Density Measurements in a Supersonic Microjet Using Miniature Rainbow Schlieren Deflectometry

Abstract

Understanding of the structure of complex supersonic flows requires high-resolution, nonintrusive measurements across the whole field. The measurement requirements are even more challenging when dealing with small-scale systems. In this study, we apply the miniature rainbow schlieren deflectometry system to measure the density field in underexpanded microjets from an orifice injector of 500 μm diameter. The injector is used to replicate the practical scenario of accidental leakage from a compressed gas storage facility. Experiments were conducted for a range of supply pressures P s , although the majority of the results are presented for P s = 860 kPa. Experimental schlieren images were analyzed to determine the density contours in an axisymmetric domain with a field of view of 1.5 mm radius and 7.5 mm length, at a spatial resolution of 25 μm. Pressure, temperature, and Mach number profiles (normalized by the values at the orifice) were also obtained along the jet centerline. Results show features similar to those observed in underexpanded macrojet from a sonic nozzle; multiple shock-cell structures with expansion and compression fans, freejet boundary, incident, normal, and reflected shock waves, subsonic slip strip, and outer shear layer. The miniature rainbow schlieren deflectometry technique is shown to provide high-quality quantitative data to explain the structural details of underexpanded microjets.