. in their paper "Flame dynamics of a variable
swirl number system and instability control" (published in
Combustion and Flame, 2013. 160(9): p. 1729-1742) disclosed design
and testing of a variable angle swirler system for varying the
swirl number in order to suppress thermo-acoustic instability in a
lean premixed swirl combustor. Their results how that with an
increase in the blade angle in a variable blade swirler, the swirl
number does not increase rapidly as compared to the theoretical
prediction, with the difference being more than a factor of 2 at
higher blade angles.
[0011] It is important to note that the swirler with movable blades
requires sophisticated and complex linkage mechanism for changing
the blade angle. Furthermore, increasing blade angle for achieving
higher swirl number introduces blockage in the flow path resulting
in enhanced pressure drop. Therefore, a variable blade angle
swirler offers limited change in swirl number at the cost of
pressure drop.
[0012] Control techniques which utilize secondary/pilot fuel or air
for suppressing thermo-acoustic instability modifies flow rate
and/or the equivalence ratio within the combustor which could in
turn unwantedly impact the heat release rate, flame stability,
engine performance and exhaust emission levels.
[0013] To date, in all modern gas turbine combustors, flames are
stabilized aerodynamically by static vane swirlers. In all these
combustors, all solid wall (liners with cooling holes) boundaries
enclosing the combustion process are almost static. This prevents
any external actuation, active control or real time modification of
the flow and flame pattern inside the combustor.
[0014] There is, therefore, a need in the art for a method for
mitigating the problem of narrow band, low frequency
thermo-acoustic oscillation in gas turbine combustors for which
there is no systematic fool proof solution.
[0015] All publications herein are incorporated by reference to the
same extent as if each individual publication or patent application
were specifically and individually indicated to be incorporated by
reference. Where a definition or use of a term in an incorporated
reference is inconsistent or contrary to the definition of that
term provided herein, the definition of that term provided herein
applies and the definition of that term in the reference does not
apply.
[0016] In some embodiments, the numbers expressing quantities of
ingredients, properties such as concentration, reaction conditions,
and so forth, used to describe and claim certain embodiments of the
invention are to be understood as being modified in some instances
by the term "about." Accordingly, in some embodiments, the
numerical parameters set forth in the written description and
attached claims are approximations that can vary depending upon the
desired properties sought to be obtained by a particular
embodiment. In some embodiments, the numerical parameters should be
construed in light of the number of reported significant digits and
by applying ordinary rounding techniques. Notwithstanding that the
numerical ranges and parameters setting forth the broad scope of
some embodiments of the invention are approximations, the numerical
values set forth in the specific examples are reported as precisely
as practicable. The numerical values presented in some embodiments
of the invention may contain certain errors necessarily resulting
from the standard deviation found in their respective testing
measurements.
[0017] As used in the description herein and throughout the claims
that follow, the meaning of "a," "an," and "the" includes plural
reference unless the context clearly dictates otherwise. Also, as
used in the description herein, the meaning of "in" includes "in"
and "on" unless the context clearly dictates otherwise.
[0018] The recitation of ranges of values herein is merely intended
to serve as a shorthand method of referring individually to each
separate value falling within the range. Unless otherwise indicated
herein, each individual value is incorporated into the
specification as if it were individually recited herein. All
methods described herein can be performed in any suitable order
unless otherwise indicated herein or otherwise clearly contradicted
by context. The use of "any" and "all examples", or exemplary
language (e.g. "such as") provided with respect to certain
embodiments herein is intended merely to better illuminate the
invention and does not pose a limitation on the scope of the
invention otherwise claimed. No language in the specification
should be construed as indicating any non-claimed element essential
to the practice of the invention.
[0019] Groupings of alternative elements or embodiments of the
invention disclosed herein are not to be construed as limitations.
Each group member can be referred to and claimed individually or in
any combination with other members of the group or other elements
found herein. One or more members of a group can be included in, or
deleted from, a group for reasons of convenience and/or
patentability. When any such inclusion or deletion occurs, the
specification is herein deemed to contain the group as modified
thus fulfilling the written description of all Markush groups used
in the appended claims.
OBJECTS OF THE INVENTION
[0020] A general objective of the present disclosure is to enable
instability-free implementation of lean premixed combustion
technology in gas turbine combustors.
[0021] An object of the present disclosure is to mitigate problem
of narrow band, low frequency thermo-acoustic oscillation in gas
turbine combustors so that the lean premixed combustion technology
may be successfully implemented in gas turbine combustors.
[0022] An object of the present disclosure is to suppress
thermo-acoustic instability without any addition or detraction in
the combustor inlet air or fuel flow rates.
[0023] An object of the present disclosure is to provide a method
for suppressing thermo-acoustic instability that does not
incorporate an automated feedback loop.
[0024] Another object of the present disclosure is to dynamically
control flow and flame structure towards achieving an
instability-free combustor.
[0025] Yet another object of the present disclosure is to provide
an externally actuated dynamic control for real time modification
of the flow and flame pattern inside the combustor for mitigating
thermo-acoustic instability.
SUMMARY
[0026] Aspects of the present disclosure relate to reduction of
thermo-acoustic instabilities in combustor of a gas turbine. In
particular, it discloses a method and apparatus to mitigate problem
of narrow band, low frequency thermo-acoustic oscillation in gas
turbine combustors so as to stabilize a lean premixed flame.
[0027] In an aspect, the disclosure provides a dynamic control
strategy for mitigating thermo-acoustic instability in a swirl
stabilized, lean premixed combustor by rotating the otherwise
static swirler that is primarily meant for stabilizing the lean
premixed flame. The otherwise static swirler is subjected to a
controlled rotation for imparting increased turbulence intensity
and higher tangential momentum to the premixed reactants towards
mitigating thermo-acoustic instability. Thus the disclosed control
technique does not suffer from unwanted impacts of the heat release
rate, flame stability, engine performance and exhaust emission
levels faced with other known control techniques which utilize
secondary/pilot fuel or air modifying the equivalence ratio within
the combustor for suppressing thermo-acoustic instability.
