To address the growing need for sustainable aviation, hydrogen fuel cell technology is being explored as a promising alternative to conventional kerosene-based propulsion systems. Compared to hydrogen combustion, hydrogen fuel cells offer a significant environmental advantage by producing zero nitrogen oxides (NOₓ) in their exhaust. For commercial viability, aircraft design must optimize both range and payload, which in the context of hydrogen fuel cell propulsion translates to maximizing system efficiency and minimizing propulsion system weight. Achieving these objectives requires accurate and flexible system modeling tools.

This thesis focuses on the development of a new hydrogen fuel cell system modeling library in Modelica, named AeroFCS. The library is a joint effort between zepp.solutions and Delft University of Technology and is designed to be compatible with the existing DeSimECS Modelica library developed by the Faculty of Aerospace Engineering. The choice of Modelica, an acausal modeling language, enables flexible system configurations by allowing input-output modifications without component model adjustments. The AeroFCS library includes a custom fuel cell model and a validated humidifier model. These are integrated with compressor, intercooler, and turbine models from DeSimECS to simulate a complete fuel cell air supply system. Additionally, a verified two-phase medium model is used to simulate humid gas properties. A Python-Modelica interface is developed to facilitate simulation post-processing and to support future system optimization studies. System simulations are conducted under specific environmental conditions and across a range of fuel cell operating pressures and currents. Operational maps are generated to assess compressor pressure ratios and mass flow rates. These maps help identify regions of optimal performance by plotting system efficiency and net power between the surge and choke lines of the compressor. Results indicate that the highest efficiencies are achieved at high pressure ratios, close to the surge limit of the compressor, for any given system power.

The AeroFCS library offers a foundational tool for simulating and optimizing hydrogen fuel cell propulsion systems, and future work will focus on improving model fidelity and increasing model functionality.

To reduce aviation’s climate impact, the industry must significantly reduce greenhouse gas emissions, necessitating the development of more sustainable and efficient propulsion technologies. The Water-Enhanced Turbofan (WET) engine is such a promising future aero-engine concept. The engine integrates a Joule-Brayton cycle with a semi-closed Rankine steam cycle. Superheated steam is injected before the combustion chamber and downstream before the core nozzle. While previous studies have explored the fundamental thermodynamics of the concept and assessed its potential for reducing climate impact, this study explores the design space of the WET engine and examines key design parameters and their influence on cycle performance using NASA’s pyCycle and OpenMDAO framework. Moreover, the high-fidelity in-house software Hexacode is used to model the heat recovery steam generator (HRSG). Based on the design exploration, a water-to-air ratio (WAR) of 0.20 is found optimal for the WET cycle, reducing the thrust-specific fuel consumption (TSFC) by 7.3% with respect to a LEAP-1A-type engine. The best design solution comes with a significantly higher bypass ratio, and lower overall pressure ratio and turbine inlet temperature. Nozzle velocity ratios higher than 1 have been demonstrated to enhance the overall engine efficiency. Besides this, the condenser is identified as the main critical component of the proposed engine concept, while the HRSG design becomes challenging at high water-to-air ratios. Fuel burn can be further reduced by increasing the OPR and steam injection temperature. Future work should focus on detailed condenser modeling and the integration of the thermodynamic cycle model with preliminary heat exchanger (HEX) design models to improve system-level performance predictions.

This study investigates optimizing working fluids and system design for Waste Heat Recovery (WHR) in turboelectric aircraft, focusing on the Onera Dragon concept. Using the PC-SAFT equation of state and QSPR models, the research identifies optimal fluids for an Organic Rankine Cycle (ORC)-based airborne WHR system. The method includes optimizing pseudo-fluids, which are mapped to real fluids based on PC-SAFT parameter proximity. A bi-objective optimization also attempts to find real-fluid optima directly. The best pseudo-fluid achieved a 1.56% fuel saving, while the best real fluid, acetyl chloride, achieved 1.37%, outperforming cyclopentane’s 0.92%. The bi-objective approach did not yield better real-fluid results. Binary mixtures of non-polar pseudo-fluids and blends of cyclopropane and cyclopentane showed no improvement, indicating no benefit from temperature glide in the condenser.

