Optimization of turbine squealer tip cooling design by combining shaping and flow injection

In this study, a turbine squealer tip is optimized by a multiobjective genetic algorithm (MOGA) with varying the squealer heights and the tip cooling configurations. The three objectives selected are the aerodynamic efficiency, the film cooling effectiveness and the surface fluid temperature variance. The multiscale methodology is implemented to reduce the computational cost and to skip the meshing of cooling holes. Two optimization approaches are compared: a) a conventional method that optimizes an uncooled shape first and then the cooling configuration sequentially, and b) a method that optimize shaping and cooling concurrently. The concurrent method is found to obtain a heat transfer performance that is not achieved by the conventional optimization. Moreover, by adding the cooling, the performance ranking of the uncooled blades in terms of the aerodynamic efficiency is changed. These observations are due to the strong interaction between the coolant and the tip leakage flow. They indicate that the coolant injected at the tip is not passive as expected in the conventional film cooling designs. By altering the tip leakage flow structure, the coolant can reduce the tip leakage loss, which contradicts the conventional wisdom that the added coolant should always lead to extra losses due to the extra mixing. More detailed observations of the flow field indicate that the influence of the squealer height towards the aerodynamic efficiency is caused by two competing effects: the blockage effect to reduce the tip leakage mass flow rate and the sudden expansion loss effect to generate additional losses. The heat transfer performance can be significantly influenced by increasing the squealer height because of the trapped coolant in the cavity.