Experimental study of intra-injector flow fluctuations induced by a LOX injector orifice driving combustion instability

Abstract

In combustion chambers of rocket engines, injector flow dynamics can be one of the triggers of injector-coupled high-frequency (HF) and low-frequency (LF) instabilities, which were observed during hot-fire tests of research combustors at DLR. The whistling flow from the LOX post orifice was hypothesized as the excitation source of the acoustic eigenmodes of the LOX posts, which led to an excitation of HF instabilities, whereas the two-phase flow across the orifice was hypothesized as reason for LF instabilities. To investigate the hypothesis of the orifice-flow induced instability, a modularized single LOX post with an optically accessible orifice module was used for water experiments. The unsteady pressures downstream of the orifice were measured by high-speed piezo sensors at cavitating and non-cavitating intra-injector flow conditions. In addition, cavitating orifice flows were directly visualized by backlight imaging with a high-speed camera through the optically accessible orifice module. The cavitating flow shows two types of flow characteristics: the hydroacoustic peaks induced by cavitation and induced by orifice whistling. The peaks originating from cavitation have complex multiple peak structures in the low frequency region, which can cause lowfrequency chugging instabilities in rocket engines. The peak Strouhal number from cavitation decreases with increasing pressure drop while the Strouhal number from whistling is mostly constant. The non-cavitating flow shows peaks at constant Strouhal number. Also, the second longitudinal mode of the post is excited under these conditions. In conclusion, two types of acoustic peaks from the intra-injector flow of the orifice were identified. The results showed that the intra-injector flow can excite LOXpost eigenfrequencies which can be coupled with the combustion chamber volume leading to combustion instabilities.