[0028] In an aspect, the rotating swirler induces vortex breakdown
and increased turbulence intensity to decimate periodic
interactions found during a particular phase of the instability
cycle on account of periodic collision of diverging flame base with
the flame segment above the dump plane in the combustor. Thus the
rotating swirler is able to avoid flame-flame interactions. The
flame-flame interactions result in emergence of strongly positive
Rayleigh indices contributing as sources of acoustic energy to
drive the self-excited instability. The rotating swirler can,
therefore, decimate the acoustic energy source, to render quiet,
instability mitigated swirling flames.
[0029] In an aspect, the disclosed concept has been tested over a
wide range of lean equivalence ratios, bulk flow velocities and
swirler rotation rates with advanced measurement and diagnostic
techniques for validating its robustness. Prominent reduction of
the fundamental acoustic mode amplitude: by about 25 dB is observed
with this control technique for the cases that were studied.
Furthermore, understanding of the physical mechanism responsible
for dynamic instability mitigation by the disclosed strategy has
been established through investigation of the transient flame
dynamics and the reacting flow field. The distinct changes
associated with the reacting flow field were observed using
Particle Image Velocimetry (PIV). An attempt has been made to probe
into the self-excited flame dynamics using high speed, intensified,
chemiluminescence imaging and identify the instability driving,
source locations from spatial Rayleigh indices map.
[0030] In an aspect, the disclosure provides a combustor to
mitigate thermo-acoustic instability by a dynamic control strategy.
The disclosed combustor can be a swirl stabilized, lean premixed
combustor incorporating a vane swirler that is primarily meant for
stabilizing the lean premixed flame. The swirler can be an axial
swirler mounted on a shaft-bearing-coupling-motor arrangement
configured to facilitate its rotation at different rpms. The
combustor can further incorporate means for dynamic control of flow
and flame structure towards achieving an instability free
combustor.
[0031] In an aspect, the disclosed concept can be implemented along
with a closed loop control mechanism based on detecting unstable
flame condition using sensors, computing fastest route to achieve
stable condition and actuating means to rotate swirler at desired
speed. In an aspect, a combustor equipped with a swirler and closed
loop mechanism to control the speed of swirler can serve as a smart
combustor that can automatically maintain stable flame condition
free from thermo-acoustic instability.
[0032] Various objects, features, aspects and advantages of the
inventive subject matter will become more apparent from the
following detailed description of preferred embodiments, along with
the accompanying drawing figures in which like numerals represent
like components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The accompanying drawings are included to provide further
understanding of the present disclosure, and are incorporated in
and constitute a part of this specification. The drawings
illustrate exemplary embodiments of the present disclosure and,
together with the description, serve to explain the principles of
the present disclosure.
[0034] FIG. 1 illustrates an exemplary perspective view of
experimental setup and diagnostic tools in accordance with
embodiments of the present disclosure.
[0035] FIGS. 2A, 2B and 2C illustrate exemplary images of the
experimental setup of FIG. 1 along with close-up view of the lower
section comprising of the main combustor and images of flame at
different rotational speeds of the swirler in accordance with
embodiments of the present disclosure.
[0036] FIGS. 3A and 3B illustrate exemplary plots of Fast Fourier
Transform (FFT) of the unsteady pressure data for one of the
experimental conditions showing three distinct modes under
conditions of stationary swirler and the rotating swirler
respectively in accordance with embodiments of the present
disclosure.
[0037] FIGS. 4A, 4B and 4C illustrate exemplary plots of variation
of the three mode amplitudes for different swirler rotational
speeds for three different experimental conditions in accordance
with embodiments of the present disclosure.
[0038] FIGS. 5A, 5B and 5C illustrate exemplary plots of variation
of frequencies of different FFT modes for three different
experimental conditions in accordance with embodiments of the
present disclosure.
[0039] FIG. 6 illustrates variation of mean chemiluminescence
intensity with different swirler rotation rates for three different
experimental conditions in accordance with embodiments of the
present disclosure.
[0040] FIGS. 7A, 7B, 7C and 7D illustrate exemplary images of mean
z-vorticity field superimposed with streamlines; and RMS of
fluctuating velocity field without and with swirler rotation for
one of the experimental conditions in accordance with embodiments
of the present disclosure.
[0041] FIGS. 8A to 8F illustrate exemplary velocity and vorticity
profiles at two different axial locations above the dump plane with
and without swirler rotation in accordance with embodiments of the
present disclosure.
[0042] FIGS. 9A and 9B illustrate exemplary flame image sequences
for complete instability cycles without and with swirler rotation
for one of the experimental conditions in accordance with
embodiments of the present disclosure.
[0043] FIGS. 10A and 10B illustrate exemplary phase averaged flame
images without and with swirler rotation for one of the
experimental conditions in accordance with embodiments of the
present disclosure.
[0044] FIG. 10C illustrates an exemplary plot of diverging flame
base angle at different phases of instability cyclein accordance
with embodiments of the present disclosure.
[0045] FIGS. 11A and 11B illustrate comparison of spatial Rayleigh
Index (RI)without and with swirler rotation respectively for one of
the experimental conditions in accordance with embodiments of the
present disclosure.
[0046] FIG. 12 illustrates an exemplary instantaneous flame image
sequence of thermo-acoustically unstable flame for one of the
experimental conditions showing the interaction of diverging flame
base with the flame segment above dump plane in accordance with
embodiments of the present disclosure.
[0047] FIG. 13 illustrates an exemplary instantaneous flame image
sequence corresponding to the instability-free flame without
Aluminium tube in accordance with embodiments of the present
disclosure.
