Effects of seal installation in the mid-passage gap between turbine blade platforms on film cooling

Abstract

Hot combustion gas in gas turbines can be ingested into the mid-passage gap between the blade platforms, resulting potentially a catastrophic damage. To prevent the ingress of hot gas, the blade platform underneath cavity is pressurized with purge air. Seals are typically installed in the mid-passage gap to minimize the purge air requirement. Under inadequate back flow margin, ingress of the main flow can occur especially at the upstream region of the mid-passage gap due to the pressure distribution characteristics on the platform. Excessive leakage flow can significantly reduce the turbine performance. This study investigated the effect of a seal installed in the mid-passage gap on film cooling effectiveness and heat transfer on the platform. Experiments were conducted by varying the ratio of the leakage flow to the main flow by 0.2%, 0.4%, and 0.6%, in which the mid-passage gap was unblocked (no seal; Opening 100% case) and partially blocked (seal; opening 50% case). The film cooling effectiveness of the platform was measured using the pressure sensitive paint method, and the heat/mass transfer coefficient of the platform was obtained using the naphthalene sublimation method. The seal in the mid-passage gap reduced or prevented completely the ingress of main flow under the given conditions. Under a high blowing ratio, the seal had little effect on the platform film cooling effectiveness. At the same mass flow rate of leakage flow, the heat transfer distributions were similar, regardless of whether a seal was installed. However, the platform with a partial seal in the opening 50% case showed a peak in the heat transfer closer to the gap than the platform without the seal, as the leakage flow reattached closer to the gap.