Study of LOx/H2 Spray Flame Response to Acoustic Excitation in a Rectangular Rocket Combustor

Abstract

This work is focused on understanding how intrinsic processes may be involved in driving high frequency combustion instabilities. The response of an experimental rocket combustor to both unperturbed and acoustically excited conditions at four different operating points was studied. The combustor has a rectangular cross-section with acoustic forcing, five shear coaxial injection elements running cryogenic oxygen-hydrogen (LOx/H 2 ), and optical access. The operating points comprise two different chamber pressures and two different hydrogen injection temperatures. The chamber pressures of 40 and 60 bar allow the influence of sub-and supercritical LOx, respectively, to be studied. The H 2 injection temperatures of ~290 K and ~50 K were selected since low H 2 temperature has long been thought to influence the stability of LOx/H 2 engines. Combustor response was compared between the four operating points under both undisturbed conditions, and forced resonance of the 1L and 1T modes. The measures used to compare combustor response were acoustic spectra, modal acoustic energy distribution, LOx jet atomization behavior, flame displacement, and chemiluminescent emission intensity. None of the parameters compared revealed a significant difference in response between the four operating points. There were two exceptions of interest. The first was in the distribution of unperturbed acoustic energy to the 1L and 1T modes. The content of both modes was lower at 60 bar, regardless of H 2 temperature, except the 1T mode was more energetic at 60 bar when using low temperature H 2 . The second was the greater decrease in intact LOx core length under 1T-mode excitation with low temperature H 2 . Both of these observations may be related to the historic tendency for transverse mode instability with decreasing H 2 temperature. Nomenclature c = bulk sound speed in combustion chamber δʹ = acoustic particle displacement D = diameter of oxygen injector (LOx post) I = OH* emission intensity Iʹ = unsteady OH* emission intensity Ī = time averaged OH* emission intensity J = injection momentum flux ratio L = length of intact LOx core M = mass flow rate N = response factor ω = acoustic frequency pʹ = acoustic pressure amplitude P cc = combustion chamber pressure 2 qʹ = unsteady heat-release rate ROF = oxidizer-to-fuel mixture ratio ρ = bulk density in combustion chamber T = injection temperature u = injection velocity uʹ = acoustic (particle) velocity VR = injection velocity ratio