Numerical analysis of blade tip leakage flow and shroud heat transfer in gas turbine engines

摘要

One of the most critical components of gas turbine engines, rotor blade tip and casing, is exposed to high thermal load. It is a significant challenge to the designer to protect the turbine material from this severe situation. Leakage flow over the blade tip is also one of the important issues to improve the turbine performance. To understand the detailed phenomena and natures of the heat transfer on the turbine blade tip and casing in association with the tip leakage flow under actual turbine operating conditions, both steady and unsteady simulations have been conducted. A single stage gas turbine engine was modeled and simulated using commercial CFD solver ANSYS CFX R.11. The modeled turbine stage has 30 vanes and 60 blades with a pressure ratio of 3.2 and a rotational speed of 9500 rpm. The predicted isentropic Mach number and adiabatic wall temperature on the casing showed good agreement with available experimental data under the close operating condition. Through the steady simulations, the typical tip leakage flow structures and heat transfer rate distributions were analyzed. The tip leakage flow separates and recirculates just around the pressure side edge of the blade tip. This coverage of the recirculating flow results in low heat transfer rates on the tip surface. The leakage flow then reattaches on the tip surface beyond the flow separation zone. This flow reattachment has shown enhanced heat transfer rates on the tip. The leakage flow interaction with the reverse cross flow, induced by relative casing motion, is found to have significant effect on the casing heat transfer rate distribution. Critical region of high heat transfer rate on the casing exists near the blade tip leading edge and along the pressure side edge. Whereas near the suction side the heat transfer rates are relatively low due to the coverage of the reverse cross flow. The effects of the tip clearance heights and rotor rotating speeds were also investigated. The region of recirculating flow increases with the increase of clearance heights. The flow incidence changes and the casing relative motion is enhanced with higher rotation speeds. As a result, the high heat transfer rate regions have been changed with these two parameters. Unsteady simulations have been performed to investigate time dependent behaviors of the leakage flow structures and heat transfer on the rotor casing and blade tip. The effects of different time steps, number of sub iteration and number of rotor vane passing were firstly examined. The periodicity of the tip leakage flow and heat transfer rate distribution is observed for each vane passing. The relative change in the position of the vane and the vane trailing edge shock alters the inlet flow conditions of the rotor part. It results in the periodic variations of the leakage flow structures and heat transfer rate distributions. The higher heat transfer rates were observed at the region where the trailing edge shock reached. The maximum amplitude of the pressure fluctuation in the tip region is about 20% of the averaged rotor inlet pressure. The maximum amplitude of the heat transfer rate fluctuation on the blade tip, caused by the unsteady leakage flow variations, reaches up to about 25% of the mean heat transfer rate. The effects of tip clearance heights and rotor speeds have also been analyzed and compared one with respect to others. Same typical patterns of leakage flow structures and heat transfer rate distribution can be obtained in both steady and unsteady simulations. However, steady simulation underpredicted the highest heat transfer rate. Because it couldn't capture the critical local high heat transfer phenomena caused by the unsteady stator-rotor interactions