FILM COOLING ON HIGHLY LOADED BLADES WITH MAIN FLOW SEPARATION - PART 1: HEAT TRANSFER

摘要

Film cooling experiments were run at the high speed cascade wind tunnel of the University of the Federal Armed Forces Munich. The investigations were carried out with a linear cascade of highly loaded turbine blades. The main targets of the tests were to assess the film cooling effectiveness and the heat transfer in zones with main flow separation. Therefore the blades were designed to force the flow to detach on the pressure side shortly downstream of the leading edge and it reattaches at about half of the axial chord. In this zone film cooling rows are placed among others for reduction of the size of the separation bubble. The analyzed region on the blade is critical due to the high heat transfer present at the leading edge and at the reattachment line after main flow separation. Film cooling can contribute to a reduction of the size of the separation bubble reducing aerodynamic losses but increases in general heat transfer due to turbulent mixing. The reduction of the size of the separation bubble might also be twofold since it acts like a thermal insulator on the blade and reducing the size of the bubble might lead to stronger heating of the blade. Film cooling should therefore take into account both: firstly a proper protection of the surface and secondly reduce aerodynamic losses diminishing the extension of the main flow separation. While experimental results of the adiabatic film cooling effectiveness were shown in previous publications, the local heat transfer is analyzed in this paper. Emphasis is also put in analyzing in detail the flow separation process. The tests comprise furthermore the analysis of the effect of different outlet Mach and Reynolds numbers and film cooling. In part two of this paper the overall film cooling effectiveness is addressed. Local heat transfer is still difficult to predict with modern numerical tools and this is especially true for complex flows with flow separation. Some numerical results with RANS and LES show the capability of a commercial solver in predicting the heat transfer.