Exploratory assessment of a combined-cycle engine concept for aircraft propulsion

This preliminary study considers a combined cycle configu­ration for aeroengines, whereby thermal energy from the exhaust of the gas turbine is partly recovered in order to obtain additional mechanical power. The waste heat recovery system is based on a closed thermodynamic bottoming cycle with supercritical car­ bon dioxide (scCO2) as working fluid, allowing to achieve a very high power density. As first step of the investigation a thermo­dynamic cycle analysis of the combined-cycle engine (CCE) is carried out. Results are compared to those of the intercooled­recuperative engine (IRE) configuration for the same operating conditions and calculated under the same modeling assumptions. The estimated nominal SFC of the proposed CCE configuration is approximately 20% lower compared to that of a conventional turbofan, and 6% lower than that of the IRE, if pressure drops in the heat exchangers are neglected. Such large gain justified further analysis, by including the preliminary sizing of main components. Once the sizing of heat exchangers is factored in, the thermodynamic benefit of the CCE is offset by the penalty due to the weight of the additional equip­ment. This is mainly caused by i) the space constraints of the turbofan nacelle, which strongly limit the recoverable thermal power, and ii) the lack of proper het exchanger technology for such a highly unconventional application. These issues, and the many other that need consideration, will be addressed in an upcoming research project encompassing a much wider scope involving new aircraft and propulsion system