The adoption of Vapour Compression Cycle (VCC) refrigeration technology in aircraft Environmental Control Systems (ECS) presents efficiency advantages over conventional Air Cycle Machine (ACM) architectures. However, the implementation of VCCs in aviation remains a challenge due to excessive system weight, negative environmental impact of refrigerants, and high control complexity. This study focuses on the Inverse organic Rankine cycle Integrated System (IRIS) at TU Delft, which investigates electrically driven VCC systems with novel refrigerant types for aircraft applications. A dynamic model of the IRIS refrigeration loop is developed to assess transient behaviour and evaluate the performance of a high-speed centrifugal compressor to be integrated into the system. The model employs a modular, physics-based approach using the Modelica/Dymola environment, incorporating the Moving Boundary method for heat exchangers and steady-state performance maps for the compressor. Model verification is performed through steady-state and transient analyses. Subsequently, a linear decentralized control strategy is designed to regulate the compressor inlet and outlet conditions, ensuring stable operation across varying test conditions. Simulation results demonstrate that the controller successfully regulates the refrigeration loop of the IRIS to reach the desired setpoints for full compressor testing, while keeping inputs within operational limits. Additionally, it is shown that key system dynamics are correctly captured by the model, providing valuable insights into the operational feasibility of centrifugal VCCs in aerospace ECS applications.

This research investigates the adjoint-based optimization of bare tube heat exchangers, aiming to minimize air-side pressure drop. The study focuses on optimizing tube shapes in an in-line bare tube heat exchanger configuration, selected for its inherent advantages in reducing pressure drop. The optimization targets flow conditions characterized by a Reynolds number of approximately 1000, based on the tube diameter. The accuracy of simulations was found to depend significantly on the turbulence modelling approach. Different models were benchmarked against empirical correlations to ensure accuracy, with the most accurate model used to recompute the flow fields of the optimized designs as a post-processing step. The optimized round tubes achieved a pressure drop reduction of up to 20%. For elliptical tubes, the optimization resulted in a pressure drop reduction of up to 10%. This 10% reduction adds to the notable improvement gained by replacing round tubes with elliptical ones.

The aviation sector faces growing pressure to reduce greenhouse gas emissions, while projections are suggesting that emissions could double by 2050 to account for increases in passengers. Liquid hydrogen fuel cell-electric (LH2FCE) propulsion offers a promising solution, with the potential to reduce climate impact by up to 90\% compared to conventional turboprop engines. However, significant challenges exist, including heat dissipation, increased drag, and increased weight penalties from heavy fuel cell stacks, cryogenic hydrogen storage and thermal management systems.
This thesis evaluates the impact on payload and range of retrofitting a regional turboprop aircraft with an LH2FCE propulsion system, by performing the preliminary design of the balance-of-plant systems, and assessing system performance by means of steady state analyses in take-off, top-of-climb and cruise conditions. A lumped parameter model was developed to simulate the fuel cell and balance-of-plant components, including an air supply and thermal management system and ram air ducts. Results show that payload reductions of approximately 58-77\% are expected for a 1500 km range compared to current turboprop aircraft, primarily due to increased mass and drag penalties, which reduce the propulsion system's specific power and lift-to-drag ratio. Sensitivity analyses were conducted, highlighting the effects of fuel cell operational parameters. It was revealed that adjustments in fuel cell temperature, pressure, and current density in the different operating conditions can enhance system performance. Additionally, it was found that most systems must be sized for top-of-climb, except for the ram air duct, which is constrained by take-off conditions. Incorporating a variable inlet design may eliminate ram air drag in cruise, although detailed drag analysis is recommended. Integration of a turbine and halving the rate-of-climb demonstrated that achieving a 1500 km range with a competitive payload (>3000 kg, 35 passengers) is feasible for retrofits, while reserving mass for non-modeled systems such as batteries. Moreover, projections for 2030 suggest that the performance of current turboprop aircraft could be matched by LH2FCE systems.
These findings highlight that LH2FCE propulsion is a viable and sustainable alternative for regional aviation, provided the reduction in payload is acceptable. With advancements in fuel cells, heat exchangers, electric motors, and liquid hydrogen storage, LH2FCE aircraft could achieve performance of current kerosene-powered turboprop aircraft by 2030.

