To reduce the impact of aviation on climate, more sustainable propulsion systems need to be developed. One proposed solution is the hydrogen fuel cell. This propulsion system requires pressurized air for optimal operation. The energy of this air can be utilized by a radial expander located downstream of the fuel cell, which transfers mechanical power to the compressor. Accurate estimation of expander parameters and design is crucial for efficient propulsion system development. Although current fluid models cover a wide range of compositions and flow regimes, there is still a need for a system-level fluid model that accounts for non-equilibrium condensation optimized for expanders. The goal of this study is to explore the consequences of considering or neglecting non-equilibrium condensation on expander performance and design. The developed fluid model is a hybrid approach combining NASA polynomials for species-specific property estimation, IF97 for dew point prediction, and a surrogate model for subcooling temperature calculation. To enable a fair comparison between equilibrium and non equilibrium condensation, a simplified one-dimensional expander model was also developed. The comparison results show that considering non-equilibrium condensation increases the projected power and mass flow while decreasing the cross-sectional area of the expander at the outlet for given inlet and design conditions. The specific work remains unchanged; however, the values of thermodynamic properties along the normalized streamwise location differ when non-equilibrium condensation is considered. The most significant differences include lower static temperature, higher density, and increased water vapor content. It is concluded that such discrepancies should be included in system-level simulations to improve propulsion system mass and efficiency. Moreover, the effects of composition changes under non-equilibrium conditions show that expander performance strongly depends on the water content, the amount of condensation, and the magnitude of subcooling. Higher water content decreases mass flow while increasing specific work and power of the expander; however, it reduces total-to-total efficiency because of increased condensation. Future work should focus on expanding the surrogate model to cover a wider range of pressure ratios in order to improve the accuracy of subcooling temperature prediction. Additionally, the fluid model could be extended to include more thermodynamic and transport properties required for more sophisticated expander models. ... Read more

Hydroisomerization of alkane isomers is an important step in the manufacture of current kerosene and sustainable aviation fuels. Zeolites are used as acid catalysts in the process. It is therefore important to have predictions of the maximum loading of hydrocarbons in zeolites. Here, a cascade model using machine learning models is used to predict the maximum loading of alkane isomers in zeolites. The cascade is composed of a gradient-boosted tree classifier stage that predicts whether adsorption occurs or not, and a regressor predicting the value of the maximum loading. The final dataset consists of 45 different molecules (both linear and branched alkanes up to C16) and 97 different zeolites structures, resulting in 4365 datapoints. Descriptors include information on the geometry and topology of zeolite channels, as well as shape and size of molecules. Extra composite descriptors are also present to provide the models a physical basis for predictions. Multiple regressors of different nature are considered: Support Vector Regressors, Gradient-Boosted Trees, extreme Gradient-Boosted Trees, and the TabPFN pretrained model. Out of all the models, TabPFN yields the highest generalization performance and lowest error. An interpretability analysis is conducted to assess whether the decisions abide by the governing physics of adposition. It is confirmed that the top descriptor choices abided by the necessary physical constraints, but also that secondary properties such as shape-based selectivity are also accounted for. It is shown that despite both classifier and regressor being insensitive to random splits in data, the regressor is prone to overfitting at low fractions of data withheld for testing. The cascade model is compared with an Artificial Neural Network for training and deployability. Despite training taking more resources for the neural network, the latter is lighter both in memory and storage when compared to the cascade. This work builds on previous research in predicting the Henry coefficient at zero loading. Using this previous model and the findings of this work, one can draw the full adsorption isotherm for any alkane, thus enabling the analysis of adsorption behaviour of alkane mixtures using IAST. ... Read more

The increasing electrification of aircraft subsystems and the integration of high-power-density components are creating significant challenges for onboard thermal management systems. These key subsystems are reaching their performance limits, motivating the development of new advanced solutions. One promising option is the adoption of innovative heat exchanger concepts featuring complex internal geometries, such as strut-based lattice structures, and triply periodic minimal surfaces (TPMS), that are only feasible to manufacture through additive processes. The aim of this work is to develop a systematic methodology to characterize the thermo-hydraulic performance of advanced heat transfer structures based on periodic unit cell (Representative Volume Elements) simulations. The resulting numerical data are then used to calibrate reduced order models suitable for the preliminary design of heat transfer devices. More in detail, the study focuses on the characterization of four heat transfer topologies over a wide range of Reynolds and Prandtl numbers and structure porosities. The performance data from the simulations were used to derive empirical correlations for the friction factor and Nusselt number as a function of the topology porosity. These correlations were then implemented into an $\epsilon-NTU$-based model to support preliminary design activities. To verify this methodology, a cold plate was analyzed and the results were validated against CFD simulations. The methodology showed adequate accuracy for preliminary design, providing reliable predictions where CFD is computationally prohibitive. Future work will expand geometric inputs beyond porosity to broaden the design space of additively manufactured heat transfer devices. ... Read more

In an effort to reduce CO2 emissions, the energy sector is undergoing a profound transformation, targeting decarbonization and more efficient energy use. In this context, power plants based on the Organic Rankine Cycle (ORC) concept have the potential to play a key role, both as primary power production systems and for the recovery and conversion of otherwise unused thermal energy.

