Film cooling performance in a simulated blade tip geometry

One of the most problematic areas in gas turbine engines is the blade tip region, especially near the trailing edge, where it is very difficult to provide sufficient cooling. In all configurations with unshrouded tips, a clearance gap exists between the turbine blade and the outer shroud. The pressure difference between the suction and pressure sides of the blade drives a sink-like flow through this gap. The combination of leakage flow from the freestream and coolant flow induces high convective heat transfer coefficients on the blade tip surface. The resultant thermal loading can be significant and detrimental to the turbine blade tip durability, leading to early failure. Film cooling can be provided by means of a series of holes located on the tip itself providing protection not dissimilar to film cooling of the main blade. However the interaction of coolant and the separation bubble resulted in a significantly different film cooling performance to that of non-tip cases. An experimental investigation of the simulated turbine blade tip is presented in here. The first section discusses PIV flow field measurements, the second covers the measurement of film cooling effectiveness and the third heat transfer measurements. All three parts investigate the effect of using different film cooling injection points and blowing ratios for injection on the blade tip itself close to the pressure surface corner Additionally, the effect of varying the corner radii between the pressure surface and the tip is reported. The experimental method uses the steady state liquid crystal technique. A Reynolds number of 30,000 based on the clearance gap hydraulic diameter for the mainflow was used.