Comparison of measurements and predictions for blowing from a turbine blade tip

The clearance gap between the tip of a turbine blade and the shroud has an inherent leakage flow from the pressure side to the suction side of the blade. This leakage flow of combustion gas and air mixtures leads to severe heat-transfer rates to the blade tip of the high-pressure turbine. The subject of this paper is the cooling effectiveness levels that result from blowing through two holes placed in the forward region of a blade tip. These holes are referred to as dirt purge holes and are generally required for manufacturing purposes and for expelling dirt from the coolant flow when, for example, operating in sandy environments. A direct comparison of experimentally measured and computationally predicted adiabatic effectiveness levels for different blowing ratios and tip gaps show good agreement for the small tip clearance, but relatively poor agreement for a large tip clearance, which is attributed to an overprediction of the jet separation and attachment phenomena. Results indicate that the cooling effectiveness is highly dependent upon the tip clearance with good cooling achieved at low coolant flows (0.19% of the passage flow) for a small tip clearance. Even at twice the coolant flow (0.38% of the passage flow) the blade tip for a large tip clearance appeared to have much less cooling benefit from the dirt purge jets relative to the small tip clearance.