Power Harvesting from Shock Waves: the Axial Bladeless Turbine

Abstract

A new class of bladeless turbines was developed which allows for power extraction from harsh environments with minimal maintenance cost. This is achieved through a wavy hub surface that promotes shocks and expansion fans and hence generates torque besides trust if used as bottoming or topping cycle. A numerical procedure to design, mesh, and model this new expansion device through steady and unsteady Reynolds Averaged Navier Stokes simulations is outlined. Then, the full three-dimensional flow field is replicated using a two-dimensional geometry to enable a simpler test section with full optical access at the Purdue Experimental Turbine Aerothermal Lab. Pressure, heat flux, and skin friction are computed via several measurement techniques to provide an accurate estimation of the uncertainties on the power, efficiency, and heat flux of the bladeless turbine. High-frequency pressure sensors (160 kHz) along with a high-frequency heat flux sensor (atomic layer thermopile) are used to characterize the unsteady phenomena on the hub and the shroud. Unsteadiness in the flow field is assessed through 10 kHz shadowgraph, density gradients are quantitatively assessed via 3 kHz Background Oriented Schlieren, and unsteady velocity components and flow angles are characterized with 1 kHz Femtosecond Laser Electronic Excitation Tagging. A reduced order model is constructed with Spectral Proper Orthogonal Decomposition to retrieve the dominant frequencies in the flow field, which could be associated with a multitude of shock-boundary layer, shock-shock, and shock-shear layer interactions.A parametric study and multi-objective optimization to maximize power extraction while minimizing pressure loss and heat flux are performed. The operational envelope and scaling of the bladeless turbine are described for several reduced mass flows, reduced speeds, and swirl angles. Based on all the gathered simulations, a guideline for the design of bladeless turbines is provided.Finally, the operation of the bladeless turbine is analyzed considering the unsteady propagation of a rotating oblique shock throughout the passage. Non-dimensional parameters to generically describe rotating shocks are discussed, and their influence on the operation of the turbine is assessed. Correction terms for the power and pressure loss during the unsteady operation of the bladeless turbine are developed with results of this section.