Radial turbines are frequently submitted to unsteady inlet flows, for example in turbocharging applications. Complex flows dominated by waves propagation take place, and advanced methodologies are required. Such complexity is hardly compatible with industrial constraints and design time scales. Also, the validity of the usual performance indicators, such as efficiency, is questionable in unsteady flows. However, the need for simplification led the community to develop modeling strategies for unsteady effects, based on hypotheses. One of those is that the rotor flow is assumed quasi-steady. This assumption is assessed by different criteria of the literature. It also enables an adaptation of performance indicators such as efficiency and pressure ratio. But the validity of such an assumption is still under discussion. The present paper is a contribution to this discussion. It focuses on a physical analysis of the physics involved in unsteady flows and the consequences that it produces on the instantaneous performance. Unsteady numerical simulations are analyzed, performed on a realistic radial turbine stage which was submitted to different transient phases. An instantaneous over-shoot of the turbine torque is observed for some transient regimes. Such a result demonstrates that the rotor flow is not immune to unsteady effects. A discussion is then conducted around the possible validity of the quasi-steady assumption for the rotor. This discussion includes the different criteria already found in the literature, and the alternative formulation proposed in this contribution, based on the ratio of time scales.

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