[0048] FIG. 14 illustrates an exemplary block diagram indicating a
closed loop feedback mechanism for maintaining thermo-acoustic
instability-free flame condition in a smart combustor in accordance
with embodiments of the present disclosure.
DETAILED DESCRIPTION
[0049] The following is a detailed description of embodiments of
the disclosure depicted in the accompanying drawings. The
embodiments are in such detail as to clearly communicate the
disclosure. However, the amount of detail offered is not intended
to limit the anticipated variations of embodiments; on the
contrary, the intention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the present
disclosure as defined by the appended claims.
[0050] Each of the appended claims defines a separate invention,
which for infringement purposes is recognized as including
equivalents to the various elements or limitations specified in the
claims. Depending on the context, all references below to the
"invention" may in some cases refer to certain specific embodiments
only. In other cases, it will be recognized that references to the
"invention" will refer to subject matter recited in one or more,
but not necessarily all, of the claims.
[0051] Various terms as used herein are shown below. To the extent
a term used in a claim is not defined below, it should be given the
broadest definition persons in the pertinent art have given that
term as reflected in printed publications and issued patents at the
time of filing.
[0052] The present disclosure relates to reduction of
thermo-acoustic instabilities in combustor of a gas turbine. In
particular, it discloses a method and apparatus to mitigate problem
of narrow band, low frequency thermo-acoustic oscillation in gas
turbine combustors so as to stabilize a lean premixed flame.
[0053] In an aspect, the disclosure provides a dynamic control
strategy for mitigating thermo-acoustic instability in a swirl
stabilized, lean premixed combustor by rotating the otherwise
static swirler that is primarily meant for stabilizing the lean
premixed flame. The otherwise static swirler is subjected to a
controlled rotation for imparting increased turbulence intensity
and higher tangential momentum to the premixed reactants towards
mitigating thermo-acoustic instability. Thus the disclosed control
technique does not suffer from unwanted impacts of the heat release
rate, flame stability, engine performance and exhaust emission
levels faced with other known control techniques which utilize
secondary/pilot fuel or air modifying the equivalence ratio within
the combustor for suppressing thermo-acoustic instability.
[0054] Thermo-acoustic instability is a complex phenomenon
influenced by several parameters including but not limited to swirl
number, combustor acoustics, damping, mode of combustion, inlet
velocity and density profiles, etc. An aero gas turbine engine
operates over a wide range of the above parameters and during
flight, these parameters could rapidly change and interact to
render a particular operating condition stable or
thermo-acoustically unstable. Hence instability-free stable regime
for one particular flow or equivalence ratio may turn out to be
highly unstable for a different flow condition. For e.g., in a high
swirl number reacting flow, a prominent Processing Vortex Core
(PVC) could be the source of instability. In such a scenario, it is
possible that reducing swirl number might assist in the instability
mitigation which could also be easily achieved by rotating the
static swirler in the opposite direction. Thus clearly, the
flexibility to dynamically change the swirl number and inlet
turbulence intensity that renders the rotating (spinning) swirler,
a superior flame stabilization mechanism when compared to a static
swirler.
[0055] In the present disclosure, a premixed flame is being
stabilized downstream of high speed, rotating swirler blades, where
the turbulence intensity could be varied and controlled. Increasing
swirler rotation rate would lead to controlled increase of
turbulent flame speed. With an increase in the turbulent flame
speed, the flame stabilization location could be changed on
average, accompanied by change in the flame structure. It is known
that flame shape and location could be primary drivers that
determine whether a given flame-combustor configuration would be
thermo-acoustically unstable or not. Accordingly, the present
disclosure provides a method and apparatus for changing the flame
location by controlling the swirler rotational frequency and using
it as a thermo-acoustic control strategy in a swirl stabilized
flame configuration.
[0056] In an aspect, the present disclosure focuses on the effect
of an actuated swirler on thermo-acoustics in an otherwise
thermo-acoustically unstable combustor as against changing the
flame dynamics with static swirlers.
[0057] Take off point for the current invention is variable angle
swirler system designed and tested by Durox et al. (mentioned in
the background section) for varying the swirl number in order to
suppress thermo-acoustic instability in a lean premixed swirl
combustor. As it is shown by Durox et al. that with an increase in
the blade angle in a variable blade swirler, the swirl number does
not increase rapidly as compared to the theoretical prediction,
with the difference being more than a factor of 2 at higher blade
angles. Moreover, a variable blade angle swirler offers limited
change in swirl number at the cost of pressure drop. Besides this,
swirler with movable blades requires sophisticated and complex
linkage mechanism for changing the blade angles. Furthermore,
increasing blade angle for achieving higher swirl number introduces
blockage in the flow path resulting in enhanced pressure drop. The
present disclosure aims to overcome disadvantage of an enhanced
pressure drop and limited variation in swirl number in the case of
variable blade angle swirler by rotating the static vane swirler
thereby providing the additional work input to the flow by
controlling the motor rpm. High rotation rates and effective design
of the swirler blades could even lead to a net pressure gain in the
combustor with an actuated swirler.
[0058] In an aspect, the rotating swirler induces vortex breakdown
and increased turbulence intensity to decimate periodic
interactions found during a particular phase of the instability
cycle on account of periodic collision of diverging flame base with
the flame segment above the dump plane in the combustor. Thus the
rotating swirler is able to avoid flame-flame interaction locations
which result in emergence of strongly positive Rayleigh indices
contributing as sources of acoustic energy to drive the
self-excited instability. The rotating swirler can, therefore,
decimate the acoustic energy source, to render quiet, instability
mitigated swirling flames.
[0059] In an aspect, the disclosed concept has been tested over a
wide range of lean equivalence ratios, bulk flow velocities and
swirler rotation rates with advanced measurement and diagnostic
techniques for validating its robustness. Prominent reduction of
the fundamental acoustic mode amplitude: by about 25 dB is observed
with this control technique for the cases that were studied.
