Since the advent of commercial aviation, advancements in propulsion system technology have been the main cause of the reduction of fuel consumption. Modern turbofan engines typically achieve a thermal efficiency of approximately 50%, implying that roughly half of the chemical energy released by fossil fuel combustion is lost to the environment as hot exhaust gas.

For gas turbine engines of stationary power plants, it is common practice to use bottoming units based on the Rankine cycle to recover part of this energy and increase thermal efficiency by up to 20%. The concept of the combined-cycle engines is also suitable for applications with low power capacity, however, organic compounds instead of water must be used as the working fluid of the bottoming unit. The combined-cycle concept is in principle also suitable for aircraft engines, however, adding an organic Rankine cycle (ORC) waste heat recovery system to an aircraft gas turbine engine is challenging because the thermodynamic benefit is counterbalanced by the increased aircraft mass and drag. The few studies conducted so far on combined-cycle aircraft engines indicate a possible net benefit on fuel consumption, however, these results are based on low-fidelity models, neglecting or only partially considering the effect of the new engine configuration on aircraft design and performance.

The work documented in this dissertation aims to provide reliable information on the feasibility of the combined-cycle engine concept based on complex system models, enabling the optimization of preliminary designs and formulating design guidelines. For this purpose, a simulation framework that considers the interaction of the gas turbine, the bottoming unit, and the aircraft was developed. This software package is named ARENA framework, and it can provide the preliminary design of combined-cycle engines optimized for minimized fuel consumption while considering their effect on aircraft design and performance.

ARENA was used to model the effect of this novel technology on the fuel consumption of three exemplary aircraft adopting different combined-cycle configurations and mission scenarios. These cases are 1) a medium-range aircraft employing a combined-cycle auxiliary power unit (CC-APU) instead of a conventional APU to provide power on the ground, 2) a medium-range turboelectric aircraft employing combined-cycle turboshaft engines (CC-TS) in place of conventional turboshaft engines, and 3) a medium-range partial-turboelectric aircraft replacing conventional turbofan engines with combined-cycle turbofan engines (CC-TF). All combined-cycle engine configurations are based on an ORC waste heat recovery unit implementing a non-recuperated cycle, using cyclopentane as the working fluid, whereby the ORC turbogenerator converts the recovered heat into electrical power. The simulated CC-APU engine consumes approximately 50% less fuel to provide ground power compared to a conventional APU, which corresponds to mission fuel savings of approximately 0.6%. The power output of the CC-APU engine is 250 kW, of which 60kW are provided by the ORC turbogenerator. The optimized ORC unit features a mass-specific power of 1.5kW/kg and an efficiency of 15%, while the overall combined-cycle efficiency is 34%. The fuel savings calculated in the case of the CC-TS engine are 1.5%, if compared to a single-cycle turboshaft engine. The combined power output is 5.4MW of which 340kW are contributed by the ORC turbogenerator. The optimized ORC unit mass-specific power is 1.3kW/kg and its efficiency at cruise is 17%, while the CC-TS engine efficiency is 53%. The CC-TF engine burns 4% less fuel if compared to a single-cycle engine. It contributes 60% of the cruise thrust and 2.6MW of shaft power of which 570 kW are provided by the ORC turbogenerator. The shaft power is converted to thrust by the electrical distributed propulsion system. The optimized ORC unit has a mass-specific power of 1kW/kg and an efficiency of 18%. The performance difference between the CC-TS and CC-TF engines is mainly due to different condenser integration architectures. The condensers of the CC-TF engine are integrated into the engine bypass duct downstream of the fan, whereas the condensers of the CC-TS engine are placed into ram-air ducts. The combination of pressure rise and thermal energy input into the bypass air stream increases the propulsive efficiency and the specific thrust of the CC-TF. According to these preliminary studies, the optimized CC-TS and CC-TF engines have no appreciable impact on the lift-to-drag ratio of the aircraft and the maximum take-off mass only increases by a few percent.
It can be concluded that according to the results of this work, the thermodynamic benefit of adopting an ORC system to recover the thermal energy of the exhaust of gas turbines onboard aircraft can outweigh the penalties of the increased aircraft mass and drag. However, the uncertainty due to modeling limitations and simplifying assumptions suggests that further research and development are needed before decisions regarding the development of this engine concept can be taken. Such a drastic change in engine configuration would only be justifiable if the fuel consumption reduction is larger than what was estimated. Further performance improvements may be possible if advanced heat exchanger technology is considered. Furthermore, as well known from theory and practice regarding ORC power plant technology, the identification of an optimal organic working fluid (pure or mixture) may result in considerable performance and operational improvements. Another research direction worth investigating is the optimization of the design of the combined-cycle engine to minimize environmental impact and not fuel consumption. Preliminary considerations show that the benefit of waste heat recovery in this case may be even larger.

