The maximum attainable performance of small gas turbines represents a strong limitation to the operating altitude and endurance of high-altitude unmanned aerial vehicles (UAVs). Significant improvement of the cycle thermal efficiency can be achieved through the introduction of heat exchangers, with the consequent increase of the overall engine weight. Since semi-closed cycle engines can achieve a superior degree of compactness compared to their open cycle counterparts, their use can offset the additional weight of the heat exchangers. This paper applies semi-closed cycles to a high-altitude UAV propulsion system, with the objective of assessing the benefits introduced on the engine performance and weight. A detailed model has been created to account for component performance and size variation as function of thermodynamic parameters. The sizing has been coupled with a multi-objective optimisation algorithm for minimum specific fuel consumption and weight. Results of two different semi-closed cycle configurations are compared with equivalent state-of-the-art open cycles, represented by a recuperated and an intercooled-recuperated engine. The results show that, for a fixed design power output, engine weight is approximately halved compared to state-of-the-art open cycles, with slightly improved specific fuel consumption performance. Optimum semi-closed cycles furthermore operate at higher overall pressure ratios than open cycles and make use of recuperators with higher effectiveness as the mass penalty of the recuperator is smaller due to the lower engine mass flow rates.

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Conventional Brayton cycles have demonstrated to be significantly less efficient than alternative propulsion systems (spark ignition, diesel, fuel cells, etc.) for low power output applications, such as for small size UAVs. The gas turbine performance could be enhanced through the introduction of heat exchangers, with the consequent increase of the overall engine weight. Semi-closed cycles have documented advantages of higher thermal efficiency and degree of compactness than traditional intercooled-recuperated open cycles. This paper discusses advantages and applicability of semi-closed cycles to a small gas turbine, designed for a medium altitude UAV mission. In particular, size and altitude effects have been accounted in the performance evaluation of two different semi-closed cycle arrangements designed for an output shaft power of 100 hp (74.57 kW). Resultant performance has been compared with equivalent simple recuperated and intercooled-recuperated open cycles. Furthermore, a final engine performance comparison has been made with data obtained from a similar analysis performed on a larger engine, with a power output of 300 hp (223.71 kW) and designed for an extremely high altitude UAV application. While promising results have been obtained for the larger case study, where semi-closed cycles have demonstrated superior performance and higher engine compactness than conventional solutions, similar trends have not been displayed for the smaller engines, as consequence of the strong size effects observed in the turbomachinery performance. For the 100 hp engine the semi-closed cycles are slightly outperformed by the open cycle engines.@en