THERMO-ACOUSTIC INSTABILITIES
IN A BACKWARD-FACING STEP STABILIZED
LEAN-PREMIXED FLAME
IN HIGH TURBULENCE FLOW
V. Sabel’nikov, C. Brossard, M. Orain, F. Grisch,
M. Barat, A. Ristori, P. Gicquel
MOTIVATIONS
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
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Pollutants emissions requirement is the key driver to the
design of Lean Prevaporized Premixed (LPP) Gas Turbines
Combustors
LPP combustors prone to occurrence of combustion
instabilities
It is important to understand the processes controlling the
combustion instabilities (their development and limit cycle
characteristics)
LEAN-PREMIXED COMBUSTION INSTABILITIES IN
HIGH TURBULENCE FLOWS: MOTIVATIONS
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
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Most of experimental studies were done on subscale combustors where
Reynolds number is ~ 104 and turbulent Reynolds number is ~ 102.
However, typical industrial gas turbine burners are characterized by
greater Re: Re ~ 105, Ret ~ 103
Borghi diagram:
ONERA / LAERTE
Stepped Combustor
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
ONERA / LAERTE STEPPED COMBUSTOR
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Water-cooled
Choked inlet, variable area exit nozzle
100x100-mm2 square section, 35-mm high step
Preheated air
Optically accessible combustion chamber (on 3 sides,
both lateral - 260x100 mm2, upper - 260x36 mm2)
Unsteady pressure transducers P1 and P4 located at
15 and 675 mm downstream of the step
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
OPERATING CONDITIONS
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Inlet velocity Uo (m/s)
40 - 65
Inlet air temperature Ta (K)
450 - 600
Fuel equivalence ratio φ
0.8 – 1.0
Air mass rate (g/s)
200 - 350
Methane mass rate (g/s)
10 -15
Re= UoH/ν
1. - 3.105
Ret = u’L/ ν
1. - 2.103
Combustor pressure P (bar)
1.12 - 1.50
Combustor power (kW)
550 - 830
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
INSTABILITY BEHAVIOR: Regime transition
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Transition from stable to unstable combustion performed
by increasing the degree of constriction of the combustor
exit nozzle, which changed its acoustic impedance and
increased the mean combustor pressure
⇒ unstable at P > 1.25 bar
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Stable combustion - opened back pressure valve
φ = 0.82 ; Uo = 61 m/s ; ma = 270 g/s ; Ta = 550K ; P = 1.13 bar
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Unstable combustion
φ = 0.82 ; Uo = 50 m/s ; ma = 270 g/s ; Ta = 550K ; P = 1.35 bar
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
FLAME DYNAMICS: VELOCITY FIELD
Particle Image Velocimetry (PIV)
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Two superimposed
laser sheets placed
along flow axis
2.5 Hz repetition rate
LaVision® system,
Davis 7.1 software
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
FLAME STRUCTURE:
Planar Laser Induced Fluorescence (PLIF)
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Pulsed laser, tuned to a
transition of OH
Laser sheet placed along flow
axis or perpendicular
Camera intensifier gate width:
200 ns
3 Hz repetition rate
50 mm
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
« STABLE » FLAME: High-speed visualization
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Reacting zone: OH* chemiluminescence emission
narrow-band interference filter: 310 nm ± 10 nm FWHM
Intensified high-speed video camera
(Photron Fastcam-ultima APX-i2)
4,000 frames/s for 1024x512 pixels frame size
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
« UNSTABLE » FLAME : High-speed visualization
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Reacting zone: OH* chemiluminescence emission
narrow-band interference filter: 310 nm ± 10 nm FWHM
Intensified high-speed video camera
(Photron Fastcam-ultima APX-i2)
4,000 frames/s for 1024x512 pixels frame size
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
INSTABILITY BEHAVIOR: Unsteady pressure traces
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Power spectrum shows pronounced peak at frequency 66Hz (1/4
longitudinal acoustic wave for the inlet duct) of high amplitude 154dB
BUT strong modulation in frequency (55-80 Hz) and amplitude, with
abrupt low-frequency drops toward zero (around 2 Hz)
prevents phase-locked acquisition with unsteady pressure traces as
usually practiced in studies at moderate Reynolds numbers
PIV/high-speed movies/pressure traces synchronization
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
INSTABILITY BEHAVIOR: instability cycle
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unsteady pressure measured at the step
PIV / High-Speed Visualization Synchronization
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High-speed camera:
slave system
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121 images every 400 ms
∆t = 3 µs
PIV acquisition trigger
500 µs
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
PIV:
master system
4000 Hz
time
2.5 Hz
VELOCITY FIELD, « STABLE » FLAME
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
Obtained for a series of 101 consecutive instantaneous velocity fields
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Average velocity field
60 m/s upstream
RMS fluctuations,
longitudinal component
10 m/s upstream
20 m/s in mixing layer
ACOUSTIC FLUCTUATIONS OF VELOCITY FOR
UNSTABLE FLAME
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
45 m/s upstream
Acoustic fluctuations
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Extreme levels of velocity
recorded upstream of the step
(instantaneous velocity fields)
Velocities up to + 200 m/s
flow reversal up to -40 m/s
FLAME STRUCTURE CHARACTERISTICS
deduced fom PLIF images
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
laser sheet placed perpendicular to the main flow, -20 mm upstream
from the step
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Thickened wrinkled flame regime characteristics: very small wrinkled
structures (down to a few hundred microns), combustion pockets
disconnected from the main flame
Highly three-dimensional, largely distributed over volume
Unstable flame: variations in spatial location
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
CONCLUSIONS
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Low-frequency (around 66 Hz) combustion instabilities were investigated at
large Re conditions in the ONERA / LAERTE stepped combustor
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Combined use of PLIF, PIV and high-speed visualization techniques provided
detailed insight into flame dynamics and structure characteristics
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Large-amplitude and extremely violent flapping of the flame evidenced: up and
down across the combustor, in and out in the inlet duct (unsteady flashback)
with very strong velocity fluctuations
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Background turbulence at large Re has important impact on the flame structure
and combustion instabilities characteristics, in particular modulation in
frequency and amplitude
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Results suggest that some modifications of turbulent combustion modeling are
needed when applying LES approach
14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 07-10 July, 2008
ACKNOWLEDGEMENT
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The authors are grateful to the Direction Générale de
l’Armement (French Ministry) and to the Scientific Direction
of ONERA for funding this study