Investigation On The Backface Clearance Flows In Deeply Scalloped Radial Turbines

Deeply scalloped radial turbine rotors "scallop" their disk to reduce weight and centrifugal stress, and are widely used in modern microturbines. The blade backface clearance and the backface cavity clearance are introduced due to the scalloped disk. The backface clearance flows lead to a complicated secondary flow structure in the blade passage, and what is worse, the high thermal load in the blade backface clearance and the hot gas ingestion in the backface cavity clearance are unfavorable for the turbine rotor durability. In this paper, the flow structure and heat transfer of the backface clearance flows in deeply scalloped radial turbines are numerically investigated, and the main contents are as follow:1. The secondary flow structure due to the clearance flows in the blade passage is investigated, and their influences on the blade outlet parameters are also discussed. It is found that the tip clearance flow, blade backface clearance flow and backface cavity clearance flow interact with each other and the secondary flow structure in the blade passage is complicated. The clearance flow can affect the flow angle at the outlet and reduce the efficiency which has a great impact on the distribution of blade outlet parameters.2. The flow structure in the blade backface clearance is investigated and the flow mechanism in the clearance is analyzed. It is shown that the radial velocity and Coriolis force are reduced significantly near the disk rim, and the pressure difference across the blade backface clearance decreases drastically. As a result, the leakage flow and the scraping flow interact with each other in the blade backface clearance, and the complex flow separation, reattachment, recirculation and impingement are also found.3. Based on the flow structure in the blade backface clearance, the heat transfer in the blade backface region is analyzed, and methods are proposed to reduce the thermal load in this region. It is illustrated that the distribution of heat transfer coefficient in the blade backface clearance is mainly governed by the flow reattachment and impingement. Although the leakage loss increases with increasing the clearance height, a larger clearance height can decrease the high heat transfer coefficient due to the impingement effect of the scraping flow. By using the suction side squealer geometry, the velocity and the flow reattachment in the blade backface clearance decrease, and the heat transfer coefficient on the blade backface is reduced remarkably.4. The backface cavity clearance flow is investigated and its influence on the turbine efficiency, axial thrust, cooling and sealing effectiveness is analyzed for three sealing geometry schemes. It is found that, although the sealing geometry with a larger disk radius has the relatively higher turbine efficiency, the axial thrust increases and the cooling and sealing effectiveness is very low. With the same disk radius, the sealing geometry with a radial clearance shows a better performance on cooling and sealing effectiveness than that with an axial clearance.