Treatment and optimization of casing and blade tip for aerodynamic control of tip leakage flow in a turbine cascade

Abstract

Various shape designs for tip clearance contribute to leakage control in turbomachinery whereas few studies so far focus on the combination treatments of casing and blade tip, their smoothness and continuousness as well. A novel circumferential casing and blade tip treatments based on the bimodal Gaussian function curve for suppression of tip leakage flow have been numerically studied in a linear turbine cascade. This configuration is then optimized by applying the Kriging model and orthogonal design for further loss reduction. As the main aerodynamic parameters, leakage flow and total pressure loss coefficient are evaluated with three configurations: flat-tip without casing treatment (BASE), casing and blade tip treatments (CBTT) and its optimization result (OPT). Compared with BASE, the results in CBTT and OPT indicate that under the effect of blocking vortex (BV), the tip leakage vortex (TLV) is divided into two parts. One becomes involved in mixing process with BV, and then joins to the upper passage vortex (UPV) downstream, the other rebuilds behind bimodal peaks. The further optimization would lead to a sharp decline of loss in CBTT and a better result in OPT. Compared with the baseline case, a reduction of 5.39% in relative total pressure loss could be noticed in the CBTT case while a decreasing of 10.87% found in the OPT case, as a result of the earlier blocking effect.