The ongoing environmental crisis has accelerated the development of radical new technologies that aim at decreasing, and possibly eliminate in the future, the carbon footprint across the industrial spectrum. Reuse and waste management are at the forefront of this transformation and enable prolonging the life-cycle of valuable raw materials, reducing the energy consumption of industrial processes, and abating the emissions related to operation of prime movers for commercial transportation and freights. Organic Rankine cycle (ORC) power systems for the conversion of thermal energy into electricity, at temperatures ranging from ≈ 120 °C to > 500 °C and with a power capacity fromfew to tens MW, are commercially available and employed to obtain CO2-free electricity from geothermal reservoirs, industrial waste heat, the exhaust of gas turbines and stationary internal combustion engines, and the combustion of biomass. However, the market potential of high-temperature and high efficiency ORC systems with power output up to several hundreds kW is arguably very large. Suitable working fluids for these cycles are made of complex molecules, and therefore the speed of sound of the expanding organic vapor is of the order of tens m/s and the flow within a single-stage radial-inflow turbine – which is often selected as an optimal compromise between operating costs, which are related to its efficiency, and investment costs, which are instead more intimately linked to its size and complexity – is bound to be highly supersonic. Its design is thus challenging not only from the fluid dynamic point of view, but also because of many other aspects related to high rotational speed, sealing and bearings technology, and rotordynamics. These high-speed machines are the ideal expander type for low-capacity systems in which Tsource ≤ 550 °C, such as long-haul trucks (≈ 10-30 kW), shipping vessels (≈ 100-500 kW), and aircraft (≈ 100-500 kW).
This dissertation documents work performed as part of the ARENA project, whose objective is the estimation of aircraft performance including combined-cycle propulsion systems making use of organic Rankine cycle for onboard thermal energy recovery. The first objective of this dissertation is to advance the current knowledge regarding the optimal design of supersonic radial-inflow turbines for such applications. The second goal is to design a prototype turbogenerator and integrate it within a test bed for the ORCHID, whose main purpose is the generation of data for verification and validation of supersonic RITs, and in the future to serve as an open test case for advanced R&D on propulsion and power technologies. The first main contribution of this work to the research field of high-temperature ORC turbines is the formulation of novel preliminary design guidelines for radial-inflow turbines, obtained with TurboSim, a preliminary design code written in Python which was extended to analyze the impact of critical turbine design parameters on losses and efficiency. The resultswere used to formulate best practices for selecting stage duty coefficients that maximize expander efficiency, including the effect of the volumetric flow ratio and of the isentropic pressure-volume exponent to account for the impact of nonideal thermodynamic effects on expander performance, which was not accounted for in previous tools. The tool was used within the ARENA project to perform the preliminary design, and estimate the efficiency and weight, of RIT for combined-cycle turboshaft and turbofan engines for novel hybrid-electric aircraft. The second main contribution is the design and realization of a test facility for supersonic radial-inflow ORC turbines, whose detailed design process to select the turbogenerator layout, its main components, and the specifications of the assembly and its individual parts are developed and applied in this dissertation. The test bed, featuring a supersonic RIT for high-temperature ORC systems of low power capacity (≈ 10 kW), will enable measurements of the fluid dynamic efficiency of high-pressure ratio RITs, and the assessment of the impact of design choices (e.g., vane count, impeller blade shape, tip clearance) on turbine performance. The results of the experimental campaigns could also provide knowledge applicable to other types of radial turbines and supersonic turbomachinery, such as turbines for rocket engines and expanders to drive fuel cell turbocompressors. Research on oil-free gas bearings and seals is also possible and highly relevant, and the powertrain assembly which couples the turbine to a high-speed electric machine for braking power will facilitate research and development of next-generation high-speed electric generators—a critical technology for more electric and all-electric aircraft.

