Coupling Behaviour of LOx/H2 Flames to Longitudinal and Transverse Acoustic Instabilities

Abstract

A rectangular combustor with acoustic forcing was used to study flame-acoustic interaction under injection conditions which are representative of industrial rocket engines. Hot-fire tests using liquid oxygen and gaseous hydrogen were conducted at pressures of 40 and 60 bar, which are sub-and supercritical conditions respectively for oxygen. To our knowledge, acoustic forcing has never before been conducted at pressures this high in an oxygen-hydrogen system. Examined samples of hydroxyl-radical emission imaging, collected using a high-speed camera during periods of forced acoustic resonance, show significant response in the multi-injection element flame. Transverse acoustic velocity causes shortening of the flame, concentrating heat release near the injection plane. Fluctuating acoustic pressure causes in-phase fluctuation of the emission intensity. Based on these observations, a theorized flame-acoustic coupling mechanism is offered as an explanation for how naturally occurring high frequency combustion instabilities are sustained in real rocket engines. Nomenclature c = bulk sound speed in combustion chamber I = OH* emission intensity I' = fluctuating component of OH* emission intensity Ī = time averaged OH* emission intensity J = injection momentum flux ratio N = response factor ω = acoustic frequency p' = acoustic pressure amplitude P cc = combustion chamber pressure p 0 = eigenmode solution acoustic pressure distribution R = Rayleigh index ROF = oxidizer-to-fuel mixture ratio ρ = bulk density in combustion chamber u' = acoustic (particle) velocity VR = injection velocity ratio

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

“…(4). High-speed OH* imaging from two tests at 60 bar were also amenable to calculation of N. Application of the above methodology to calculate global N values from the twodimensional images is detailed by Hardi et al 46 .…”

Flame Visualization within a Rocket Combustion Chamber without Film Cooling System

“…On the experiment side, many researches have been conducted their works to reveal the effects of pressure oscillation on heat release oscillation by adding forced oscillation using aerodynamic measures such as siren wheel, and conducted flame observation. 1,2) In these experiments, window glasses were usually protected by film cooling with either fuel or inert gas, so that the flow field would be affected somehow.…”

“…However, the flow field within the chamber for the visualization should be identical to that within the actual chamber to reproduce the self-starting oscillation, i.e., film cooling as well as recessed window installation should be avoided. 1,3) In the present study, we fabricated a new chamber for the visualization, made of quartz tubes without film cooling for a single injector experiment with liquid hydrogen and liquid oxygen. This 'without cooling' restriction set a limitation to allowable heat flux to the combustor wall (glass tubes in this case), so that the observation was limited to the vicinity of the coaxial injector in the streamwise direction (about 40 mm from the face plate), however, this region would be very important to investigate the physics of the combustion oscillation.…”

Acoustic response of an injection system to high-frequency transverse acoustic fields

“…Research activities on the Common Research Combustor at the German Aerospace Center (DLR) highlighted the importance of the characterization of the acoustic modes of the injection system/ chamber, in the interpretation of experimental results, [15][16][17] and a numerical study on the same experimental set-up also highlighted the impact of coupled acoustic cavities on resonance frequencies. 18 Injectionchamber coupling phenomenon has been also treated by Hardi et al [19][20][21] The use of an acoustic baffle in the hydrogen manifold was necessary in that case to reduce acoustic coupling. The impact of acoustic coupling between the injection system and combustion chamber in an another DLR sub-scale model combustor has been recently investigated by Urbano et al 22 who used high-performance large-eddy simulation in combination with computational acoustics to simulate combustion instabilities in this combustor.…”