EXPERIMENTAL INVESITIGATIONS AND NUMERICAL ANALYSIS ON HEAT TRANSFER OF A NGV ENDWALL AT ENGINE REPRESENTATIVE REYNOLDS AND MACH NUMBERS

In this work, investigations on detailed heat transfer characteristics over an endwall surface for a turbine nozzle guide vane were conducted. Experiments were performed in a linear cascade test facility using infrared thermography technique over a range of engine-representative inlet Reynolds numbers from 1.40×10 to 4.20×10 with a high free-stream turbulence level. Accordingly, numerical simulations were carried out to be compared and complementary to the measurements. Besides, in order to demonstrate the effects of exit Mach number on endwall heat transfer in isolation, simulations were also conducted at various vane exit Mach numbers ranging from 0.616 to 1.016 while the inlet Reynolds number was kept constant at the design point of Rein=3.46×10. High heat transfer coefficients were observed in the leading edge region and near and downstream of the throat areas. Increasing inlet Reynolds number could result in higher heat transfer level on the endwall surface. Numerical results agreed well with the measurements, and the predicted flow field in the vane passage highlighted the links between the heat transfer patterns and the flow structures near the endwall. The computational results also revealed that vane exit Mach number mainly dominated the heat transfer in the aft part of the endwall and higher Mach numbers led to lower heat transfer coefficients. INTRODUCTION Great efforts have been devoted to thermal analysis and designs of gas turbine stages in order to increase the cycle efficiency of gas turbines continuously. In particular, though it is a technical challenge to measure the thermal loads in a real engine vane passage, accurate knowledge of local thermal load patterns under engine-representative conditions is critical for engine designers to validate and optimize the cooling schemes and further to ensure the durability and reliability of the designs. This demonstrates the importance of providing accurate experimental data related to each specific configuration for engine designer. The flow inside a vane passage is characterized by highly three-dimensional and complex flow structures due to the generation of secondary flows and the radial and circumferential pressure gradient, which have strong impacts on flow development and heat transfer on the vane endwalls. Previously, many studies documented the influence of those vortical structures on endwall heat transfer and the detailed reviews regarding the flow and heat transfer issues near the endwall of a vane passage were made by Chyu (2001) and Simon et al. (2006). To date, the relationship between the secondary flow and heat transfer as well as cooling is attracting constant attention over the years. Blair (1974) could be the pioneer to detect the heat transfer over endwalls and discovered the highly strong links to the vortical structures evolving within the passage. Later, the links was confirmed by the work of Takeishi et al. (1990). With the deep understanding of the secondary flow mechanism in the vane passage, Friedrich (1997) summarized the common understanding of high heat transfer coefficient regions on the endwall based on the existing experimental work. Those regions are 1) the leading edge where the horseshoe vortex (HV) forms, 2) the suction surface shoulder where the lift-off line of the suction side of the HV and the dominated passage vortex meet, 3) the pressure surface corner where the boundary layer is thin due