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.

Liquid hydrogen has been identified as a low-emission alternative to hydrocarbon fuel sources for aircraft. Although there has been a notable increase in the number of studies and technology readiness of liquid hydrogen technologies in the recent decade, there is still more research and development necessary to realize commercially viable hydrogen-fueled aviation. Achieving zero boil-off by using active cooling has been identified as a potential method of decreasing the mass and volume of a future liquid hydrogen fueled aircraft. However the lack of literature or data on the thermal performance and specific power of cryocoolers for this application makes it difficult to assess its feasibility. Therefore, a study was performed to develop a reverse turbo-Brayton cryocooler simulation tool, focused on the three heat exchangers of this cooler type. This paper presents the methodology for the development of this tool alongside a case study on a liquid hydrogen fuel tank for TU Delft's flying-V concept. From the case study it has been concluded that the considered RTBC design offers a reduction in the liquid hydrogen mass, however not significant enough to offset the cryocooler's own weight. An improved performance and specific power of the individual components would be required to achieve a net weight reduction.

A significant trend in aero engine design has been the rise in turbine inlet temperatures, as well as the drive to produce raise efficiency. Since the 1960s, turbine inlet temperatures have exceeded turbine material limits, with turbine cooling systems being used to bridge the gap. Modern engines require substantial amounts of cooling air, prompting a need to understand further the impact of turbine cooling on turbine and engine performance in cycle calculations.

Current models employed for performance analysis of cooled turbines are based on technical assumptions that are several decades old. This raises questions about the possibility of adapting models to more accurately represent modern engine technology. As such, this thesis aims to develop a cooled turbine model (CTM) for use in PyCycle, an open-source engine cycle analysis platform. The CTM is based on the cooled turbine blade row model defined by Young and Wilcox, which employs empirical constants to estimate cooling mass flow rates. The CTM can estimate the cooling flow requirements and the associated entropy rise for the turbine, accounting for the irreversibility in the turbine cooling process.

The implemented CTM models a complete turbine stage and multi-stage turbines based on three key aspects: the thermodynamics of a cooled turbine row, the work extraction in an equivalent uncooled turbine stage, and the conversion of thermodynamic properties between an absolute and rotating frame of reference. The CTM was verified and validated using three other cases, and it was found to accurately capture the effects of turbine cooling on bulk flow properties.

Following the implementation of the CTM, a study into the (semi-)empirical parameters and constants was performed to update existing parameters for modern engines. The limited availability of data relating to flow velocities and Mach numbers in aero-engine turbines forms a significant obstacle to the accurate specification of some empirical parameters. An estimation for the average Stanton number over turbine blades based on the gas temperature and Reynold’s number was derived and validated with the Von Karman Institute’s LS-89 turbine cascade results. The impact of updating the empirical parameters used in the CTM
has been assessed by formulating the cooling flowestimation as an optimization problem.
Using the updated ranges for the empirical parameters, the optimization study showed the potential for a significant reduction in the estimated cooling fraction.

Hydrogen Polymer Electrolyte Membrane (PEM) fuel cell technology is becoming increasingly popular in the transportation sector, especially the automotive industry. It can power electric drives by converting hydrogen into electricity in a chemical reaction with oxygen, whose products consist only of water. Therefore, hydrogen PEM fuel cell-based powertrains are free of any harmful emissions, enabling clean propulsion technology. An important part of an automotive fuel cell system is the air processing subsystem, which usually features an electrical air compressor (E-compressor) to supply the necessary oxygen for the chemical reaction. This E-compressor can absorb between 10-30% of the fuel cell gross power. One way to decrease this share is to expand the fuel cell air exhaust flow in a turbine that contributes to power the E-compressor, essentially realizing an electric turbocharger (E-turbocharger). E-turbochargers for fuel cells are still in the development phase, therefore there is a lack of experience and knowledge on their benefit. In this thesis, a model of an existing automotive PEM fuel cell air processing system has been developed using the AVL Cruise M software. In addition, two prototype PEM fuel cell E-turbochargers (ETC1 and ETC2, respectively) have been modeled and integrated into the main air processing system model. Steady-state simulations have been performed for three different power levels. Moreover, the turbine inlet temperature of the ETC1-based model was varied in the range of 60°C to 150°C to quantify what benefits exhaust heating with waste heat brings in addition to the inherent turbocharging performance gains. The results show that compared to the baseline system, the model featuring ETC1 achieved an increase in net power between 3.04-6.78% or a decrease in fuel consumption between 3.19- 7.67%, depending on the power level. For the ETC2-based model, from low to high load, the net power increased between 1.88-2.04%, or fuel consumption decreased between 2.07-3.17% compared to the baseline system. When increasing the turbine inlet temperature to 150°C, the model showed an additional decrease in fuel consumption of 2.1 percentage points at full power. The models could be improved by implementing a first-principle-based model of the E-compressor and E-turbocharger, which could have been developed in this project if more detailed information on the heat flows within these components was available.

