A Superposition Technique for Predicting Overall Effectiveness With Multiple Sources of Individually Characterized Internal and External Coolant Flows

Abstract

High-performance gas turbine engines rely on intricate cooling schemes to protect turbine components from the high-temperature freestream gas. Determining the optimal placement of film cooling holes in optimal locations while remaining mindful of the consumption of precious coolant air is therefore an interesting design challenge. For several decades, a method of superposition has been in use that provides a reasonable prediction of the adiabatic effectiveness in a region influenced by multiple rows of film cooling holes acting together if the adiabatic effectiveness distributions resulting from individual rows are already known. This classical superposition technique has been used with success to at least provide a first cut at determining where coolant holes might be placed on turbine components. One of the shortcomings of the method is that it strictly applies only to the prediction of adiabatic effectiveness which is indicative of the adiabatic wall temperature, not the actual surface temperature of the turbine component. Indeed, the overall effectiveness, which is indicative of the actual surface temperature, is somewhat more complicated as it is influenced not only by external cooling but also by internal cooling. In the present work, a method of superposition of overall effectiveness data is proposed, thereby allowing prediction of the actual surface temperature distribution on a component. Experimental results show that it is possible to use knowledge of the overall effectiveness distributions resulting from individual internal cooling and associated film cooling holes acting alone to determine how they are likely going to act when combined. This technique shows promise for a turbine designer to use superposition for actual surface temperature prediction, not just adiabatic wall temperature prediction, and therefore higher quality initial cooling designs.