The pressure-fed rocket cycle is well suited to small engines, but suffers from mass penalties when scaling up to larger systems with higher chamber pressures, making the pump-fed cycle a natural solution to achieve higher performance. Radial inflow turbines (RITs) are of particular interest as they may achieve high pressure ratios in a single stage, leading to compact, high-power-density designs suitable for small-scale open cycle rocket engines.

This thesis investigates the feasibility of RITs for a hypothetical successor to Delft Aerospace Rocket Engineering’s Stratos V rocket. Vaned and vaneless RITs were designed in AxStream and validated against CFD in Ansys CFX. Both satisfied the 24 kW power requirement, enabling the rocket to outperform pressure-fed and electric pump-fed configurations, but exhibited large efficiency losses from tip leakage, high outlet Mach numbers and severe flow separation. The vaneless turbine achieved lower mass flow and higher power, whereas the vaned turbine showed more potential for optimisation through improved stator and volute design. A preliminary design of a more typical axial turbine suggests higher efficiency and practicality, but was not yet validated.

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.

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.

The focus of this thesis is the preliminary design optimization and performance analysis of non-cavitating centrifugal turbopumps using a 1D lumped-parameter method. Centrifugal pumps are critical components in many engineering applications due to their high efficiency in converting mechanical energy into fluid pressure. This study presents the development and validation of a computationally efficient model to design and predict the flow properties along different stations of a centrifugal pump. The methodology minimizes the reliance on expensive CFD simulations, making use of multiple design parameters to control the impeller, diffuser, volute, and exit cone configurations to ensure a robust and efficient performance.

A common challenge in turbopump design is the prediction and mitigation of cavitation, which occurs when the fluid static pressure falls below its vapor pressure. This leads to the formation and subsequent collapse of vapor bubbles, which can cause severe damage to pump components. Given the complex physics involved, this work makes use of reduced-order models to predict, and avoid cavitating regimes through design adjustments.

This thesis provides a comprehensive framework for optimizing the design of centrifugal turbopumps by integrating validated loss models and a reduced-order modeling approach. The proposed design methodology is applied to develop a centrifugal pump for the TU Delft ORCHID facility, demonstrating its feasibility and effectiveness. This research contributes a practical modeling tool for centrifugal turbopump optimization and performance analysis.

Climate change is a pressing issue. Hydrogen aircraft are an excellent alternative to current kerosene aircraft, and appear to be the most viable long-term solution for net-zero flight. One of the main challenges for hydrogen aircraft is to sustain a low boil-off rate. This research explores the solution of active cooling of the LH2 mixture in the fuel tank. It focusses on the construction of a thermodynamic model for cryogenic liquid hydrogen fuel storage in aircraft, a system model for single-stage Reverse Turbo-Brayton Cryocoolers (RTBC), and the conceptual design of the RTBC’s miniature high-speed compressor. This is used for the integrated modelling of the RTBC, compressor and LH2 fuel tank for an exploration study of the carbon neutral hydrogen concept of the long-range Flying-V aircraft. Results show that the RTBC might offer a valuable addition to the Flying-V design space for boil-off control in addition to careful design of the insulation and tank shape.

In the scope of global decarbonisation, hydrogen has been identified as an essential energy carrier. Not only as an energy storage solution for further electrification in vehicles but also as a primary fuel source in processes that rely on chemical energy which cannot be easily electrified. However, at ambient conditions, hydrogen is also a very sparse gas with a low energy density. This requires it to be either liquified or compressed as a gas for most storage and transport solutions, both of which are rather energy-intensive processes. In the case of compressed hydrogen this leads to a significant amount of potential energy stored in the storage tanks. In current applications, the high pressure hydrogen is then expanded through a valve to reach the system’s desired working pressure. A process in which most of the potential energy stored in the compressed is lost to thermal effects.

The research in this thesis is focused on identifying a suitable application in which an expander could be applied to recover part of the potential energy stored in compressed hydrogen and to provide a conceptual design and a thermodynamical analysis of this expander to assess its efficiency and feasibility and has been caried out in collaboration with Ayed Engineering GmbH.
The literature study provided an overview of the current hydrogen landscape and the potential applications in which an expander could be applied. These have been evaluated and led to the selection of a hydrogen refuelling station as a suitable application for this purpose.

A system model has been developed to replicate a refuelling process in a hydrogen refuelling station and to serve as an operating environment for the to-be-designed expander. Based on the operating parameters of the hydrogen refuelling station, a number of expander designs have been developed, each optimized for the operating conditions in the station at a certain timestep. Empirical loss models have been applied to assess the performance of each of these designs across the entire operating range. From this, it was possible to find which potential design would provide the best overall performance. This selected design and the operating conditions at its optimal performance point have then been used in a 3D CFD validation case using Ansys CFX.

