Laboratory studies of film cooling performance for turbine section airfoils typically quantify adiabatic effectiveness and occasionally the heat transfer coefficient for the film cooling configuration. In this study the normalized airfoil metal surface temperatures are obtained directly by using a test model that has a material conductivity scaled to the external and internal heat transfer coefficients so that the Biot number for the model is similar to that for the actual airfoil. These results provide an experimental test case of the conjugate heat transfer involved in turbine airfoil cooling. In this study, conventional adiabatic effectiveness and the overall cooling effectiveness (normalized surface temperature for the matched Biot model) were measured for a generic blade leading edge using three rows of shaped holes. Distinct differences were found between the adiabatic effectiveness and overall cooling effectiveness. Also included is a practical application of this experimental method for which the degradation of overall cooling effectiveness due to a plugged cooling hole is examined.

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The clearance gap between a turbine blade tip and its associated shroud allows leakage flow across the tip from the pressure side to the suction side of the blade. Understanding how this leakage flow affects heat transfer is critical in extending the durability of a blade tip, which is subjected to effects of oxidation and erosion. This paper is the second of a two-part series that discusses the augmentation of tip heat transfer coefficients as a result of blowing from film-cooling holes placed along the pressure side of a blade and from dirt purge holes placed on the tip. For the experimental investigation, three scaled-up blades were used to form a two-passage, linear cascade in a low-speed wind tunnel. The rig was designed to simulate different tip gap sizes and film-coolant flow rates. Heat transfer coefficients were quantified by using a constant heat flux surface placed along the blade tip. Results indicate that increased film-coolant injection leads to increased augmentation levels of tip heat transfer coefficients, particularly at the entrance region to the gap. Despite increased heat transfer coefficients, an overall net heat flux reduction to the blade tip results from pressure-side cooling because of the increased adiabatic effectiveness levels. The area-averaged results of the net heat flux reduction for the tip indicate that there is (i) little dependence on coolant flows and (ii) more cooling benefit for a small tip gap relative to that of a large tip gap.

One significant issue concerning the impingement heat transfer with a jet array is related to the so-called “crossflow”, where a local jet performance is influenced by the convection of the confluence from the impingement of the jet/jets placed upstream. As a result, the heat transfer coefficient may vary along the streamwise direction and creates more or less nonuniform cooling over the component, which is undesirable from both the performance and durability standpoints. Described in this paper is an experimental investigation of the heat transfer coefficient on surfaces impinged by an array of six inline circular jets with their diameters increased monotically along the streamwise direction. The local heat transfer distributions on both the target surface and jet-issuing plate are measured using a transient liquid crystal technique. By varying the jet hole-size in a systematic manner, the actual distribution of jet flow rate and momentum within a jet array may be optimally metered and controlled against crossflow. The effects on the heat transfer coefficient distribution due to variations of jet-to-target distance and inter-jet spacing are investigated. The varying-diameter results are compared with those from a corresponding array of uniform jet diameter.

Durability of turbine blade tips has been and continues to be challenging, particularly since increasing turbine inlet temperatures is the driver for improving turbine engine performance. As a result, cooling methods along the blade tip are crucial. Film-cooling is one typically used cooling method whereby coolant is supplied through holes placed along the pressure side of a blade. The subject of this paper is to evaluate the adiabatic effectiveness levels that occur on the blade tip through blowing coolant from holes placed near the tip of a blade along the pressure side. A range of blowing ratios was studied whereby coolant was injected from holes placed along the pressure side tip of a large-scale blade model. Also present were dirt purge holes on the blade tip, which is part of a commonly used blade design to expel any large particles present in the coolant stream. Experiments were conducted in a linear cascade with a scaled-up turbine blade whereby the Reynolds number of the engine was matched. This paper, which is Part 1 of a two part series, compares adiabatic effectiveness levels measured along a blade tip, while Part 2 combines measured heat transfer coefficients with the adiabatic effectiveness levels to assess the overall cooling benefit of pressure side blowing near a blade tip. The results show much better cooling can be achieved for a small tip gap compared with a large tip gap with different flow phenomena occurring for each tip gap setting.