Furthermore, understanding of the physical mechanism responsible
for dynamic instability mitigation by the disclosed strategy has
been established through investigation of the transient flame
dynamics and the reacting flow field. The distinct changes
associated with the reacting flow field were observed using
Particle Image Velocimetry (PIV). An attempt has been made to probe
into the self-excited flame dynamics using high speed, intensified,
chemiluminescence imaging and identify the instability driving,
source locations from spatial Rayleigh indices map.
[0060] FIG. 1 illustrates an exemplary perspective view 100 of
experimental setup incorporating features of the disclosed
combustor along with diagnostic tools to validate the proposed
concept in accordance with embodiments of the present disclosure.
The combustor can incorporate a swirler 102 mounted on a
shaft-bearing-coupling-motor arrangement for facilitating its
rotation at different speeds. The shaft-bearing arrangement 104 can
be housed within a stainless steel circular pipe 116 of suitable
inner diameter and height. In the exemplary embodiment shown in
FIG. 1 the swirler diameter is 30 mm (with a hub diameter of 10
mm). The stainless steel circular pipe 116 housing the
shaft-bearing arrangement 104 has inner diameter 31 mm and height
100 mm.
[0061] The swirler 102 can be recessed from the exit of the pipe
116 by a distance of 3 mm. One end of the shaft 106 of the
shaft-bearing arrangement 104 can be attached to the swirler 102
and its other end can be connected to a motor such as stepper motor
110 through a coupling such as love joy coupling 108. The stepper
motor 110 can have adequate power/torque rating depending on size
of the swirler 102. In the exemplary set up shown in FIG. 1 the
stepper motor 110 has torque rating of 1.7 kgf-cm. The stepper
motor 110 can be powered by a transformer 164 and its rotational
speed can be controlled by stepper driver circuit which in the
exemplary set-up uses sine pulses from a waveform generator
integrated with oscilloscope 162 (Agilent InfiniiVision
DSO-X-2002A). Air and Methane in appropriate ratio can enter into a
perforated plate fitted plenum chamber where they are premixed. Air
and methane can be separately metered by mass flow controllers. In
the exemplary set-up of FIG. 1 air flow supplied by an external
screw compressor and Methane (99% purity) is controlled by
calibrated AlicatMass Flow Controllers (Air-0-500 SLPM range;
Methane 0-50 SLPM range). Uncertainty in mass flow controllers
(MFC) is given by .+-.0.8% of displayed reading in the MFC+0.2% of
full scale range of the MFC.
[0062] The air-fuel mixture can be delivered into the combustor
through four inlets 112 placed at the bottom portion of the
stainless steel circular pipe 116. The swirl stabilized premixed
flame is confined within a tube such as cylindrical quartz tube 114
in FIG. 1 having inner diameter 46 mm and height 60 mm. Besides
this, another tube such as aluminium tube 152 having inner diameter
44 mm and length 2 m can be mounted above the quartz test section
to generate the self-excited thermo-acoustic instability. The
cylindrical quartz tube 114 and aluminium tube 152 can be connected
by a threaded cap(not shown here)which has a provision for igniting
the reactants.
[0063] Transient pressure data from the combustor in the
experimental set up of FIG. 1 can be acquired at a rate of 10 kS/s
using a pressure sensor such as Kistler piezo-resistive pressure
sensor 160 (Model no: 4260A045BIBA07D1; range: 0-3 bar absolute
pressure) placed just upstream of the swirler 102. The uncertainty
in the pressure sensor 160 used in the set-up is .+-.0.2% of full
scale range. The unsteady pressure time series can be monitored and
recorded for a span of 3 sec. through an oscilloscope 162.
[0064] In order to capture data to validate the disclosed concept,
a camera such as high speed camera 156 (Photron FASTCAM SA5)
equipped with intensified relay optics-IRO (LaVision) and UV lens
is utilized for observing the line of sight integrated, unfiltered,
flame chemiluminescence dynamics associated with the
thermo-acoustic instability and its transition with swirler
rotation. The flame videos are captured for 2 seconds at 5000 fps
with a resolution of 1024.times.1024 pixel.sup.2. Besides this,
distinct changes in the reacting flow field, with and without
swirler 102 rotation are analysed quantitatively using PIV. The PIV
measurements can be performed at the symmetry x-y plane of quartz
tube shown in FIG. 1 which is illuminated by a 532 nm Nd:YAG laser
158 (Litron Nano SPIV) sheet of sub-millimeter (.about.0.5 mm)
thickness. The reacting flow field is seeded uniformly by 1-5
micron sized alumina particles. The PIV images are captured using
high resolution CCD camera 154 (LaVision Imager Intense) at a frame
rate of 4 Hz and processed using DAVIS 8 software. The swirler
rotation rates are varied from 0 to 1800 rpm in steps of 200 for
all three cases.
[0065] FIGS. 2A, 2B and 2C illustrate exemplary images 200, 225 and
250 respectively of the experimental setup of FIG. 1 along with
zoomed view (in image 225) of the lower section comprising of the
main combustor 202, and close up images 250 of the flame in the
combustion chamber at different swirler speed in accordance with
embodiments of the present disclosure. The 2 m long Aluminium pipe
152 shown in the image 200 has been used as a duct for exciting for
generating self-excited thermo-acoustic instability. Image 225
shows the combustor comprising the quartz test section (tube) 114,
fuel-air mixture inlets 112, stepper motor 110 and love joy
coupling 108 etc. The image 250 shows an exemplary flame emission
images when the swirler 102 is stationary i.e. at 0 rpm and
rotating at three different rpms.