Current design methods for oblique manifolds were first presented in the paper of London et al. in 1968. This method presents a single equation to shape the dividing manifold of several manifold configurations. However, current literature shows a potential to improve the current design method by implementing advanced techniques. This paper aims to bridge this gap by presenting an innovative design methodology incorporating these advanced techniques. The proposed design method integrates free-form deformation and adjoint-based methods within a 1st-order shape optimization framework. The objective function focuses on the minimization of mass flow mal-distribution, evaluated by an incompressible Reynolds-Averaged Navier-Stokes solver. Through three initial tests, this paper demonstrates a consistent improvement in the objective for all obtained designs. Noteworthy is the finding that small changes to the initial design lead to significant enhancements, evidenced by an 82.5\% decrease in the mass flow mal-distribution. Off-design testing further substantiates the effectiveness of the design method, showcasing superior performance under varying initial conditions for the optimized designs. Further testing not only exhibits improved objectives but also reveals common features among results for diverse initial designs. This paper shows a novel and effective design methodology for consecutive manifold systems, laying a robust foundation for an improved design method.

The environmental emergency is one of the most critical challenges of modern times. The exponential increase of industrial activities is the primary cause of anthropogenic climate change, with negative consequences for the environment, society and economy. In the transport sector, decarbonization is the most significant challenge. In aviation, the environmental objectives are to halve international CO2 emissions by 2030, and to reach net-zero carbon emissions by 2050. The achievement of these goals implies a step-change in the traditional practice of aircraft design, with many resources invested in research for promoting the use of fossil fuel-free propulsion systems and electrified auxiliary systems.

In this framework, the research presented in this dissertation is on methods for automated design optimization with applications to a novel electrically-driven Environmental Control System (ECS) for aircraft cabin cooling. The ECS is the main consumer of non-propulsive power onboard aircraft. The founding idea of the project, which has been carried out in collaboration with several companies, is to replace the traditional ECS equipping airliners, which is based on Air Cycle Machine (ACM) technology, with Vapour Compression Cycle (VCC) systems powered by high-speed centrifugal compressors. Work performed within this project demonstrated that such a VCC-based ECS can be more efficient and lighter than an ACM-based ECS in the case of mainstream passenger airplanes. The main goal of the research documented in this dissertation is to provide design methods and guidelines for the optimal design of aircraft ECS whose core is a VCC system powered by novel high-speed electrically-driven compressors and using low-Global Warming Potential (GWP) working fluids in place of the conventional R-134a refrigerant. Additionally, the study encompasses the analysis of the impact of the selected working fluid on the optimal design of the main system components, i.e., the heat exchangers and the centrifugal compressor. For this purpose, a novel integrated design optimization framework has been developed: it allows to perform the multi-objective optimization of the aircraft ECS across different points of the aircraft operating envelope. This method enables the concurrent automated optimization of thermodynamic cycle, preliminary component sizing and working fluid selection. Moreover, the successful application of the method to this complex case demonstrates that the approach is generally applicable to any thermal energy conversion system.

