The performances of automotive turbocharger turbines have long been realized to be quite different under pulsating flow conditions compared to that under the equivalent steady and quasi-steady conditions on which the conventional design concept is based. However, the mechanisms of this phenomenon are still intensively investigated nowadays.
This paper presents an investigation of the response of a stand-alone rotor to inlet pulsating flow conditions by using validated unsteady Reynolds Averaged Navier-Stokes solver (URANS). The effects of the frequency, the amplitude and the temporal gradient of pulse waves on the instantaneous and cycle integrated performances of a radial turbine rotor in isolation were studied, decoupled from the upstream turbine volute. Numerical method was used to help gaining the physical understandings of these effects.
A validation of the numerical method against the experiments on a full configuration of the turbine has been performed prior to the numerical tool being used in the investigation. The rotor is then taken out to be studied in isolation. The results show that the turbine rotor alone can be treated as a quasi-steady device only in terms of cycle integrated performance; however, instantaneously, the rotor behaves unsteadily which increasingly deviates from the quasi-steady performance as the local Strouhal number of the pulsating wave is increased. This deviation is dominated by the effect of quasi-steady time-lag; at higher local Strouhal number, the transient effects also become significant. Based on this study, an interpretation and a model of estimating the quasi-steady time lag have been proposed; a criterion for unsteadiness based on the temporal local Strouhal number concept is developed, which reduces to the Λ criterion proposed in the published literature when cycle averaged; this in turn emphasizes the importance of the pressure wave gradient in time.