[0066] In an embodiment, the 2 m long Aluminium pipe 152 has been
used as a duct for exciting narrow band thermo-acoustical
longitudinal modes at about 75 Hz. Rotation of swirler within
1350-1800 rpm could significantly mitigate these excited
thermo-acoustic longitudinal modes over different bulk flow
Reynolds number (.about.4000-10000) and equivalence ratios
(0.676-0.732). In another embodiment, test was carried out in the
above described set-up on a model combustor that stabilizes a
premixed flame using 30 degree vane angle swirlers. When the
swirler 102 was static, the flame could thermo-acoustically
self-excite at a frequency of around 75 Hz due to 2 m long tube
downstream of the quartz test section.
[0067] It was found, both from pressure transducer signals and
phase averaged flame images obtained by image processing of high
speed flame chemiluminescence videos, that on rotating swirler 102
within the range of 1200-1800 rpm, the amplitude of the 75 Hz mode
and large scale longitudinal motion of the flame and flame base
angle were greatly diminished. The mitigation of the dominant
instability mode is shown in FIG. 3. This clearly suggests
successful mitigation of the self-excited thermo-acoustic
instability using rotation of swirler. Results of the experiments
carried out with the experimental set-up of FIGS. 1 and 2A under
different operating conditions i.e. bulk flow conditions have been
discussed further in succeeding paragraphs.
[0068] The disclosed method highlights utilization of
rotation/actuation of swirlers in combustors that stabilize
flame(s) in propulsion or power generation gas turbine engines
towards i) instability mitigation ii) dynamic and systematic
augmentation or reduction of swirl number iii) enhanced turbulence
iv) better atomization of liquid sprays v) better mixing of fuel
and air vi) easier implementation in practical combustors with
relatively easier mechanical arrangement, among other possible uses
in this field to a person having ordinary skill in the art.
[0069] In an embodiment, in order to establish efficacy of the
disclosed concept, above described experimental setup was used to
carry our experiments under bulk flow conditions corresponding to
three different experimental cases are presented in Table 1
below:
TABLE-US-00001 TABLE 1 Experimental Conditions Bulk Reynolds
Equivalence Swirler Experimental number Ratio rotation rates
Conditions Case (Re) (.PHI.) (rpm) I 4085 0.676 0-1800 in steps of
200 II 7274 0.705 0-1800 in steps of 200 III 10269 0.732 0-1800 in
steps of 200
[0070] As shown in the table 1 above, swirler rotational speed was
varied from 0 to 1800 rpm in steps of 200 for all three cases
namely Case I, II and III. For Case I, Bulk Reynolds number (Re) is
4085 and equivalence ratio (.PHI.) is 0.676. Likewise for Case II,
Bulk Reynolds number (Re)is 7274 and equivalence ratio (.PHI.) is
0.705. And for Case III, Bulk Reynolds number (Re) is 10269 and
equivalence ratio (.PHI.) is 0.732.
A. Pressure Measurements
[0071] In an embodiment, the experiments were used to
quantitatively establish efficacy of the rotating swirler in
mitigating thermo-acoustic instability from the pressure amplitudes
at discrete frequencies. Initially, when the swirler 102 was
stationary (i.e. 0 rpm), three distinct modes were observed from
the Fast Fourier Transform (FFT) of the unsteady pressure data
corresponding to case II, as shown in graph 300 of FIG. 3. Here the
first longitudinal mode exhibits a sharp distinct peak at 76 Hz. In
addition, the second and third modes occur at 206 and 342 Hz
respectively with much weaker amplitudes. Plot 350 of FIG. 3 shows
the FFT of unsteady pressure data when the swirler 102 is rotated
at 1800 rpm where significant attenuation in the amplitude of the
first mode is evident.
[0072] FIGS. 4A, 4B and 4C illustrate exemplary plots 400, 430 and
460 respectively depicting the variation of three mode amplitudes
for different swirler speeds corresponding to three cases in
accordance with embodiments of the present disclosure. It can be
seen from the plot 400 where variation in the ensemble averaged
first mode amplitude (in dBSPL) with swirler rotation rate is
shown, that for all the three cases, with an increase in swirler
rpm, the amplitude of the first mode decreases monotonically from
about 150 dB observed with stationary swirler. A reduction in the
first mode amplitude by about 25 dB is observed with a maximum
swirler rotation rate of 1800 rpm for these cases which explicitly
demonstrates effectiveness of the proposed mitigating strategy.
[0073] In an aspect, electrical power expended in rotating the
swirler at 1800 rpm is less than 1% of the thermal power output
from the combustor, and therefore, the proposed strategy does not
result in any significant drain on the power output of the turbine
where the combustor may be used, thereby establishing basic
feasibility of the proposed dynamic control concept.
[0074] The influence of swirler rotation rate on the second and
third mode amplitudes of pressure data is shown in plots 430 and
460 respectively. The plot 430 indicates that the second mode
amplitude exhibits a slightly increasing trend beyond 900 rpm for
the three cases. On the other hand, as evident from plot 460, third
mode amplitude does not reveal any prominent trend with swirler
rotation rate for these cases. However, on comparing plot 430 and
460 with plot 400, it is clearly evident that the amplitude of
second and third acoustic modes are comparable with that of the
first mode at higher swirler rotation rates where the mitigation of
the first mode is significant. To summarize, an increase in the
swirler rotation rate does not energize the amplitudes of second
and third acoustic modes appreciably, though mitigation of the
first dominant mode is significant.
[0075] FIGS. 5A, 5B and 5C illustrate exemplary plots 500, 530 and
560 respectively of variation of frequencies of different FFT modes
for the three different experimental conditions in accordance with
embodiments of the present disclosure. The plot 500 shows effect of
swirler speed on frequency of the first mode and it can be noticed
that the first mode frequency increases with swirler rotation rate.