ORC power plants are diverse and complex, and are characterized by the strong interdependency between the thermodynamic cycle characteristics, equipment design, and properties of the working fluid. For base-load power production applications, where the return on investment is closely correlated with power plant efficiency, it can be beneficial to adopt advanced thermodynamic cycle configurations, such as those based on the use of a zeotropic mixture as working fluid or a supercritical pressure in the heating process. These configurations aim to mitigate the heat transfer irreversibilities that arise because the working fluid undergoes phase change at constant temperature. The benefit that these configurations have on power plant efficiency is thus proven. However, this efficiency improvement comes at a cost: as the temperature differences driving the heat transfer process are reduced, the required heat transfer area-and thus, the cost of the component- increases. Consequently, a complex tradeoff exists between plant power output and plant capital expenditure. It therefore remains uncertain whether adopting such configurations, particularly the use of mixtures as working fluids, is also advantageous from an economic standpoint.

The objective of the research documented in this dissertation is to address this question. The main contributions include detailed preliminary design models of heat exchangers and turbomachinery, and the development of a new integrated ORC design framework, called WoPycle. This framework enables the simultaneous optimization of thermodynamic cycle parameters, preliminary component design, and molecular structure of the working fluid. The optimization of the molecular structure is achieved by relying on the PCP-SAFT equation of state model, equipped with group contribution methods, which enable the prediction of the fluid properties solely from the molecular groups that compose the molecule. These groups are thus treated as optimization variables, thereby allowing the identification of the optimal molecular structure of the working fluid or mixture components, as well as the discovery of novel compounds, which have not yet been considered so far due to the lack of adequate models.

