Design of a Semi-Empirical Tool for the Evaluation of Turbine Cooling Requirements in a Preliminary Design Stage

There is an everlasting demand for higher cycle efficiencies in gas turbine engines. One way to obtain a higher efficiency is to increase the overall pressure ratio (OPR) of the engine. In order to benefit from the efficiency increase whilst maintaining the same specific gas power, the turbine inlet temperature (TIT) has to increase with it. However, with the extremely high TIT used in modern day gas turbine engines, the melting point of the materials used in nozzle guide vanes (NGV) and high pressure turbine (HPT) blades is already exceeded. Therefore, the vanes and blades rely on active cooling on the in- and outside to prevent it from melting. However, the application of cooling air in the turbine is detrimental to the engine performance and hence is desired to be kept at a minimum. Next to application of a turbine cooling system with high heat transfer coefficients, a lower cooling air massflow can be obtained by using coolant of a lower temperature. The use of cryogenic fuels in hybrid engines currently investigated in for example the AHEAD project, can prove to be useful in that aspect; a heat exchanger can be applied to exchange heat between the coolant and the fuel to reduce the temperature of the cooling air. In this study, a semi-empirical turbine cooling program is developed which is able to calculate the thermal performance of a cooling system configuration and assess the effects of the cooling air on engine performance. For the determination of the heat transfer coefficient and pressure drop in the turbine cooling program, semi-empirical correlations from various journal articles are used rather than relying on computational intensive CFD calculations. Also, the tool is able to predict the cooling air requirement for any turbine cooling configuration to a reasonable level of accuracy, enough to be useful during the engine preliminary design phase. This turbine cooling program is used to study the effects of the cooling air on engine performance and research the feasibility of using pre-cooled coolant in a hybrid engine configuration. This main conclusions of this study are that, as expected, cooling air is detrimental to the engine performance. However, the magnitude of the turbine cooling requirements and hence the detrimental effect on the engine performance is commonly underestimated. Therefore, the proper prediction of turbine cooling requirements is important in early design stages, especially for engines operating at higher OPR and TIT. Furthermore, the cooling air requirements decrease if cooling air of a lower temperature is used. Re-using spent cooling air to cool the next row of vanes/blades turns out to have a negative effect on the engine performance. The findings in this report can prove to be important for future engine design where OPR and TIT values are expected to keep rising. Also, the assessment of the effect of coolant pre-cooling and re-use of coolant come in handy for the intended application in the AHEAD hybrid engine.