An increase in the swirler rpm enhances the turbulence intensity at
the burner exit close to the dump plane. This observation is also
confirmed by the PIV measurements which will be discussed in
subsequent paragraphs. Increased turbulence intensity can augment
turbulent flame speed through increased flame surface area,
especially near the wall boundary layers. On the other hand, it can
be noticed from plots 530 and 560 that the frequencies of second
and third acoustic modes exhibit negligible variation with swirler
rotation rate for the three cases.
B. Flow Field Measurements with PIV
[0076] The investigation of reacting flow velocity field with
swirler actuation is expected to shed more light into the
underlying physical mechanism responsible for the mitigation of
thermo-acoustic instability by the proposed concept. In this
regard, vorticity .omega..sub.z superimposed with streamlines and
u'.sub.rms, associated with the reacting flow field are analyzed in
detail. The statistics of the velocity field are realized from an
ensemble average of 200 individual PIV scans along the symmetry
(x-y) plane of the quartz section spanning the flame region.
[0077] The mean z-vorticity field of the reacting flow superimposed
with streamlines is shown in FIG. 7A for Case II without swirler
rotation. The streamlines exhibit an elongated vortex tube residing
above the dump plane in the Outer Recirculation Zone (ORZ).
Furthermore, the Inner Recirculation Zone (IRZ) has not evolved for
this case owing to a low geometric swirl number of about 0.4.
Interestingly in FIG. 7B, when the swirler is rotated at 1800 rpm,
the formation of IRZ with two counter-rotating vortices, as in a
Vortex Breakdown Bubble (VBB) can be observed from the streamlines
close to the burner exit. The swirler rotation increases the swirl
number thereby facilitating vortex breakdown leading to the
formation of a coherent IRZ. Furthermore, size of corner vortices
and its vorticity strength in the ORZ diminishes with swirler
rotation, which can be observed from the comparison of FIG. 7A and
FIG. 7B. In addition to this, enhanced turbulence intensity with
swirler rotation is also evident close to the ORZ and shear layers.
This can be noted from the comparison of FIG. 7C and FIG. 7D.
[0078] A comparison of y-component of mean velocity (U.sub.y),
u'.sub.rms, and .omega..sub.z profiles at the axial locations of 1
mm and 3 mm above the dump plane are shown in FIGS. 8A to 8F for
the swirler rotation rates of 0 and 1800 rpm. From FIGS. 8A and 8B,
it can be observed that U.sub.y slightly reduces close to the dump
plane region with the actuation of swirler at 1800 rpm. However, in
contrast to the static swirler case, U.sub.y is enhanced close to
the IRZ with swirler actuation due to the formation of vortex
bubble breakdown. The swirler rotation also enforces a pronounced
enhancement in u'.sub.rms near the ORZ and in the shear layer as
compared to the static swirler case (see FIGS. 8C and 8D). In
addition, an overall reduction in corresponding values near the
ORZ, with swirler actuation, can be noted from FIGS. 8E and 8F.
C. High Speed Chemiluminescence Imaging
[0079] The instantaneous flame image sequences for complete
instability cycles, corresponding to Case II for the static and
actuated swirler (0 and 1800 rpm respectively), are shown in FIGS.
9A and 9B respectively. The sinusoidal variation in the mean
spatial heat release rate during one instability cycle can be
observed clearly from FIG. 9A. Large scale longitudinal motion of
the self-excited flame is also evident without swirler rotation.
FIG. 9B depicts the instantaneous flame image sequence during one
dampened instability cycle with the swirler rotation rate of 1800
rpm. Here, an increase in mean spatial heat release rate and
corresponding reduction in its peak to peak amplitude during one
cycle can be noticed with swirler rotation as compared to the
static swirler.
[0080] To better observe the statistical nature of the self-excited
flame dynamics, the phase averaged images at 0.degree., 90.degree.,
180.degree., 270.degree. corresponding to Case II, without swirler
rotation is depicted in FIG. 10A. The phase averaging is performed
over 79 cycles. A periodic change in the spatial chemiluminescence
intensity along with fluctuation (expansion and contraction) in
diverging flame base angle at different phases can be observed for
the static swirler case from FIGS. 10A and 10B. The periodic
fluctuation in the diverging flame base angle can be attributed to
the instability in the absence of vortex bubble breakdown and IRZ,
which is evident from FIG. 7A.
[0081] The phase averaged flame chemiluminescence images at
(0.degree., 90.degree., 180.degree., 270.degree.) with swirler
rotation rate of 1800 rpm are shown in FIG. 10B. Here, the phase
averaging is performed over 80 cycles. With swirler rotation,
increase in the diverging flame base angle can be observed. Earlier
reported LES numerical study by Stone and Menon (Proceedings of the
Combustion Institute, Vol. 29, No. 1, 2002, pp. 155-160) made
similar observation on the flame shape with an increase in swirl
number in a dump combustor configuration. Besides this, fluctuation
in the diverging flame base angle, over different phases, is
however diminished which can be confirmed from FIGS. 10B and 10C.
Increased diverging flame base angle is attributed to VBB and
formation of coherent IRZ due to an increased swirl number, as
confirmed earlier by PIV measurements in FIG. 7B. Significant
reduction in the flame heat release rate unsteadiness can also be
inferred indirectly from the spatial chemiluminescence intensity of
the phase averaged images in FIG. 10B as compared to 10A. The phase
averaged images clearly reveal significant suppression in the
periodic longitudinal motion of the flame with swirler rotation,
indicating a prominent mitigation in the thermo-acoustic
instability. This can be attributed to the overall change in the
mean flame topology with swirler rotation due to the formation of
stable IRZ and enhanced turbulence intensity in the flame
stabilization region.
D. Driving Regions of Thermo-acoustic Instability--Analysis of
Rayleigh Index Map
[0082] Probing the spatial distribution of the local Rayleigh Index
(RI) of the thermo-acoustically unstable flame can help in
identifying the locations which drive the instability in a given
combustor configuration. In order to compute the spatial RI,
pressure and high speed chemiluminescence representative of heat
release rate, must be simultaneously obtained. This is accomplished
by simultaneously triggering the data acquisition from the pressure
transducer and high speed IRO camera at a common sampling rate of 5
kS/s. The spatial RI is computed for the flame corresponding to
case II for the swirler rotation rates of 0 and 1800 rpm.
