Coupling optimization of casing groove and blade profile for a radial turbine

In order to suppress the tip leakage loss and increase the isentropic efficiency in a radial turbine with low aspect ratio blade, a new coupling flow control technology including casing groove and NACA blade profile is proposed and optimized numerically. The coupling effects of casing groove size L-S, groove number Num(slot), maximum thickness location of profile MLT and leading parameter of profile LP on isentropic efficiency of a radial turbine are obtained with Design of Experiment (DOE) and response surface model. A genetic optimization is achieved and the coupling flow control mechanism is revealed. Complex variation of isentropic efficiency is observed with the coupling effect of groove size and groove number, and maximum isentropic efficiency variation of 0.6% is obtained with the increase of L-S when Num(slot) is 3. The coupling effect between L-S and LP, is also significantly influenced by groove number. The lower limit of LP for maximum isentropic efficiency is decreased from 7 to 3 with the increase of groove number. The optimization result illustrates that the isentropic efficiency can be increased by 0.9% at design point when the L-S, Num(slot), MLT and LP are 0.8 mm, 2, 0.1, and 0.8 respectively. A maximum isentropic efficiency increment of 2.0% is also found when total pressure ratio is 2.1. The optimal casing groove forms a flow opposite to the tip leakage flow to suppress the impact range of the secondary flow near suction surface. At the same time, the optimal NACA profile also decreases the tip leakage velocity near trailing, suppresses the tip leakage vortex generation, and reduces the trailing edge loss.