Experimental Investigations on Supersonic Ejectors

Abstract

A supersonic ejector is used to pump a secondary gas using a supersonic primary gas flow by augmentation of momentum and energy in a variable area duct. The internal compressible flow through an ejector has many complex gas dynamic features, like compressible shear layers and associated shock interactions. In many practical applications, ejectors are operated in the choked flow regimes where higher operating pressure ratios and mass flow rates are encountered. On the other hand, rather low entrainment and subsonic secondary flow dynamics (referred as the mixed regime of operation) dominate the dilution and purging applications of ejectors. The fundamental understanding of the flow dynamics associated with gaseous mixing process in the ejector especially in the mixed operational regime is still unclear. Obtaining a comprehensive understanding of the flow through a supersonic ejector in the mixed regime through experimental investigations is the prime focus of the present study. A new supersonic ejector test facility is designed, fabricated and established in the laboratory during the course of this study. The effects of using different gases in the secondary flow have been investigated. Two novel methods to improve the ejector by enhancing mixing are also implemented. Optical diagnostic tools (Time-resolved Schlieren and laser scattering) and wall static pressure measurements are used to investigate the dynamics of mixing process inside the ejector. State of the art image processing codes are developed to determine the length in the ejector for which the primary and the secondary flows are separate, referred here as the non-mixed length from the results of the flow visualization studies. Exhaustive experiments are carried out on the two dimensional rectangular supersonic ejector by varying the mass flow rates of primary and secondary flows, primary stagnation pressure, for two locations of the nozzle in the ejector. The non-mixed length determined from quantitative flow visualization tools is found to lie within 4.5 to 5.2 times the height of the duct (20 mm). The non-mixed flow length determined from flow visualization studies corroborates well with the wall static pressure measurements. A significant reduction of non-mixed length of about 46.7% is caused by shock wave-boundary layer interactions in the supersonic nozzle at over-expanded conditions. Further, the effects of differences in molecular weight and ratio of specific heats on the performance are also studied using cylindrical supersonic ejector at low entrainment ratios (0.008 to 0.06). In these studies air is used as the primary fluid while argon and helium are used in the secondary flow segment of the ejector. The results indicate that Argon has better entrainment characteristics compared to helium. Two novel supersonic nozzles (the tip rig nozzle and Elliptic Sharp Tipped Shallow lobed nozzle) are also devel- oped to enhance mixing in the ejector. About 30% enhancement of entrainment ratio is observed with the newly designed nozzle geometries. Illustrative numerical simulations are also carried out to complement the experimental studies.