[0083] The Rayleigh Index is given by Equation. (1) below:
RI = 1 T .intg. 0 T p ' q ' dt ( 1 ) ##EQU00001##
[0084] Here, T corresponds to the time period of one cycle. p'
denotes fluctuating component of pressure (N/m.sup.2) and q'
denotes fluctuating component of heat release rate (W/m.sup.2)
[0085] In the present work, spatial RI map is computed by
integrating p'q'over 91 cycles for the thermo-acoustically unstable
flame. The equation for computing RI integrated over n cycles is
given by Equation (2) below:
RI = 1 nT .intg. 0 nT p ' q ' dt ( 2 ) ##EQU00002##
[0086] FIGS. 11A and 11B illustrate comparison of spatial RI
without and with swirler rotation respectively for one of the
experimental conditions in accordance with embodiments of the
present disclosure. The region which drives the instability
corresponds to the zone where the diverging flame base interacts
with the flame segment above the dump plane as can be seen in FIG.
11A. A comparison of spatial Rayleigh indices in FIGS. 11A and 11B
clearly reveals a prominent reduction of the intensity of the
instability sources in the flow, with swirler rotation. In order to
understand the origin of the instability driving source in RI map
of FIG. 11A without swirler rotation, the dynamics of self-excited
flame is further investigated. The time sequence of instantaneous
images of the thermo-acoustically unstable flame corresponding to
case II without swirler rotation is shown in FIG. 12. At particular
phases (0.degree., 90.degree. in FIG. 10A) of the instability
cycle, the diverging flame base and the flame segment above the
dump plane approach each other due to opposing directions of the
tangential velocity of IRZ and ORZ vortices which is evident from
FIG. 12. These two flame regions eventually collide and merge,
resulting in the annihilation of flame surface area. During second
half of the instability cycle at certain phases, diverging flame
base and the flame segment retreat away from each other thereby
generating new flame surfaces (refer phase averaged images
corresponding to 180.degree., 270.degree. in FIG. 10A). In an
earlier study, Lee et al. (in their paper titled "An experimental
estimation of mean reaction rate and flame structure during
combustion instability in a lean premixed gas turbine combustor,"
published in Proceedings of the Combustion Institute, Vol. 28, No.
1, 2000, pp. 775-782.) utilized mean reaction rate computed from
OH-PLIF images for estimating the spatial RI in the case of swirl
stabilized natural gas premixed flame in a dump combustor. During
the instability cycle, they noticed that periodic interaction of
IRZ and ORZ and found that IRZ and ORZ act as driving regions for
instability whereas shear layer region damps the same.
[0087] It is important to note that the variation in diverging
flame base angle in the case of static swirler (due to the absence
of a continuous VBB and IRZ) is the main factor which contributes
to this flame-flame interaction. The periodic annihilation and
generation of flame surface area due to the interaction of
diverging flame base with the flame segment above the dump plane
ultimately leads to the unsteadiness in the heat release rate.
This, when coupled with pressure fluctuations, feeds energy to the
acoustic oscillations thereby driving the thermo-acoustic
instability.
E. Thermo-acoustic Instability Triggering Precursors
[0088] In an aspect, an attempt has also been made to investigate
the origin or precursor of the flame-flame interaction discussed in
the previous section by probing the dynamics of swirl flame without
the presence of Aluminium tube. The flame thus stabilized in the
absence of Aluminium tube is always instability-free but otherwise
is conditioned to the same experimental parameters as case II. The
experimental conditions corresponding to this instability-free
flame is denoted by case II(b) and shown in Table 2 below:
TABLE-US-00002 TABLE 2 Details of Experimental Conditions for Case
II and Case II(b) Bulk Presence State at Reynolds Equivalence of
tube 0 rpm number Ratio above quartz (Static Case (Re) (.PHI.) test
section swirler) II 7274 0.705 Yes Thermo- acoustically unstable
II(b) 7274 0.705 no Instability- free
[0089] Investigation of case II(b) is critical as it provides an
unambiguous reference for identifying the inherent features of the
flow and understanding the mechanism that could trigger the
transition from an instability-free state to thermo-acoustically
unstable state under suitable conditions.
[0090] The instantaneous image sequence showing the dynamics of
instability-free flame corresponding to case II(b) is shown in FIG.
13. In the case of instability-free flame, corner vortex interact
with the flame segment above the dump plane, which leads to its
stretching by roll-up into the ORZ as observed from FIG. 13. There
is no large scale fluctuation in the diverging flame base angle in
case II(b) as compared to case II due to the absence of
instability. This flame-vortex interaction dynamics observed in the
instability-free flame assists the flame segment above the dump
plane to interact strongly with the periodically fluctuating
diverging flame base in case II. Hence, this flame segment roll-up
into the ORZ can be considered as a possible precursor which leads
to the flame-flame interaction for sustaining instability under
favourable conditions.
F. Mitigating Mechanism of Thermo-Acoustic Instability with the
Rotating Swirler
[0091] As discussed in earlier paragraphs, rotation of the static
swirler enhances swirl number resulting in vortex bubble breakdown.
This creates a stable IRZ which was absent when the swirler
remained static. Consequently, a stable but increased diverging
flame base angle is also realized accompanied by the suppression in
its periodic fluctuation. This prevents the periodic annihilation
and generation of flame surfaces due to the interaction between the
diverging flame base and the flame segment above the dump plane at
particular phases during an instability cycle. Thus, the strong
driving source (positive RI) for sustaining thermo-acoustic
instability is progressively decimated with swirler rotation.
