Contribution to the understanding of cold pulsating flow influence in the efficiency of small radial turbines for turbochargers

In the present paper an unsteady approach to determine the performance of a small radial inflow turbine working under cold pulsating flow is presented. It has been concluded that a reasonably good characterization of turbine behavior working with pulsating flow can be obtained using in a quasi-steady way models of the turbine isentropic efficiency and turbocharger mechanical efficiency. Both models have been fitted using data obtained from a steady flow characterization procedure. Turbocharger measured parameters from the cold pulsating flow campaign have been compared with the ones obtained from one-dimensional gas-dynamics computational modeling. The modeling approach is based on quasi-steady isentropic and mechanical efficiency models. Reasonably good accuracy in compressor and turbine variables prediction has been obtained for most of the operative conditions. Influence of amplitude and frequency of the pulsating flow over the instantaneous and average turbine efficiency has been studied to put some light on the analysis of the involved physical phenomena. The main conclusion is that the biggest effect of unsteady flow on turbine efficiency is through the influence on blade jet to speed ratio. It has been also concluded that for the same average blade to jet speed ratio pulses amplitude does not influence turbine efficiency when it is closed but yes at other VGT positions. The effect of pulses frequency is less evident and only influences VGT performance at the highest VGT openings.