Role of hydrodynamic shear layer stability in driving combustion instability in a premixed propane-air backward-facing step combustor

Abstract

This paper presents a global hydrodynamic stability analysis of flow fields in a backwardfacing step combustor, assuming weakly nonparallel flow. The baseline experiments in a "long" combustor of length of 5.0 m shows the presence of two combustion instability states characterized by coherent low-and high-amplitude acoustic pressure oscillations. The analysis is performed for propane-air mixtures at three values of φ = 0.63, 0.72, and 0.85, which correspond to quiet, low-amplitude and high-amplitude instability states in the long combustor experiments. Base flow velocity and density fields for the hydrodynamic stability analysis are determined from time-averaged particle image velocimetry measurements made after the length of the duct downstream of the step has been shortened to eliminate acoustic pressure oscillations. The analysis shows that the shear layer mode is self-excited for the φ = 0.72 case with an oscillation frequency close to that of the long combustor's fundamental acoustic mode. We show from an analysis of the weakly forced, variable density Navier-Stokes equations that self-excited hydrodynamic modes can be weakly receptive to forcing-suggesting that the low-amplitude instability in the long combustor is due to semi-open loop forcing of heat-release oscillations by the shear layer mode. At φ = 0.85, the analysis shows that the flow is hydrodynamically globally stable but locally convectively unstable. Spatial amplification of velocity disturbances by the convectively unstable flow causes high-amplitude combustion instability in the long combustor case. These results show that combustion instability can be sustained by two different mechanisms by which acoustic and hydrodynamic modes being either strongly coupled result in fully closed loop forcing, or weakly coupled result in semi-open loop forcing of the flame by a self-excited hydrodynamic mode.

Thermogravitational and hybrid convection in an obstructed compact cavity

“…13a and 13b with Figs. 5 and 7 would immediately reveal, disturbances are now located essentially in the right portion of the cavity and there is strong evidence for their essentially vertical-shear-driven nature (the interested reader being also referred to the similar studies recently carried out by Ali and Dey [78] and Hemchandra et al [79]). After discussing in the previous section the dynamics of hybrid forced-buoyancy convection as a result of cold fluid injection through an inflow section located on top of the left sidewall, we finally address the case for which the inflow section is positioned relatively close to the floor.…”

Impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame

“…The results showed the FTF to be very sensitive especially to variations of the flame speed and the inlet duct temperature. Hemchandra et al 12 analyzed the role of hydrodynamic shear layer instabilities for CI. They found that acoustic-hydrodynamic coupling can lead to CI by closed and semi-open loop mechanisms.…”

“…Hemchandra et al. 12 analyzed the role of hydrodynamic shear layer instabilities for CI. They found that acoustic-hydrodynamic coupling can lead to CI by closed and semi-open loop mechanisms.…”

“…2 To investigate CI numerous experimental, numerical, and theoretical studies have been performed. [3][4][5][6][7][8][9][10][11][12] These instabilities are often described by a coupling of heat release oscillations with acoustic modes of the combustion chamber. Recent findings suggest that they are also generated by intrinsic thermoacoustic modes.…”

Characterizing hydrogen-fuelled pulsating combustion on thermodynamic properties of a combustor

“…It is found that the primary driving mechanism of 1k Hz combustion instability is the acoustically coupled pulsating motion of the inner H 2 /O 2 flame or periodic ignition of H 2 /O 2 mixture. Recently, Hemchandra et al 11 discovered that there are two different mechanisms driving combustion instability. One is a strong coupling between acoustics and hydrodynamic modes.…”