Numerical investigation of the unsteady aerodynamics of blade tip leakage flow inside gas turbine engines

In today's modern gas turbine engines, the region between the rotor and the stationary shroud has the most extreme fluid-thermal conditions in the entire turbine, and is characterized by a periodically unsteady three-dimensional flow field. Due to the pressure difference across the blade tip, leakage flow enters the gap region from the pressure side and exits from the suction side. Tip leakage flow consists of hot mainstream gas and is highly undesirable since it does not turn, and so does not produce any work. Also, high heat transfer rates in the tip gap region occur as a result of leakage flow due to the formation of very thin boundary layers, which may lead to over-heating of the stationary shrouds. The purpose of the present work is to conduct an unsteady study of the tip leakage flow adjacent to the shroud in real gas turbine engines using an in-house industrial computational fluid dynamics (CFD) code. A turbine stage consisting of the nozzle guide vane (NGV) and rotor was modeled. The effect of tip clearance height, inlet turbulence intensity, inlet total temperature, and rotor angular velocity on the tip leakage aerodynamics will be investigated. To the best of the author's knowledge, time-accurate simulations have not been performed in order to study the effects of flow parameters on tip leakage flow aerodynamics. In addition, the trials of using a commercial CFD package to obtain heat transfer calculations on the shroud will be presented. It was found that the size of the separation bubble on the pressure side of the blade tip is dependent on the inlet total temperature and rotor angular velocity. Also, when the relative height of the separation bubble is large, a small re-circulation zone was found at the suction side of the blade tip. In all cases, flow re-circulation was found near the trailing edge and was due to the combined effect of the shroud relative motion with the secondary cross-flow from the adjacent blade passage.