Design optimization of unshrouded rotor tip cavity of a high pressure turbine stage with low aspect ratio was carried out to maximize the turbine stage efficiency. Cavity shapes were parameterized with 4 design variables including rim thickness, cavity depth, cavity front blend radius and cavity aft blend radius. Initially the CCD method was utilized for sampling experimental points and the Kriging method was chosen to construct an approximation model. The optimum points derived from the approximation model were assessed by CFD analyses to verify the approximation model. The approximation model was refined repeatedly by adding more experiment points to minimize difference of CFD result and predicted value from the approximation model at the optimum point.
The optimization result showed that there is an optimum ratio of cavity depth to tip clearance height, while the optimum design suggests cavity front blend radius and cavity aft blend radius be as small as possible within the design range. As the tip clearance height increases, the optimized tip cavity depth increases. However, the rim thickness has little effect on the optimum tip cavity depth. Without the tip cavity, leakage flow at fore part of the blade suction surface develops large vortex flow from the starting point of the unguided turning region due to adverse pressure gradient. The tip cavity prevents the early leakage flow from flow to the suction surface, which suppresses the leakage flow dissipation to the loss. It results in efficiency improvement. The effect of the tip cavity on the efficiency increases at the larger tip clearance.
On the other hand, the cavity rim thickness effect on the efficiency becomes noticeable when the tip cavity depth is over than the optimum value. The rim thickness effect mainly appears on the tip leakage flow after the blade throat. The leaked flow after the blade throat generates a high loss region near the blade tip, especially when the rim thickness is small. The loss from the thick tip cavity rim gradually increases as the tip clearance increases. However, the rim thickness effect is most sensitive when the tip clearance is small. The loss generation mechanism due to the rim thickness is totally different to the tip cavity depth effects on the total pressure loss.