The Effect of Clearance Flow of Variable Area Nozzles on Radial Turbine Performance

Abstract

The flow behind the variable area nozzle for radial turbines was measured with a 3-hole yaw probe and calculated with CFD. Two nozzle throat-areas were investigated: the smallest and the largest opening for the variable nozzle. Test results agreed with the calculated results qualitatively. The leakage flow through the tip clearance of the nozzle vane significantly affected the flow field downstream of the nozzle vane with the smallest opening. However, the effect on leakage flow on the flow field downstream of the nozzle vane with the largest opening was very weak. In the flow field of the largest opening nozzle, the effect of wake s dominant. The effect of the clearance of the nozzle vane on the turbine performance was estimated by a 1D-model and the strong influence on the turbine efficiency was confirmed at smallest opening. The flow fields in the impeller downstream of the nozzle vane at the smallest opening with and without the nozzle clearance were investigated with CFD. The setting angle of the nozzle vane without clearance was adjusted to match the operating point of the turbine with the nozzle clearance. In order to extract the specific work from the impeller, the nozzle vane with the vane clearance requires the larger vane setting angle than that without clearance. The increase of the vane setting angle increases the incidence loss and deteriorates turbine efficiency.