Preliminary Results on Coaxial Jet Spread Angles and the Effects of Variable Phase Transverse Acoustic Fields (Postprint)

Abstract : An experimental study on the jet spreading angle of N2 shear coaxial jets at sub-, near-, and supercritical pressures is presented. The jet spreading angle is an important parameter which characterizes the mixing between two flows forming a shear layer. The present results are compared with previous experimental data, CFD results, and theoretical predictions. The angle measurements are made directly from at least 20 backlit images. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. The chamber pressure ranges from 1.5 to 5.0 MPa to span subcritical to supercritical pressures. The chamber to outer jet density ratio varies from 0.17-4.8 and the momentum flux ratio between the outer and the inner jet varies from 0.37 to 30. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. For the ranges of conditions studied it is found that the tangent of the jet spreading angle is roughly constant and approximately 0.19 with std. dev. of 0.02. The value is lower than those predicted by different theories for planar mixing layers of variable density for gaseous flows. The second part of the paper focuses on the initial results obtained by combining two piezo-sirens which generate a transverse acoustic field to excite the coaxial jet. The resonant frequency studied is approximately 3kHz and delta P/P varies from 1-1.6%. These two acoustic sources can have an arbitrary phase between them so the position of the jet with respect to the pressure and velocity field can be adjusted. The main parameter investigated is the length of the dark inner jet core. The initial results indicate an effect of the phase angle on the dark core length but the differences are statistically significant only in the extreme cases.