Experimental Evaluations of the Relative Contributions to Overall Effectiveness in Turbine Blade Leading Edge Cooling

Gas turbine components are protected through a combination of internal cooling and external film cooling. Efforts aimed at improving cooling are often focused on either the internal cooling or the film cooling; however, the common coolant flow means the internal and external cooling schemes are linked and the coolant holes themselves provide another convective path for heat transfer to the coolant. Measurements of overall cooling effectiveness, phi, using matched Biot number models allow evaluation of fully cooled components; however, the relative contributions of internal cooling, external cooling, and convection within the film cooling holes are not well understood. Matched Biot number experiments, complemented by computational fluid dynamics (CFD) simulations, were performed on a fully film cooled cylindrical leading edge model to quantify the effects of alterations in the cooling design. The relative influence of film cooling and cooling within the holes was evaluated by selectively disabling individual holes and quantifying how phi changed. Testing of several impingement cooling schemes revealed that impingement has a negligible influence on phi in the showerhead region. This indicates that the pressure drop penalties with impingement may not always be compensated by an increase in phi. Instead, internal cooling from convection within the holes and film cooling were shown to be the dominant contributors to phi. Indeed, the numerous holes within the showerhead region impede the ability of internal surface cooling schemes to influence the outside surface temperature. These results may allow improved focus of efforts on the forms of cooling with the greatest potential to improve performance.