WoPycle was used to perform the integrated design and optimization of ORC power plants that convert thermal energy from a low-temperature heat source into electricity, considering both multicomponent working fluids and supercritical cycle configurations. Results show that, due to the high cost of the air-cooled condenser and the distribution of exergy losses among the main components of the plant, the thermodynamic benefit of using a zeotropic mixture—which leads to a 6–7% increase in net power output—does not sufficiently reduce the Levelized Cost of Electricity (LCOE) of the plant. Therefore, for this application, a zeotropic mixture is not recommended as the working fluid, as the additional practical challenges, specifically controlling the concentration of the components during plant operation, far outweigh the advantages. On the contrary, adopting a supercritical cycle configuration appears more promising, as heat transfer irreversibilities are reduced in the Primary Heat Exchanger (PHE), which is a comparatively more affordable component. In principle, an LCOE reduction of 6-7% is achievable, provided that the working fluid flows on the shell side of the PHE, as this minimizes the cost of the component. Nevertheless, supercritical power plant operation with this arrangement is not documented in the literature. Thus, the practical feasibility remains uncertain.
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The improvement of the efficiency of aircraft propulsion systems and the reduction of fuel consumption are key objectives in the development of next-generation propulsion systems. The feasibility and performance of promising concepts such as combined-cycle engines, hybrid-electric propulsion, and hydrogen-fueled systems often depend on complex thermal management or cooling systems, where compact heat exchangers (HXs) are critical components. More in detail, the integration of HXs in airborne systems is limited by strict space and volume constraints as well as by the impact of pressure losses and mass on the performance of the propulsion system. Therefore, sub-optimal HX designs can offset or even negate potential performance gains.
This dissertation documents research work performed as part of the ARENA project, which aims to develop methodologies for assessing the fuel-saving potential of airborne organic Rankine cycle thermal energy harvesting systems serving as bottoming units to gas turbine engines, leveraging unconventional propulsion system configurations. The first objective of the present research is to advance the state-of-the-art regarding methodologies for the optimal design and integration of aerospace-grade compact HXs into novel propulsion systems. The second objective is to investigate the potential of the Meredith effect, whereby the drag introduced by an air-cooled HX is offset by the acceleration of the heated airflow through a nozzle, for airborne ORC units, and to develop methods for the optimal design of the ramair cooling system.
To this end, an HX model for single- and multi-pass configurations, applicable for sizing and rating problems of components operating with sub- and supercritical working fluids, was implemented in the in-house software HeXacode and validated with experimental data. Preliminary design tools based on this HX model have been embedded within the system design framework for integrated system-and-component optimization of aero engines featuring an ORC bottoming unit. The system design framework was used to optimize the design of combined-cycle auxiliary power unit (CC-APU), turboshaft (CC-TS), and turbofan (CC-TF) configurations. The results demonstrate that achieving optimal performance requires balancing HX thermal effectiveness, pressure loss, and mass. However, this integrated optimization approach is computationally time-consuming. Therefore, a data-driven surrogate modeling methodology has been developed to predict the performance of optimized HXs under variable operating conditions and design specifications. This method reduces the computational cost associated with system optimization studies by more than half.
Results of optimal combined cycle engine configurations demonstrated that the thermal energy rejected by the condenser to the ramair stream can increase propulsive efficiency and specific thrust. This thrust gain is primarily influenced by the total pressure losses in the duct, ram-air temperature increase, and total to static nozzle pressure ratio. Therefore, the thrust that can be generated by the ram air cooling duct is much larger in the case of the CC-TF, whereby the condenser is positioned aft of the fan and outlet guide vane within the bypass duct, compared to the CC-TS configuration.
Due to the significant influence of the ram air cooling duct on the aerodynamic performance of the propulsion system, an accurate duct model is required for preliminary design studies. Therefore, a CFD model of the ram air duct has been developed, whereby the HX is modeled as a porous zone while a steady state RANS solver is used for the airflow within the duct. The CFD model has been used to develop a lumped parameter model of the ram air duct. The accuracy of the developed lumped parameter model is comparable to that of the CFD model, and enables fast evaluation of the duct performance at both design and off-design conditions. The lumped parameter model is also used to quantify the influence of HX and duct geometric design, heat rejection, and total pressure losses on drag recovery through the Meredith effect, providing new guidelines for ram air cooling systems for airborne applications.
The methodologies developed in this research work are directly applicable to waste heat recovery systems, fuel-cell-based electric propulsion or turbo-electric propulsion systems, and support early-stage preliminary design assessments. These results contribute to reducing the uncertainty in adopting combined-cycle and other novel propulsion systems, and to guiding future development of lightweight, high-performance HXs for low-emission aviation. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

Decarbonizing aviation requires the development of novel propulsion systems that would be powered by renewable energy stored in batteries, green hydrogen, and e-SAF (a type of sustainable aviation fuel). To increase the viability of these carbon-neutral solutions, minimizing mission energy consumption will remain the key driver of the design of next-generation aircraft systems and their components. Additionally, increasing importance is placed on thermal energy recovery and thermal management, which necessitates the design of high-performance thermal components, namely heat exchangers and heat sinks.

This dissertation documents research on shape optimization using the discrete adjoint method and CAD-based parametrization for the design of aerospace-grade heat exchangers. The main outcome of this work is the development of the optimization framework to concurrently optimize multiple heat transfer surfaces parametrized using a CAD method based on Non-Uniform Rational Basis Splines (NURBS) and the discrete adjoint method available in the open-source computational fluid dynamics (CFD) software SU2. The application of the design method is demonstrated for two-dimensional and three-dimensional heat transfer surfaces in configurations representative of aircraft condensers and evaporators, as well as heat sinks for thermal management. In this regard, two formulations of surface sensitivity are proposed such that the resulting optimal solutions can feature identical shapes using averaged sensitivities or non-identical shapes when optimized concurrently, albeit independently. Additionally, the feasibility of integrating the CFD-based method in system-level design and its potential for enhancing system performance are investigated.

The results obtained using the design method show that the application of this framework can achieve geometries of thermal components with reduced pressure drop and enhanced heat transfer coefficient compared to conventional designs. The automated design chain applied to a two-dimensional configuration representing tubular heat exchangers reduced the pressure drop significantly while constraining the heat transfer rate. Using three-dimensional shape optimization of pin-fins with conjugate heat transfer resulted in an unconventional fin shape that led to a simultaneous reduction in total pressure losses and an increase in heat transfer coefficient. These performance improvements of about 20% corresponding to optimal geometries obtained from shape optimization can lead to significant gains in the performance of the system, as demonstrated by its application in the early phase of system-level design reported in this work. Future developments on such a design method have the potential to conceive designs of the next-generation heat exchangers that could be deployed in propulsion systems, enabling carbon-neutral aviation. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more