Thermoacoustic limit cycles in a premixed laboratory combustor with open and choked exits

Hield, Peter and Brear, Michael and Jin, Seong-Ho (2009) Thermoacoustic limit cycles in a premixed laboratory combustor with open and choked exits. Combustion and Flame, 156 (9). pp. 1683-1697. ISSN 0010-2180

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Full text URL: http://dx.doi.org/10.1016/j.combustflame.2009.05.0...

Abstract

This paper presents an experimental and theoretical investigation of the response of a turbulent premixed
flame during thermoacoustic limit cycle in a simple, laboratory combustor. The flame dynamics are examined using high-speed pressure transducers and CH* chemiluminescence. The so-called ‘interaction index’ and time delay between the acoustic velocity fluctuations at the flame holder and the flame’s overall heat release fluctuations are then determined. A wide range of operating conditions, traversing
the combustor’s flammability limits in Mach number and equivalence ratio, are studied for four different
combustor exits, including one where the exit is choked. In all cases the time delay correlates very well with the amplitude of the velocity fluctuations. There is also some correlation between the interaction index and these velocity fluctuations, but this is less clear. These results suggest a novel, nonlinear flame model, derived entirely empirically. An existing low-order thermoacoustic model is then extended to include convection and dispersion of entropy fluctuations downstream of the flame, enabling the effect of the choked nozzle to be examined. The novel nonlinear flame model is integrated into the low-order thermoacoustic model, and used to investigate the experimentally observed thermoacoustic limit cycles.
The model correctly simulates the observed switch to a low-frequency, entropically driven instability observed when the combustor exit is choked.

Item Type:Article
Additional Information:This paper presents an experimental and theoretical investigation of the response of a turbulent premixed flame during thermoacoustic limit cycle in a simple, laboratory combustor. The flame dynamics are examined using high-speed pressure transducers and CH* chemiluminescence. The so-called ‘interaction index’ and time delay between the acoustic velocity fluctuations at the flame holder and the flame’s overall heat release fluctuations are then determined. A wide range of operating conditions, traversing the combustor’s flammability limits in Mach number and equivalence ratio, are studied for four different combustor exits, including one where the exit is choked. In all cases the time delay correlates very well with the amplitude of the velocity fluctuations. There is also some correlation between the interaction index and these velocity fluctuations, but this is less clear. These results suggest a novel, nonlinear flame model, derived entirely empirically. An existing low-order thermoacoustic model is then extended to include convection and dispersion of entropy fluctuations downstream of the flame, enabling the effect of the choked nozzle to be examined. The novel nonlinear flame model is integrated into the low-order thermoacoustic model, and used to investigate the experimentally observed thermoacoustic limit cycles. The model correctly simulates the observed switch to a low-frequency, entropically driven instability observed when the combustor exit is choked.
Keywords:Premixed combustion, Thermoacoustic instability
Subjects:H Engineering > H141 Fluid Mechanics
H Engineering > H341 Acoustics
H Engineering > H311 Thermodynamics
Divisions:College of Science > School of Engineering
ID Code:4620
Deposited By: Seong-Ho Jin
Deposited On:20 Aug 2011 08:39
Last Modified:02 Sep 2014 09:13

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