Characterisation of Acoustic Energy Content in an Experimental Combustion Chamber with and without External Forcing

Abstract

The influence of injection conditions on rocket engine combustion stability is investigated for a sub-scale combustion chamber with shear coaxial injection elements and the propellant combination hydrogen-oxygen. The experimental results presented are from a series of tests conducted at sub and super critical pressures for oxygen and for both ambient and cryogenic hydrogen. The stability of the system is characterised by the root mean squared amplitude of dynamic combustion chamber pressure for both the non-forced and externally forced combustion chamber configurations. It was found for both the externally forced and non-forced configurations the injection velocity had the strongest influence on combustion chamber stability. Through the use of multivariate linear regression the influence of hydrogen injection temperature and hydrogen injection mass flow rate were best able to explain the variance observed in the injection velocity ratio relationship. For the externally forced configuration a nonlinear regression model was better able to predict the variance, suggesting the influence of nonlinear behaviour. The response of the system to variation of injection conditions was found to be small, suggesting that the combustion chamber investigated in the experiment is naturally highly stable.