Effects of structural parameters of laidback fan-shaped hole on film cooling effectiveness on convex surface

To burden higher turbine inlet temperature, shaped holes are introduced to the film cooling of turbine blade. The structural parameters of shaped holes are the key variables for the cooling design. Relevant literatures conduct studies mainly on the flat surface, which excludes the influence of blade curvature. And their conclusions are inconsistent, appealing for further mechanism study. In this paper, the effects of structural parameters of laid- back fan-shaped hole are experimentally investigated on a convex surface. Pressure sensitive paint technique is used to measure the adiabatic effectiveness. Three parameters, including forward expansion angle (0-12 deg), lateral expansion angle (2-20 deg) and length of the diffuser (2-14 mm), at five blowing ratios (0.5, 0.8, 1.0, 1.3 and 1.5) are tested. Numerical simulation is also employed to present the mechanism of jet-mainstream inter- action. It is shown that, at the blowing ratio over 0.8, the optimal forward and lateral expansion angle lies in the medium range. Cooling effectiveness increases before the optimal value due to better film attachment but de- teriorates after then due to coolant separation within the hole. In contrast, at the blowing ratio of 0.5, the results become different because the coolant attaches well enough that the change of the coolant dissipation rate ap- pears more dominant. The competition of two factors, weaker kidney vortex entrainment and larger shear surface, leads to an unpredictable trend. Finally, suggestions are provided for the parameter selection on the blade suction surface. As a comparison, the same studies are numerically conducted on a flat surface. Distinctly, at high blowing ratio, the effectiveness increases monotonously without a drop at too large expansion because the absence of radial pressure gradient worsens jet attachment and magnifies the effect of promoting jet attachment.