The results indicate that employing a radial inlet turbine for the recovery of potential energy stored in compressed hydrogen in a hydrogen refuelling station can indeed lead to notable operational cost savings but also that there are several practical challenges which remain to be solved. Specifically, the extremely high rotational speeds that are required in operation and the miniature design features of the turbine design. To address these, a sensitivity analysis of several system parameters and design assumptions has been included to provide guidance for future work.

Analysing and developing turbomachinery at off-design conditions requires the usage of robust and efficient Computational Fluid Dynamics (CFD) solvers. With the introduction of the computer, many advancements have been made in the field of numerical methods involving these flow problems. These numerical methods are used in order to solve these complex flow structures that are formed due to the interaction between the rotating machinery and the fluid. With the emergence of new design paradigms using computer-aided optimisation, novel solver methods are required which are able to deal with the increase in computational cost and the convergence difficulties following off-design conditions. One of the CFD solvers that is used for turbomachinery design is the open-source software SU2. SU2 is currently developed partially at the Delft University of Technology, where an increase in solver performance with respect to turbomachinery aerodynamics is greatly desired. The current work provides a numerical study of SU2's current performance with respect to turbomachinery analysis, as this is currently unknown.

The current performance of SU2 is to be analysed using steady RANS turbomachinery simulations. The research conducted by Xu et al. \cite{Xu2020} will be used as reference data in order to validate SU2's performance together with data obtained from the CFD solvers CFX and Numeca. The research conducted by Xu et al. resulted in the development of their NUTSCFD solver, which showed strong performance with respect to turbomachinery analysis. Four test cases are set up and used in order to analyse SU2. The test cases that are considered for the numerical study include: the NACA 0012 airfoil, the LS89 turbine cascade, the MTU centrifugal compressor and the 1.5 stage ETH turbine. The first three test cases are validated using the results obtained by the NUTSCFD solver, where the ETH turbine is validated using CFX and Numeca. Following these test cases, SU2's performance is analysed using the residual behaviour. Xu et al. dedicate their performance increase to be the result of the Newton-Krylov method that is implemented in their NUTSCFD solver. SU2 also includes a Newton-Krylov solver, but its performance with respect to turbomachinery is also unknown. A performance assessment with respect to SU2's Newton-Krylov method involving turbomachinery analysis is therefore conducted as well.

The results obtained using the NACA 0012 and LS89 test cases show a discrepancy in solver performance involving SU2. With respect to SU2's Newton-Krylov solver, the NACA 0012 test case shows a reduction in non-linear iterations where this is not found for the LS89 test case. Both results showed however a large difference in performance when compared to the NUTSCFD solver, where SU2's standard solver was also unable to match NUTSCFD. The results obtained following these test cases have led to the development of the MTU test case, where the MTU test case was to be used in order to provide a more accurate comparison between SU2 and NUTSCFD. Instead, SU2 was unable to run the MTU test case, where the solver showed stalling behaviour...

In the pursuit of reducing the climate impact of aviation, there has been an increased interest in the adoption of renewable energy technologies. Examples of revolutionary technologies include hydrogen fuel cell systems and waste heat recovery using the organic Rankine cycle. The design of viable energy conversion systems for aviation poses unique challenges in terms of efficiency, weight and size. To this end, research on small-scale turbomachinery operating at high rotational speeds is increasingly pursued in the context of, for example, fuel cell air compressors or organic Rankine cycle turbines. Such machines typically call for oil-free operation to avoid contamination of the process fluid. Gas foil bearings can prove to be an enabling technology due to their reliability, oil-free operation and compatibility with high rotational speeds.

The use of gas foil bearings to support organic Rankine cycle turbines requires lubrication with complex working fluids at operating conditions near the saturated vapour line or thermodynamic critical point. Although there has been an increased interest in high-pressure gas lubrication in recent scientific literature, there is still a lack of understanding of the effects of non-ideal compressible flows on the performance of (gas foil) journal bearings. In order to further address this knowledge gap, this work focuses on the modelling of such bearings lubricated with dense fluids made by complex molecules like those adopted for waste heat recovery at high temperatures in aviation.

The compressible Reynolds equation governing the thin-film flow within the bearing is discretized using a finite difference method. The solution to the non-linear problem is obtained using a relaxation method in which a thermodynamic software program updates the non-ideal thermodynamic state properties after each iteration. The load-carrying capacity of the bearing is obtained by integrating the resulting steady-state pressure field around the rotor shaft. A perturbation method is applied to obtain the stiffness and damping coefficients used in a linearized rotor-dynamic analysis. The developed computational tool allows for the analysis of bearing performance under varying operating conditions.

The conclusions of this work emphasize the challenge of generating sufficient load-carrying capacity and rotor-dynamic stability associated with high-pressure gas lubrication in journal bearings. Bearings operating with compressible lubricants near the thermodynamic critical point are typically characterized by turbulent thin-film flows with non-negligible molecular interactions. Reduced peak pressures within the gas film are anticipated, resulting in a reduced load-carrying capacity as compared to ideal gas lubrication flows. It is shown that non-ideal thermodynamic effects have an impact on rotor-dynamic stability by affecting the steady-state attitude angle of the bearing.

