The transient response of a radial turbine stage with a variable geometry system is evaluated. Mainly, the consequences of the variations of the aerodynamic performance of the stage on the response time are checked. A simple quasi-steady model is derived in order to formalize the expected dependences. Then an experimental campaign is conducted: a brutal step in the feeding conditions of the stage is imposed, and the response time in terms of rotational speed is measured. This has been reproduced on different declinations of the same stage, through the variation of the stator geometry, and correlated to the steady-state performance of the initial and final operating points of the transient phase. The matching between theoretical expectation and results is surprisingly good for some configurations, less for others. The most important factor identified is the mass-flow level during the transient phase. It increases the reactivity, even far above the theoretical expectation for some configurations. For those cases, it is demonstrated that the quasi-steady approach may not be sufficient to explain how the transient response is set. 1. Introduction Many applications make use of the interesting properties of radial inflow turbines, namely, the compactness and the ability of reaching high pressure ratios for acceptable efficiencies. The description of the radial turbines operating in steady flows has been widely presented in the literature [1, 2], together with an extensive description of the flow pattern, even for off-design conditions (e.g., in [3]). Some design processes have also been presented to ensure the good performance of the energy-recovering process of the stage (see [4, 5]). The favourable pressure gradient due to the expansion of the flow through the turbine stage stabilizes the boundary layer behaviour and makes separations unlikely if the guidance of the flow is appropriate. As a consequence, a one-dimensional description of the flow is adequate to reach an accurate description of the radial turbine functioning [4]. This is transposable to operational and geometric off-design operation [6]. But the problem gets more complex when the inlet conditions are time dependent. Most of the contributions analysing radial turbines operating in unsteady flows are found in the literature dedicated to possible turbocharging applications. The turbine stage experiences some severe pulsating inlet conditions due to its position in the exhaust manifold. The research activity in that field is currently very active. A review of the state-of-the-art and of the main
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