On the Effect of a Transverse Acoustic Field on a Flush Shear Coaxial Injector

Abstract

An experimental study on the effects of an externally-imposed transverse acoustic field in a flush shear coaxial jet is presented. In this case the inner jet recess is zero and both the inner and outer jet exit planes coincide. Since recess is a design variable used when designing new injectors, this study complements previous studies from this group where the injector geometries included a recess. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N2 as the working fluid, the chemistry effects are separated from the fluid mechanic effects of a transverse acoustic field on coaxial jets. The acoustic field is generated by two piezo-sirens whose resonant frequency is ~3kHz. The acoustic pressures generated are about 0.2-0.8% the value of the chamber pressure. The phase angle between these two sources is varied at 45 degree intervals. Two values of pressures are studied, 1.5 MPa (Reduced Pressure, Pr=0.44) where the flow is subcritical and 3.6 MPa (Pr=1.06) where the pressure is nearcritical. The outer to inner jet velocity ratio varies from ~1.5 to 17 and the outer to inner jet momentum flux ratio (J) varies from ~0.09 to 20. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. At least 3000 backlit images were taken at 20 kHz for each run. These images are the main analysis tool to study the jet behavior. The most dramatic effects resulting in about 90% reduction of the length of the inner jet core were obtained at nearcritical conditions for the J=1.7 and 3.5 cases.