Numerical investigation on effects of uncertain nozzle vane clearance on variable geometry radial turbine performance

In order to evaluate the effects of uncertain nozzle vane hub and shroud clearances on the performance in a variable geometry radial turbine, firstly, the numerical results of turbine for fully open nozzle vanes condition were compared with experimental data, and then several steady numerical simulations were carried out for the turbine under partially open nozzle vanes condition to predict the differences on turbine performance induced by uncertain hub/shroud clearances. At last the unsteady computations were performed for the even clearance model and peak efficiency model to study the aerodynamic interaction between nozzle vanes and rotor. The results show that the turbine efficiency reaches up to the maximum when the clearance value near the shroud wall is roughly 5%~15% of total clearance value while the minimum efficiency appears when all the clearance is concentrated on the shroud side. The difference between maximum and minimum efficiency is almost 6%. The clearance leakage flow along with different clearance model alters inlet flow angle to downstream rotor, and changes the loss for the rotor suction surface separation vortex. Moreover, the hub and shroud uncertain clearances configuration of nozzle vanes can add the risk of rotor blade failure by means of changing the shock wave stood on the suction surface of nozzle vane, which leads to fluctuating load to the rotor blade.