The method was applied to the design of the ECS of two different aircraft to demonstrate its capabilities: a large passenger rotorcraft and a single-aisle short-haul aircraft, i.e., the A320. Results show that it is possible to design an efficient VCC system for aircraft ECS that is powered by an electrically-driven centrifugal compressor and uses low-GWP refrigerants as working fluids. In particular, in the case of a small-capacity ECS for large rotorcraft, it was demonstrated that the use of high-molecular complexity refrigerants, such as haloolefins, enables the design of lighter and more efficient VCC systems if compared to the state-of-the-art. The test case of the airliner ECS provided the specifications to further develop and test the methodology: a so-called physics-based equation of state model was adopted for the computation of the thermodynamic properties of the working fluid. Molecular parameters allow to define the fluid, therefore they can be optimized as part of the global optimization of the design of the system. Parameters are constrained so as to define a realistic molecule, though non-existing, called pseudo-fluid. Actual working fluids whose molecular parameters are similar to those of the optimal pseudo-fluid}are selected in a following step of the design procedure. Optimal working fluids are therefore natural refrigerants. The use of these working fluids with null GWP would reduce the environmental footprint of the considered environmental control systems, while enabling an (albeit small, in the considered case) reduction of specific fuel consumption.

Complementary to the numerical investigation, a novel experimental setup called IRIS (Inverse organic Rankine cycle Integrated System) was designed, realized and successfully commissioned at the Propulsion & Power laboratory of Delft University of Technology. The setup was conceived to enable testing and performance analysis of VCC-based aircraft ECS and to validate in-house software for system and components design. The setup hosts two main test sections: one to test compressors and another to test air-cooled condensers. The results of the commissioning show that it is possible to continuously operate the IRIS setup in steady-state conditions at temperature levels which are very close to those at the design point, thus achieving a Coefficient of Performance (COP) equal to 3.76±0.48.

Hydrogen fuel cells are set to play an important role in decarbonizing the aviation sector. They operate at higher efficiencies than turbine engines, but their low operating temperature requires a dedicated thermal management system. The added drag, weight and parasitic power of such a system has a high impact on aircraft performance and its design must be integrated with overall propulsion system development. Among cooling options, flow boiling has the potential of increasing the cooling system performance, but design methods are lacking in the literature. To address this problem, this thesis presents preliminary design methods able to size both conventional single-phase and novel two-phase cooling systems and tests them on a reference regional turboprop aircraft. Sizing results show that pumped multi-phase systems result in lower system mass and drag than conventional single-phase cooling while vapour compression systems are a less attractive option due to their high power demand.

The need for more efficient aircraft has led to the conception of innovative engine configurations, such as the combined-cycle engine proposed by Delft University of Technology or the Water-enhanced turbofan developed by MTU and Pratt & Whitney. The evaluation of the potential benefits of these novel engine concepts requires detailed performance studies considering weight and drag penalties as figures of merit. At present, no weight estimation tools with sufficient level of accuracy and flexibility are publicly available, apart from WATE++, a tool developed by NASA. However, this software is subject to export control restrictions and cannot be used outside the USA. For this reason, the development of a new component-based preliminary engine design tool, "Weight EStimation of gas Turbine engines" (WEST), was started. The goal of WEST is to predict the weight of novel engine architectures with a reasonable level of accuracy, accounting for design parameters like turbine inlet temperature, overall pressure ratio, mass flow rate, and turbomachinery configuration, with a minimal set of geometry inputs specified by the user. A previous work demonstrated that WEST can effectively predict the weight of turbofan gas generators. As only the main components are modeled, the WEST estimates account for approximately 70-90% of the actual engine weight.

The capabilities of WEST were expanded in this study to allow for the design of small-scale turboshaft engines. To this purpose, a methodology to design radial compressor disks was implemented, and successfully verified against FEM results. Regarding the modeling of the complete turboshaft, it was found that the predictions of the tool account for only 60-70% of the actual engine weight. Such result was, however, expected, as WEST does not take into account the particle separator and the integrated gearbox, which may account for up to $30\%$ of the engine's total weight.

