Numerical investigation of the effect of different nozzle clearance at hub and shroud side on variable nozzle turbine performance

It is clear that when the nozzle vanes opening is adjusted to minimum area, the increased leakage flow has greater influence on the performance of variable nozzle turbine(VNT). While working with the engine, the nozzle clearances will not remain fixed and same with those under the design condition. In this paper 3-D numerical simulations of a VNT were performed to investigate the effect of different nozzle vane clearances at hub and shroud side on the turbine performance. The steady simulation results show that the turbine efficiency can reach to maximum while the shroud clearance value is about 10% of total clearance and then decrease with the increase of nozzle shroud gap. The VNT internal flow was analyzed for two typical ratios of clearance at shroud side to total clearance. It shows that although a non-uniform flow distribution at nozzle exit for the ratio of 0.15, it reduce the flow loss on the rotor suction side compared to that for equal clearance. It can be illustrated that for the ratio of 0.5, due to the centrifugal force, the low-energy fluids near the rotor hub side will migrate from hub side toward shroud side and more loss is generated during their transportation along rotor passage. The unsteady numerical simulations of this VNT with those two typical ratios were also undertaken. The change of nozzle clearance distribution also plays an important role of shock wave near the vanes exit. Compared to equal clearance at both hub and shroud, the strength and the length of the shock wave can be increased when the ratio value is 0.15, which leads an enhanced unsteady load of turbine rotor near the leading edge. This may increase the risk of HCF failure of the turbine rotor.