The present study was designed to gain deeper insight of the thermofluiddynamic fields in a compressor wheel. The compressor is part of an ultra-micro-turbogroup developed by the University of Roma 1, co-funded within the frame of a National Research Project, conceived as a portable energy conversion system delivering electricity to a broad range of small devices, for medical, military or emergency use. The expected electrical power output is 400 watts. The turbogroup, fuelled by natural gas, has a single-shaft configuration and is coupled with a reversible electric motor-generator, granting a more compact overall system, encapsulated in a relatively small self-contained assembly. The compressor wheel has an outlet diameter of 38 mm, spins at 175,000 rpm and delivers the fluid to the vaneless diffuser endowed with a static pressure of 155 kPa. The mass flow rate is 0.02 kg/s, and the total inlet temperature is 300 K and the total temperature of the flowing out fluid is about 400 K. The rotor, featuring 6 full- and 6 splitter blades, is coupled with a vaneless diffuser followed by a toroidal volute that collects the almost purely radial fluid at diffuser exit and delivers it to the regenerator. The numerical simulation based upon the k-ε turbulent model has been run within the commercial software Fluent®, on a very fine mesh constructed on a geometry obtained from a commercial turbocharger manufacturer. A critical analysis has been performed on the turbulent structures that evolve within the rotor channel, to better understand the dissipative mechanisms. To this goal, the thermal and viscous entropy generation rate, have been computed on the converged solution of the thermal-fluiddynamic field, and their maps have been inspected to study the development of turbulent structures. The results of this study confirm that an accurate analysis of the local entropy generation rates provides useful hints on how to introduce design improvements and to achieve a higher stage efficiency.

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