Detailed heat transfer distributions of narrow impingement channels for cast-in turbine airfoils

Gas turbine operation at elevated temperatures ensures increased thermal efficiency and useful power output. However, the industrial tendency to push further firing temperatures is limited by the capabilities of the current turbine blade cooling technologies which approach their limits. Nowadays, the capabilities of the foundry industry to produce high integrity and dimensionally accurate castings allow the production of integrally cast turbine airfoils with the so-called double-wall cooling technology. Cast-in turbine airfoils include a more distributed form of cooling, where the coolant can be injected in the form of impingement jets within the wall rather than the hollow of the airfoil increasing dramatically the heat exchanged capabilities. Narrow impingement cooling passages can be therefore generated close to the external hot gas flow. In this study detailed heat transfer distributions for all internal surfaces of narrow impingement channels are evaluated. The test models consist of a single row of five impingement jets investigated over a range of engine representative Reynolds numbers. Effects of jet-to-jet spacing, channel width and height and impingement jet pattern are independently examined composing a test matrix of 22 large-scale geometries. For the evaluation of the heat transfer coefficients, the transient liquid crystal technique is used considering also thermocouple thermal inertia effects. Given that the use of different impingement geometries could play an important role on the distribution of convection coefficients, and hence, on the homogeneity of blade metal temperatures, the experimental results of this study can be used along with the external heat load for the development of thermal design models for integrally cast turbine vanes and blades.