Effect of Liquid Disintegration on Flow Instability in a Recessed Region of a Shear Coaxial Injector

A liquid jet disintegration phenomenon in the recessed region of a shear coaxial injector was examined to find the relation between the difference of the recess depth and flow instability which may induce combustion instability of liquid rocket engines. An injector with a rectangular cross section and a recess for a central post, which modeled the shear coaxial injector element employed in liquid rocket engines, was constructed. The injector was made of transparent acrylic glass to allow observation of the disintegration phenomenon in the recess. Cold-flow tests with water and nitrogen gas with ambient pressures of 0.2, 0.3 and 0.4 MPa were conducted. Results showed that a condition arose in which the flow in the recess was choked by two-phase flow. The choked flow was accompanied by vibration of the central post which caused a significant change of the disintegration pattern from moderate disintegration to violent disintegration. A similar transition from a fiber-type flow to a super-pulsating disintegration flow reported by Chigier and Reitz was also observed under non-choked conditions for a coaxial injector without a recessed region. The boundary of the transition was found to depend on certain values of ReL/(WeG) 0.5 for each recess depth, including the two-phase choked flow condition. This means that the transition from a fibertype flow to super-pulsating disintegration leads to the transition from a non-choked flow to a choked flow.