Effects of Inlet Conditions on the Turbine Performance of a Radial Turbine

Abstract

For a turbocharger working under internal combustion engine operating conditions, the flow will be highly pulsatile and the efficiency of the radial turbine will vary during the engine cycle. In addition to effects of the inflow unsteadiness, there is also always a substantial unsteady secondary flow component at the inlet to the turbine depending on the geometry upstream. These secondary motions may consist of swirl, Dean vortices and other cross-sectional velocity components formed in the exhaust manifold. The strength and the direction of the vortices vary in time depending on the unsteady flow in the engine exhaust manifold, the engine speed and the geometry of the manifold itself. The turbulence intensity may also vary during the engine cycle leading to a partially developed turbulent flow field. The effect of the different perturbations on the performance of a radial nozzle-less turbine is assessed and quantified by using Large Eddy Simulations. The turbine wheel is handled using a sliding mesh technique, whereby the turbine wheel, with its grid is rotating, while the turbine house and its grid are kept stationary. The turbine performance has been compared for several inflow conditions. The results show that an inflow-condition without any perturbations gives the highest shaft power output, while a turbulent flow with a strongly swirling motion at the inlet results in the lowest power output. An unexpected result is that a turbulent inflow yields a lower shaft power than a turbulent inflow with a secondary flow formed by a pair of Dean vortices. The flow field for the different cases is investigated to give a better insight into the unsteady flow field and the effects from the different inlet conditions.