An Analytical and Experimental Investigation of Potential Mechanical Work Extraction from Supersonic Jet Flow on High Speed Mini Turbines

Abstract

In the present study, an inexpensive method of mechanical power extraction from supersonic jets was investigated. Detailed experiments were carried out to measure the mechanical torque generated by a 45o planar bladed mini-turbine with a radius of 12.8 mm. The mini-turbine was exposed to supersonic flow of air exhausting from the exit of a sonic nozzle (diameter of 0.711 mm) into still air. Experiments were carried out by placement of the mini-turbine in various configurations with respect to the supersonic jet flow centerline. These experiments were conducted in the Adaptive Aerostructures Laboratory at the University of Kansas using compressed air with a supply pressure of around 6.12 to 6.80 atm. The mini-turbine was tested up to speeds of 50,000 RPM. In addition, the experiments were carried out to determine the mechanical shaft power of the mini-turbine and the overall efficiency of the power conversion from fluid aerodynamic power of the supersonic jet flow to the mechanical shaft power measured. Ten different configurations were tested by positioning the mini-turbine at x/Dturbine = 0.5 and x/Dturbine = 0.25 (x/Dturbine defined as the lateral distance of the supersonic jet from the mini-turbine centerline axis normalized by mini-turbine diameter), +/- 3o and +/- 1.5o inclination angle, and +/- 2.5o and +/- 1.0o directional angle offset relative to the nozzle exit supersonic jet centerline axis, respectively. The effect of nozzle exit area variation on mechanical shaft power extraction and aero-mechanical efficiency of power conversion in each of the test configurations was tested for three different areas; Ae = 0.397 μm2, 0.75Ae = 0.298 μm2 and 0.5Ae = 0.199 μm2. Additional studies were performed to determine the effect of varying the axial location (y/De) of the mini-turbine along the supersonic jet centerline axis measured downstream from the ejector nozzle exit. These experiments were conducted for the three nozzle exit area cases mentioned above. Shadowgraph images were captured for the supersonic jet shock structure and jet spread at the three different nozzle exit areas specified. Results obtained from the experimental runs showed that the mechanical shaft power of the mini-turbine was in the range of 1-2 W and the aero-mechanical efficiency of power conversion achieved were approximately 25% to 40% for the mini-turbine operating at speeds of approximately 35,000 to 50,000 RPM. The aero-mechanical efficiency of power conversion increased when the nozzle exit area was reduced and as the mini-turbine was placed further downstream of the nozzle exit, respectively. These results were attributed to the fact that the jet spread and shear layer growth rate increased at the far-field locations downstream of the nozzle exit. The flow images captured showed visible diamond shock structures in the near-field region for the underexpanded nozzle exit flow and the degree of underexpansion decreased as the nozzle exit area was decreased. Reasonable values of the mini-turbine shaft power extracted from supersonic jet flow and the aero-mechanical efficiency of power conversion were achieved from the present study.