Film-Cooling effectiveness of combined hole slot geometries on a turbine rotor blade surface

Abstract

A film cooling experiment was conducted for a novel hole structure, namely, the combined hole slot. The basic geometric shape of the combined hole slot is connecting adjacent sidewalls of two parallel diffuser holes internally to form a continuous slot exit. Two combined hole slot geometries constructed based on circular and vertical slot cross-sections were tested and compared with a conventional fan-shaped hole. The film-cooling effectiveness was measured on a turbine rotor blade surface using a linear cascade and pressure sensitive paint technique. The mainstream Reynolds number was 520,000 based on the cascade outlet velocity and the axial chord length. The mainstream turbulence intensity was 3.6%. Coolant-to-mainstream density ratios of 1.5 and 1.0 were tested under five blowing ratios ranging from 0.5 to 2.5. Although an obvious bifurcation occurs at the hole exit, both combined hole slot geometries remarkably increase the film-cooling effectiveness relative to that of the fan-shaped hole, in which the case based on a vertical slot cross-section performs better than the case based on a circular cross-section, regardless of the density ratio. In comparison, the increase in film effectiveness on the suction surface due to the combined hole slot geometries is greater than that on the pressure surface, especially for medium to high blowing ratios. The combined hole slot geometry is generally more suitable for using branch holes with a weak lateral expansion, in which a tighter merging effect can be achieved at the centerline.