Aerothermal Investigation of Backface Clearance Flow in Deeply Scalloped Radial Turbines

A numerical investigation of flow structure and heat transfer in the backface clearance of deeply scalloped radial turbines is conducted in this paper. It is found that the leakage flow is very strong in the upper radial region whereas in the lower radial region, the scraping flow dominates over the clearance and a recirculation zone is formed. Pressure distributions are given to explain the flow structure in the backface clearance, and it is found that due to the sharp reduction of radial velocity and Coriolis force, the pressure difference in the lower radial region is reduced drastically, which is the mechanism for the domination of the scraping flow and the corresponding recirculation zone. There are two high heat transfer coefficient zones on the backface surface. One is located in the upper radial region due to the reattachment of the leakage flow and the other is located in the lower radial region caused by the impingement of the scraping flow. Increase of the clearance height reduces the high heat transfer coefficient caused by the impingement of the scraping flow, although it increases the leakage loss. On the other hand, the high heat transfer coefficient in the upper radial region can be reduced remarkably by using the suction side squealer geometry.