In the application of turbocharging and organic Rankine cycles (ORCs) the radial turbine can operate at high pressure ratio leading to the presence of choking inside the turbine. Information in this map region is needed to estimate the overall machine performance and to achieve an optimum matching of engine-turbocharger or an integration into ORCs. As a preliminar study before doing experimental tests in a turbocharger gas stand and before starting modeling activities, 3D CFD simulations were run for this paper. The occurrence of choking in the stator passage, in the vaneless space and in the rotor passage is categorized and analyzed under different operating conditions and VGT openings. The appearance of the shock wave depends strongly of the stator vane position and pressure ratio. In the analyzed turbocharger VGT turbine no choking has been identified in the throat of the stator vanes. In technically relevant closed VGT position choking was identified in the vaneless space. Whereas at open vane position the shock wave appears at the rotor throat and the Mach number can increase from the blade tip to the hub or from the hub to the tip depending on the rotational speed. Furthermore, tip leakage flow plays an important role, being responsible of subsonic regions in the rotor outlet although a big percentage of the section is choked.

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