Prediction Methodology of an Optimum Turbofan Engine Cycle

Abstract

This paper summarizes the steps of sizing a turbofan engine for a specified flight condition. This is done by minimizing the specific fuel consumption, which is achieved by finding optimum pressure ratios for the fan and the low pressure turbine. Turbine inlet temperature is assumed to be given, while the bypass ratio is a fall-out from the analysis. The user has the option to either input the high pressure compressor pressure ratio or to determine an optimum value that maximizes the thermal efficiency of the engine. Following a series of steps described herein, this optimum cycle can be found for any flight altitude and Mach number combination, depending on the characteristics of the cycle and the component efficiencies. This analysis is limited to turbofan engines with separate nozzles for the fan exhaust flow and the core exhaust flow. In addition, both nozzle flows are fully expanded to atmospheric pressure. Using a memorandum written by R. O. Bullock in 1973 while working at the Airesearch Manufacturing Company, this process was expanded by creating a specific tool capable of calculating various performance parameters of interest and creating a series of curves, where the specific fuel consumption is minimized for each specified flight condition. This analysis method enables the analyst to determine the bypass ratio, as well as the pressure ratios, without having to re-run the analysis for each different case. Furthermore, these curves yield the trends that will minimize the specific fuel consumption, as well as determine the fan and low pressure turbine pressure ratios necessary to achieve this minimum.