Waste heat recovery (WHR) from aeroengines via compact organic Rankine cycle (ORC) units may increase the fuel efficiency of air transportation. Heat exchangers are arguably the key components of ORC systems for aeronautical applications and their design must be optimized to guarantee the best trade-off between fluid pressure drop, weight and induced aircraft drag. At present, no heat exchangers design guidelines are available for waste heat recovery systems aboard aircraft. This study, thus, contributes to defining a proper design methodology for ORC systems of such applications. The chosen test case is a supercritical ORC system with cyclopentane as the working fluid, which recovers waste heat from the auxiliary power unit of an aircraft. The exhaust gas temperature and mass flow rate of the power unit are known and kept constant in the analysis, and so are the ambient conditions, which define the cold sink of the ORC turbogenerator. Three design strategies targeting minimum mass and maximum net power output of the ORC unit have been assessed. In the first one, the multi-objective optimization is performed by prescribing a priori the geometry and frontal area of the heat exchangers. Thus, only the cycle parameters are optimized. The second method tackles, instead, the simultaneous optimization of the geometric parameters of the condenser and the cycle parameters. It was found that the integrated design allows for system mass reduction by 10 - 12% for a given ORC power output, highlighting the importance of performing the simultaneous optimization of the thermodynamic process and the heat exchanger geometry. Finally, the third method addresses the same optimal design problem by leveraging a reduced-order model of the condenser to predict the optimal design space of this component. The generated Pareto front obtained with this method is very similar to that found by optimizing simultaneously the complete condenser geometry and the cycle parameters. The mean deviation is about 2%. With just one heat exchanger surrogate model, the Pareto front was generated in one fourth of the computational time. This is due to the lower number of optimization variables and the faster objective function evaluation.

This paper presents a preliminary study about a combined-cycle engine based on a turboshaft engine and an organic-Rankine-cycle (ORC) bottoming unit to be used onboard an aircraft with a turboelectric propulsion system. The aim is to analyse whether benefits with respect to mission fuel consumption can be derived by employing such a combined-cycle
engine when compared to a simple-cycle turboshaft engine. For this purpose, a multidisciplinary optimization framework is developed, incorporating models for the engine, ORC system, ORC turbine, heat exchangers, and mission analysis. This framework is coupled with an optimizer to identify the optimal combined-cycle engine design for minimum mission fuel consumption. The results suggest that fuel savings of around 1.5% are possible with the optimized system if compared to the aircraft employing turboshaft engines. Heat exchanger volume is identified as the most constraining parameter when it comes to combined-cycle performance. The analysis of the results suggests as aspects which might lead to further improvements the evaluation of other ORC architectures, working fluids and heat exchanger topologies.

The use of mixtures as working fluids of organic Rankine cycle (ORC) waste heat recovery (WHR) power plants has been proposed in the past to improve the matching between the temperature profile of the hot and the cold streams of condensers and evaporators, thus to possibly increase the energy conversion efficiency of the system. The goal of this study is to assess the benefits in terms of efficiency, environmental (GWP) and operational safety (flammability) that can be obtained by selecting optimal binary mixtures as working fluids of air-cooled ORC bottoming power plants of medium-capacity industrial gas turbines. Furthermore, two thermodynamic cycle configurations are analyzed, namely the simple recuperated cycle and the so-called split-cycle configurations. The benchmark case is a combined cycle power plant formed by an industrial gas turbine and an air-cooled recuperated ORC power unit with cyclopentane as the working fluid. The results of this study indicate that binary mixtures provide the designer with a wider choice of optimal working fluids, however, in the case of the recuperated-cycle configuration, no improvement in terms of combined cycle efficiency over the benchmark case can be achieved. The split-cycle configuration leads to an increase of combined cycle efficiency of the order of 1.5%, both in case of pure and blended working fluids. Furthermore, for this cycle configuration the use of Novec 649 as working fluid is advantageous because it is environmentally and operationally safe, and it does not involve any penalty in terms of combined cycle efficiency if compared to the benchmark case. Additionally, the use of this fluid would lead to a more compact turbine, as the corresponding thermodynamic cycle would determine a turbine volume flow ratio that is half of the value of the benchmark case and a specific enthalpy difference over the expansion that is one fifth.

This paper presents a preliminary study about a combined-cycle engine based on a turboshaft engine and an organic-Rankine-cycle (ORC) bottoming unit to be used onboard an aircraft with a turboelectric propulsion system. The aim is to analyse whether benefits with respect to mission fuel consumption can be derived by employing such a combined-cycle engine when compared to a simple-cycle turboshaft engine. For this purpose, a multidisciplinary optimization framework is developed, incorporating models for the engine, ORC system, ORC turbine, heat exchangers, and mission analysis. This framework is coupled with an optimizer to identify the optimal combined-cycle engine design for minimum mission fuel consumption. The results suggest that fuel savings of around 4% are possible with the optimized system if compared to the aircraft employing turboshaft engines. Heat exchanger volume is identified as the most constraining parameter when it comes to combined-cycle performance. The analysis of the results suggests as aspects which might lead to further improvements the evaluation of other ORC architectures, working fluids and heat exchanger topologies.