Film cooling characteristics on blade platform with a leakage flow through mid-passage gap

Abstract

Each turbine rotor assembly is composed of several blades, and there is a mid-passage gap between blade platforms. Purge air is supplied through the mid-passage gap to prevent the ingress of hot combustion gas which could lead to thermal damage to the blade platforms. The leakage flow (coolant) egested through the mid-passage gap affects the heat transfer of the blade platform and provides a cooling to the platform. This study investigated the effects of leakage flow on film cooling effectiveness and heat transfer on the platform with a mid-passage gap. The effects of main flow ingestion through the gap on the film cooling effectiveness and heat transfer on the platform were measured for different mass flow ratios (MFRs) of the leakage flow. The film cooling effectiveness was measured using the pressure sensitive paint method, and the heat/mass transfer was measured using the naphthalene sublimation method. The measured mass flow ratios of the leakage flow to the main flow were 0.2%, 0.4%, and 0.6%. At low mass flow ratios (MFR = 0.2%, 0.4%), the main flow ingested under the upstream region of the mid-passage gap and the leakage flow egested through the downstream region of the gap. The egested leakage flow provided a high film cooling effectiveness on the suction side downstream region. However, the leakage flow was egested from the gap with an injection angle of 90° and reattached to the surface, resulting in a high heat transfer in the suction side region near the gap. At a high mass flow ratio (MFR = 0.6%), the leakage flow egested through the entire mid-passage gap, but the leakage flow penetrated into the main flow resulted in a low film cooling effectiveness. As the leakage flow interacted with the main flow and formed a vortex, this resulted a region with film cooling effectiveness decrease and heat transfer increase.