The need for more efficient aircraft has spawned the conception of a variety of novel engine and aircraft architectures, such as the combined-cycle engine proposed by researchers at Delft University of Technology. To evaluate the potential benefits of these concepts comprehensively, in-depth performance simulations are required. Furthermore, penalties with regard to weight and drag are critical considerations, thus new engine technologies must be evaluated within their full airframe-level integration and mission profile. Accurate weight estimations of a variety of design alternatives are therefore necessary in order to properly assess the validity of such technologies and to reach an optimal design with respect to the tradeoffs between weight, drag, and efficiency. No weight estimation tools with sufficient accuracy are publically available at present, thus a new, component-based preliminary engine design tool, named ‘Weight Estimation of Aeronautical Gas Turbine Engines’ (or WEST), was developed. WEST can be used to predict the weight of novel engine architectures to a reasonable degree of accuracy, all the while accounting for sensitivity to design parameters such as turbine inlet temperature, overall pressure ratio, mass flow rate, power, and choice of turbomachinery configuration. Similar methodologies are able to design components worth between 60 and 110% of the actual weight of an existing engine, whereas WEST was able to account for about 70-90%, thus exceeding initial expectations and improving the reliability of the estimations. This tool can therefore be used to estimate the weight of a variety of existing engines, and is sufficiently flexible to model novel architectures as well, since the results are based on the application-specific design of real engine components. Among the potential uses of WEST, a promising one would be to evaluate the weight of a large set of designs for a particular application. Upon these estimates, single-equation surrogate models could be developed using statistical regression, allowing for these equations to be used as a more computationally-efficient weight estimation methodology in a wide range of aircraft-level design and optimization studies. These equations would build upon the work described in this paper and could be used to accelerate the development and introduction of new aircraft-related technologies.

Solid Oxide Fuel Cell (SOFC) systems for aviation applications have been considered for their use as APUs or powertrains, as standalone systems or in combination with gas turbines, and both fueled by reformed Jet-A or by hydrogen. Most existing studies focus on powertrain performance modelling. In this work, traditional tube-and-wing regional aircraft sizing methodologies are modified to account for liquid hydrogen fuel and SOFC-GT-Battery hybrid electric powertrains. Methodologies for powertrain modelling, power sizing, energy sizing, weight calculation and system integration have been derived, implemented, verified, validated when possible, and used to assess several study cases. Considering state-of-the-art metal-supported planar ITSOFC technology installed in a 50-seat regional aircraft as test case, it is concluded that current SOFC system power density is not sufficient to achieve MTOW values similar to conventional kerosene-powered regional aircraft. This work serves as a baseline for the integration of sizing methodologies for SOFC-GT-Battery powertrains into aircraft conceptual-to-preliminary design tools.

Industrial processes are estimated to be responsible for about 20 % of the total greenhouse gas emissions within the European Union (EU). The majority of the industrial energy demand is related to the thermal energy required for process heating. Thus, there is a need to make radical changes to the industrial heating supply to achieve net-zero CO2 emissions. In this regard, high-temperature heat pump systems are being studied as an alternative to conventional fossil fuel-based systems. This project focuses on the modeling and analysis of thermodynamic characteristics of the two heat pump concepts, namely, reverse Rankine cycle based and reverse Brayton cycle-based systems. Both heat pump concepts are compared for high-temperature applications in terms of performance, design feasibility of the key components, and ease of integration with industrial processes.

Range extended electric vehicles (REEV) are being considered as a possible solution for the electrification of passenger vehicles. The application of a recuperated gas turbine (GT) for REEVs shows potential in terms of power-to-weight ratio, emissions and cost. The recuperator plays a major role in achieving target efficiency, while being a main contributor to cost and packaging. The topology and subsequent design are key factors for the success of the concept.

A topology was selected based on literature research of different recuperator concepts for small gas turbines. A sizing model for this topology was developed to obtain a recuperator geometry for a set of boundary conditions. Furthermore, the influence of different gas turbine cycle design parameters, as well as heat exchanger surface geometries, on the resulting geometry was analyzed. The research shows that an integral GT cycle design can be used to obtain a heat exchanger that fulfills REEV requirements.