High amplitude transverse acoustic coupling under cold flow conditions

International audienceHigh frequency combustion instabilities have technological importance in the design and development of liquid rocket engines. These phenomena involve a strong coupling between transverse acoustic modes and combustion. They are currently being investigated by combining experimentation and numerical simulations. On the experimental level, the coupling is examined in a model scale system featuring a multiple injector combustor (MIC) comprising five shear coaxial injectors fed with liquid or transcritical oxygen and gaseous methane. This system is equipped with a novel actuator (VHAM for Very High Amplitude Modulator). This device was designed to obtain the highest possible levels of transverse oscillation in the MIC. These experiments are performed on the Mascotte facility (ONERA). After a brief review of the VHAM, cold flow experiments using this modulator are reported. The VHAM is here used under cold flow injection of liquid oxygen and gaseous nitrogen at a mean pressure of 7.5 bar. Experiments are first carried out without modulation. A linear frequency modulation is then used to identify the chamber eigenfrequencies. The system is finally excited with a continuous wave modulation at the 1T or 1T1L eigenfrequencies. Powerful transverse modes are created by the VHAM (with levels of around 190 dB), allowing a study of the influence of acoustic interactions with jet break-up, atomization and spray dispersion under conditions consistent with those existing when real rocket engines become unstable. Observations using backlighting and high speed imaging at a rate of 24 kHz, indicate that atomization is enhanced, notably reducing the core length. Another effect of the strong acoustic field is to create a global motion of the jets from the top of the chamber to the bottom. Hence, an asymmetry in oxygen density field between the top and the bottom of the chamber appears. This is considered as one of the phenomena that drives combustion instabilities