[0092] Along with the formation of IRZ, the swirler rotation also
enhances the turbulence intensity in the flame stabilization
region. The well-known effect of enhanced turbulent intensity is
increased turbulent flame speed which shifts the mean flame
position to an upstream location near the swirler. The increased
turbulent flame speed also assists the flame segment above the dump
plane to move closer towards the ORZ. As a result, the continuous,
augmented heat release in the dump corner region weakens the
strength of the vortices residing in the ORZ due to enhanced
dilatation and kinematic viscosity. Sustenance of increased
diverging flame base angle due to the formation of stable IRZ
accompanied by the upstream propagation of flame segment above the
dump plane due to enhanced turbulence intensity reduce the size and
strength of ORZ vortices, eventually disrupting their interaction
with the ORZ flame. Thus, these two mechanisms: formation of stable
IRZ due to enhanced swirl number together with enhanced turbulence
intensity, interact to render a much more stable flame topology at
higher rotation rates: the favourable outcome of this methodology
of mitigating instability by actuating the swirler in a
combustor.
[0093] In an aspect, based on proof of concept resulting from the
experimental set-up of FIGS. 1, 2A and 2B, the proposed rotating
swirler such as 102 can be implemented in combustors for gas
turbine engines to mitigate thermo-acoustic instabilities and lead
to introduction of lean premixed combustion technology in gas
turbines with its resultant benefits such as reduced pollutant
emission, negligible soot production to name a few.
[0094] In an aspect, the disclosure provides a combustor to
mitigate thermo-acoustic instability by a dynamic control strategy.
The disclosed combustor can be a swirl stabilized, lean premixed
combustor incorporating a vane swirler that is primarily meant for
stabilizing the lean premixed flame. The swirler can be an axial
swirler mounted on a shaft-bearing-coupling-motor arrangement
configured to facilitate its rotation at different rpms. The
combustor can further incorporate means for dynamic control of flow
and flame structure towards achieving a instability free
combustor.
[0095] In an aspect, the disclosure provides a gas turbine engine
incorporating at least one combustor; the at least one combustor
incorporating at least one vane swirler and means to rotate the at
least one swirler at varying rotational speeds, wherein rotating
the swirler mitigates thermo-acoustic instabilities. The at least
one combustor can further incorporate means to detect
thermo-acoustic instabilities such as comprising pressure sensors
to sense pressure in the combustion chamber and processors to
process the sensed pressure to identify occurrence of
thermo-acoustic instabilities; and control means to change
rotational speed of the swirler based on detected occurrence of
thermo-acoustic instabilities.
[0096] FIG. 14 illustrates an exemplary block diagram 1400
indicating a closed loop feedback mechanism for maintaining
thermo-acoustic instability free flame condition in a smart
combustor in accordance with embodiments of the present disclosure.
As shown in the block diagram 1400, a combustor that may be
operating under stable conditions can become unstable due to change
in any of the operating conditions as shown at 1402. The resultant
unstable condition as exemplified by increased amplitude of
pressure transducer signal shown at 1412 of the combustor can be
detected by sensors as shown at 1404. Based on detected condition
the closed loop feedback mechanism can, as shown at 1406, compute
fastest route to a stable condition. Using a control mechanism the
computed route can be implemented as shown at 1408, by actuating
flame holder of the combustor towards a target frequency. Change in
frequency can result in restoration of stable condition for the
changed operating parameters as shown at 1410 and exemplified by
reduced amplitude of pressure transducer signal 1414. The combustor
can remain stable as long as operating condition are maintained,
and any future change in the parameters can be handled by the
closed loop feedback mechanism to keep the combustor in stable
condition.
[0097] Thus the disclosed method and apparatus overcomes limitation
of known method and apparatus having a variable blade angle
swirler. The variable blade angle swirler offers limited change in
swirl number at the cost of pressure drop. Besides this, it also
requires a complicated mechanism to control the swirl angle. The
disclosure provides a concept that does not suffer from the above
stated disadvantages i.e. an enhanced pressure drop and limited
variation in swirl number can be overcome in a rotating swirler. In
an aspect, the disclosed method and apparatus overcomes these
deficiencies by an additional work input to the flow through
controlling the motor rpm, respectively. High rotation rates and
effective design of the swirler blades could even lead to a net
pressure gain in the combustor with an actuated swirler. In another
aspect, and as stated earlier power expended in rotating the
swirler even at its maximum speed is insignificant (less than 1%)
compared to the thermal power output from the combustor, and
therefore the proposed strategy does not result in any significant
drain on the power output of the turbine where the combustor may be
used, thereby establishing basic feasibility of the proposed
dynamic control concept.
[0098] While the foregoing describes various embodiments of the
invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof The scope of
the invention is determined by the claims that follow. The
invention is not limited to the described embodiments, versions or
examples, which are included to enable a person having ordinary
skill in the art to make and use the invention when combined with
information and knowledge available to the person having ordinary
skill in the art.
ADVANTAGES OF THE INVENTION
[0099] The present disclosure can enable instability-free
implementation of lean premixed combustion technology in gas
turbine combustors.
[0100] The method and apparatus of the present disclosure mitigate
problem of narrow band, low frequency thermo-acoustic oscillation
in a lean premixed combustor. This can enable successful
implementation of the lean premixed combustion technology in gas
turbine combustors.
[0101] The method and apparatus of the present disclosure suppress
thermo-acoustic instability without any addition or detraction in
the combustor inlet air or fuel flow rates.
[0102] The present disclosure provides a method for suppressing
thermo-acoustic instability that does not incorporate an automated
feedback loop.
[0103] The method and apparatus of the present disclosure
dynamically controls flow and flame structure towards achieving an
instability-free combustor.
[0104] The method and apparatus of the present disclosure provide
an externally actuated dynamic control for real time modification
of the flow and flame pattern inside the combustor.
* * * * *