LOW-FREQUENCY COMBUSTION INSTABILITY MECHANISMS IN A SIDE-DUMP COMBUSTOR

This article describes a study of a two-dimensional two-inlet side-dump combustor fed with a mixture of air and propane. The present results concern symmetric operating conditions with respect to the two inlets. Stable and unstable regimes which depend on the inlet velocity and the equivalence ratio have been identified. Schlieren visualization, radical imaging with an intensified CCD camera, and simultaneous pressure, inlet velocity and C2 emission light measurements, have been used to characterize the combustor behavior. Imaging of the flowfield has provided an insight on the flame structure and its interaction with the entering jets. The geometry of the flowfield inside the combustion chamber with or without instability was symmetric with respect to the combustor centerline. For stable combustion, the flowfield was characterized by the presence of two zones of intense heat release located on both sides of the jet impingement region and were distributed along the combustor centerline. Two low-frequency unstable modes (a fuel-rich regime and a fuel-lean regime with an instability frequency around 500 Hz) were studied using a conditional imaging technique. These instabilities were characterized by the excitation of the quarter-wave mode of the combustor and were associated with a complex evolution of the jets and the flame. Jet oscillations were due to the kinematic superposition of the lateral entering jets and longitudinal velocity fluctuations generated by heat release oscillations in the dome region. It was found that unsteady heat release occurs in two different ways: pulsating combustion in the dome region and convection of reaction zones downstream of the jet-impingement region. Flame oscillations were induced by a periodic impingement of the jets on the centerplane of the chamber. Pressure fluctuations in the test section were roughly in phase with the global C2 emission, indicating that the in stabilities were sustained by energy addition to the acoustic field. A two-dimensional distribution of the Rayleigh index computed for each unstable mode indicated that the fuel-lean mode was driven by the unsteady heat release in the dome region whereas the fuel-rich mode was driven by the flame oscillations downstream of the jet-impingement region. The transition from the fuel-lean to the fuel-rich instability featured a shift of driving mechanism. This study shows that even in our idealized geometry the coupling mechanisms leading to low-frequency combustion instabilities are not unique and illustrates the difficulty of devising predictive models.