Effect of film-hole shape on turbine-blade heat-transfer coefficient distribution

Detailed heat transfer coefficient distributions on the suction side of a gas turbine blade were measured using a transient liquid crystal image method. The blade has only one row of film holes near the gill-hole portion on the suction side of the blade. Studies on three different kinds of film-cooling hole shapes were presented. The hole geometries studied include standard cylindrical holes and holes with a diffuser-shaped exit portion (i.e., fan-shaped holes and laidback fan-shaped holes). Tests were performed on a five-blade Linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on the cascade exit velocity was 5.3 x 10(5). Upstream unsteady wakes were simulated using a spoke wheel-ape wake generator. The wake Strouhal number was kept at 0 and 0.1. The coolant-to-mainstream blowing ratio was varied from 0.4 to 1.2. The results show that unsteady wake generally tends to induce earlier boundary-layer transition and enhance the surface heat-transfer coefficients. When compared to the cylindrical hole case, both the expanded hole injections have much lower heat-transfer coefficients over the surface downstream of the injection location, particularly at high blowing ratios. However, the expanded hole injections induce earlier boundary-layer transition to turbulence and enhance heat-transfer coefficients at the latter part of the blade suction surface.