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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.

Rapid mixing and spreading of supersonic jets are two important characteristics in supersonic ejectors, noise reduction in jets and fuel mixing in supersonic combustion. It helps in changing the acoustic and thermal signature in supersonic exhaust. The supersonic nozzles in most cases result in compressible mixing layers. The subsonic nozzles form incompressible mixing layers but at high Mach numbers even they form compressible mixing layers. Compressible mixing layers have been found to have much lower mixing and spreading rates than incompressible mixing layer Birch & Eggers (1972). In order to enhance the spreading and mixing of mixing layers from supersonic nozzles various active and passive methods have been deviced. Active methods include fluid injection, fluid lobes and plasma actuation. Passive methods are mostly based on modifying the nozzle geometry such that the fluid expansion is ideal or the shock cell is broken. Many nozzles with exotic shapes have been developed to obtain mixing enhancements in supersonic jets Gutmark et al. (1995). To achieve enhanced mixing an innovative nozzle named as the Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle has been developed in L.H.S.R., I.I.Sc., India Rao & Jagadeesh (2014). This nozzle has a unique geometry involving elliptical lobes and sharp tips. These lobes are generated using a simple manufacturing process from the throat to the exit. This lobed and sharp tipped structure introduces stream wise vortices and azimuthal velocity components which must help in enhanced mixing and spreading. The ESTS lobed nozzle has shown mixing enhancement with 4 lobes. The spreading rate was found to be double of the reference conical nozzle. This thesis is motivated by the need to investigate the flow physics involved in the ESTS lobed nozzle. The effect of varying the number of lobes and the design Mach number of the nozzle on the mixing and spreading characteristics will be further discussed. Visualisation studies have been performed. The schlieren and planar LASER Mie scattering techniques have been used to probe the flow. Instantaneous images were taken at axial planes with the reference conical and ESTS nozzles with three, four, five and six lobes. The nozzles are for design Mach number 2.0 and 2.5. The stagnation chamber pressure was maintained to obtain over expanded, ideally expanded and under expanded flows. LASER scattering was obtained by seeding the flow with water to observe the behaviour of the primary flow. The condensation of moisture due to the cold primary flow mixing with the ambient air was exploited to scatter laser and observe the flow structures in the mixing layer. A comparison of the images of the reference conical nozzle and the ESTS lobed nozzles shows changes in the mixing layers due to the ESTS lobed nozzles. The image of the reference conical nozzle shows a distinct potential core and mixing layers all along the length of the image. For the ESTS lobed nozzles this distinction becomes unclear shortly after the nozzle exit. Thus mixing of the primary flow and ambient air is seen to be enhanced in the case of all the ESTS lobed nozzles. The flow in the case of the ESTS lobed nozzles if found to be highly non axis symmetric. The starting process of the nozzles has been visualised using time resolved schlieren. Image processing was performed on the nozzles to quantify the spread rate. The shock structure of the nozzles has been studied and found to be modified due to the lobed geometry. The level of convolution of the mixing layer due to the lobed structure has been studied using fractal analysis. The four lobed nozzle was found to have the highest spread rate and th most convoluted shear layer. Hence this nozzle was further studied using background oriented schlieren and particle image velocimetry to quantify the flow field. These experimental results have been compared with CFD simulations using the commercial software CFX5. The computations and experiments don’t match accurately but the trends match. This allows for simulations to be used as a good first approximation. The acoustic properties of a jet are dependent on the flow structure behaviour. The ESTS lobes have been found to change the flow structure. Hence the ESTS lobed nozzle was predicted to change the acoustic signature of the flow. The acoustic measurements of the flow were carried out at National Aerospace Laboratories, Bengaluru. The screech of the overexpanded flow was seen to be eliminated and the overall sound levels were found to have been reduced in all cases. Thus the lobed nozzle was found to have acoustic benefits over the reference conical nozzle. Thus the ESTS lobed nozzle has been studied and compared with the conical nozzle using several methods. The changes due to the lobed structure have been studied quantitatively. Future studies would focus on the change in thrust due to the lobed structure. Also new geometries have been proposed inspired by the current design but with possible thrust benefits or manufacturing benefits.