The modelling of a gas foil journal bearing used to potentially support the turbine of the organic Rankine cycle hybrid integrated device (ORCHID) of the TU Delft has finally been considered. The results show the utility of a numerical model in assessing bearing performance and understanding the associated physics. This work can be used as a basis for future analysis and design of gas foil journal bearings lubricated with high-pressure process fluids.

Developments in computational capabilities and the always increasing demand for higher performance of internal flow applications has meant that Computational Fluid Dynamics (CFD) has become an essential tool within the design process. A key point of interest is to couple the fluid solver with numerical optimisation techniques in order to obtain a more automated design process that can handle a vast amount of different design aspects. The open source CFD suite SU2 has emerged as an enabler for aerodynamic shape optimisation involving a large number of design variables, due to its efficient, accurate and flexible discrete adjoint solver. 
For the adjoint-based aerodynamic design optimisation of internal flow applications the deformation of the volumetric mesh has to be performed in an robust and efficient manner. Often small wall clearance gaps and periodic domains are encountered in internal flow domains, which could potentially lead to the deterioration of the mesh. Sliding boundary node methods can be applied in order to maintain the mesh quality in case of small wall clearance gaps. Additionally, periodic boundaries can be displaced in a periodic manner following the applied deformation in order to prevent low quality cells near the periodic interface. Therefore, it would be of interest to implement the sliding boundary node methods and periodic conditions in a Radial Basis Function (RBF) interpolation method, one of the most robust mesh deformation methods available. 
Additionally, the computational efficiency should be considered, since high computational times should be prevented for large and complex three-dimensional cases with a high number of design variables. 
The aim of this thesis project is therefore to develop a robust and computationally efficient mesh deformation method suitable within the discrete adjoint optimisation framework of SU2 for internal flow applications by means of developing an implementation of the RBF interpolation method including sliding boundary node algorithms, periodic boundary conditions and data reductions methods.
The sliding is achieved by replacing the interpolation condition for the sliding nodes with a planar slip condition. Or alternatively, by freely displacing the sliding nodes based on the known deformation and subsequently projecting the nodes back onto the boundary. The periodic displacement of the boundaries is ensured by making the distance function of the RBF periodic. The periodic nodes are then treated as internal nodes to allow them to move.
The developed RBF-SliDe tool is able to generate higher minimum mesh qualities compared with the regular RBF interpolation method. The sliding of the boundary nodes reduces the degree of skewing of the mesh elements in case of drastic deformations, resulting in a higher minimum mesh quality. Furthermore, the introduction of the periodic displacement prevents low quality skewed or compressed mesh elements, as the periodic boundaries move along with the deformation. 
The Aachen turbine stator blade is considered as a realistic three-dimensional test case. For this stator blade an optimised geometry was available, which was obtained with an adjoint-based aerodynamic optimisation performed with SU2. Therefore, the resulting minimum mesh quality is compared to the one obtained with the more conventional linear elasticity equation method as used in SU2. The minimum mesh quality obtained with the RBF-SliDe tool is nearly three times higher compared to the minimum mesh quality of the linear elasticity equations methods. This highlights the potential of the periodic sliding RBF interpolation method in terms of preserving the mesh quality.

Propeller propulsion systems combined with boundary layer ingestion (BLI) have been proposed as a propulsive system to reduce the CO₂ emissions of aircraft by 50% by 2050. BLI leads to increased disturbances at the propeller disk and causes more noise due to fluctuating blade loading. Using the APPU project as a test case, unsteady inflow is used to determine whether a metric to lower blade loading fluctuation on an airfoil level is possible.

Therefore, an optimisation was created using an Euler solver reliant on the harmonic balance method and the adjoint method to lower the computing time of the simulation. The combination of these methods has proven successful in turbomachinery applications. The optimisation scheme uses the modelled APPU inflow conditions to optimise for the drag coefficient while lowering the root mean squared error of the lift coefficient during the simulations by applying a constraint in various optimisation runs.

The use of CFD-based optimizations is becoming a standard for turbomachinery design. Although the use of such optimizations can lead to great improvements of the machine performances, there are still several shortcomings in the common design procedure of turbomachines. In fact, this entails the optimization of the component shape, and only a retrospective check on manufacturing feasibility and structural viability. This might lead to several design iterations and lot of time wasted.
This thesis project aims at developing a multidisciplinary optimization method where the structural behaviour of the part can be checked already in the first part of the design process. This is done by using the "cold" design as first guess of the optimization, and by performing a fluid-structure interaction to predict the deformations of the part, and simultaneously investigating the behaviour of the flow field, in order to obtain the performances achieved by the operating "hot" design.