Shipping is one of the most cost-effective and environmentally sustainable modes of transportation. Given that approximately 60% of a typical ship’s propulsive power is used to overcome frictional drag, implementing practices to reduce this resistance stands to yield substantial economic and environmental benefits, (Larsson & Raven, 2010). A promising drag reduction technique for a ship is air lubrication. Damen Shipyards Group is currently making this technology commercially available as the Damen Air Cavity System (DACS). The system reduces the frictional resistance of a ship by creating stable air cavities on the bottom hull of a ship. The air cavities cause a reduction in friction drag by decreasing the wetted area of a ship’s bottom. During the development of the DACS system, it was observed that air cavities also change the inflow into the propeller. Both the changed inflow and the frictional drag reduction affect the propulsive efficiency and required propulsive power of the vessel. This thesis aims to provide a better understanding of how the propulsive performance of a ship is affected by air cavities. Additionally, a key application for air lubrication systems is on inland waterway vessels, which frequently operate in shallow waters. However, the impact of shallow water conditions on the performance of the air cavity system is currently unknown. The research goals of this thesis are investigated with the help of computational fluid dynamics (CFD). A literature review identified the most feasible method to model a ship with air cavities i.e. representing the cavities as surfaces with a slip boundary condition. The influence of the air cavities on propulsive performance is investigated by studying the change in nominal wake field, thrust deduction, and propeller efficiency. Three ships were investigated: a cargo ship, a cruise ship, and an inland ship. The CFD results of the cargo ship were compared to sea trial data. Model test data was available for comparison of the cruise ship results. First, a grid convergence study and a sensitivity analysis were performed to investigate the accuracy of the numerical simulations and the power predictions. The biggest source of numerical uncertainty arose from the pressure drag. From the sensitivity analysis, it was found that the uncertainty of the power prediction is mainly affected by the uncertainty of the thrust prediction, followed by the uncertain propeller geometry for the cargo ship. For the cargo ship, it was found that the air cavities cause a significant frictional drag reduction. It was also found that the air cavities caused a decrease in pressure drag because they decreased flow separation at the stern. Furthermore, a strong decrease of the nominal wake fraction was observed for this ship, because the air cavities change the boundary layer on the bottom of the ship. A change in propeller efficiency was also observed because the propeller working point changes. The magnitude of the change was larger than for the other ships because this ship has a controllable pitch propeller running at a fixed rpm. When comparing sea trial data to the CFD results, it was found that CFD underpredicts the power, especially at higher speeds. Next to this, CFD predicted a larger reduction in power than measured during the trials. The prediction of the cavity length was identified as the most likely cause for the difference. A good comparison between model tests and CFD results was found for the cruise ship. It was found that the drag reduction and change in propulsive performance could be predicted reasonably accurately by modeling the air cavities as surfaces with a slip boundary condition. Furthermore, it was observed that the air cavities caused little change in propulsive efficiency on this ship. This is because the propellers of the cruise ship are located further away from the boundary layer of the ship and the wake field is therefore only slightly affected by the air cavities. On the inland ship, it was observed that the pressure drag and flow separation at the stern were influenced due to the air cavities. However, no comprehensive conclusions could be made due to scatter in the data. This is most likely due to the uncertainty present when modeling flow separation. Furthermore also for this ship, a decrease in propulsive efficiency was found because the air cavities decreased the wake fraction of the ship. Additionally, CFD simulations were conducted for the inland ship at varying water depths to assess how shallow water conditions impact the performance of the air cavity system. Since the ship’s frictional drag increased in shallow water, the total drag reduction from the air cavities also increased. Next to this, a change between pressure and flow separation when comparing air on and air off was observed. Also here, no strong conclusion could be made due to scatter in the data. Lastly, it was found that the change in wake field caused by the air cavities is larger in shallow water than in deep water. ii Based on the results of the three ships studied it can be concluded that an air cavity system affects the propulsive performance of a ship. It was found that an air cavity system reduces the wake fraction of a ship because it limits the growth of the boundary layer on the bottom of a ship. The reduction increases for ships with a high block coefficient and a large air-covered area. For a twin screw ship, the change of the wake field is less significant. The change in propeller efficiency depends on the resistance reduction, possible change of the wake field, and the original working point of the propeller. Furthermore, it was found that the thrust deduction effect is not affected by the air cavity system. It can also be concluded from the results of the cruise ship that the flow around a ship with air cavities can modeled reasonably accurately provided that the shape of the air layer under the ship is known. More research is recommended on how air cavities change the flow separation and pressure drag of a ship.

Supercritical CO₂ (SCO₂) is a promising alternative to traditional working fluids in heat pumps and power cycles due to its high density, thermal efficiency, and stability. These properties allow for the design of more compact and efficient equipment. However, accurately modeling supercritical heat transfer, especially near its pseudocritical point, is challenging, because of extreme variations in its thermophysical properties. As a result of these inaccuracies in modeling, large fault margins must be taken into account, leading to overdesign of equipment.

Current methods for estimating heat transfer in such systems include empirical measurements, computational fluid dynamics (CFD), and Nusselt correlations. Although measurements provide accurate data, it is not a scalable solution. CFD methods offer this scalability. However, CFD cannot be applied in complex scenarios such as modeling of SCO₂ due to the trade-off between computational cost and accuracy. Nusselt correlations have low computational demand and are a scalable solution but come with low accuracy. Therefore, Nusselt correlations are not suitable for the design of equipment.

To overcome these limitations, this research applies a new model to predict heat transfer to SCO₂ flowing upward in a heated vertical tube. Current prediction methods use artificial neural networks (ANN). This research applies a convolutional neural network (CNN) for its ability to capture spatial context from the heat transfer trajectory. This property enables CNN to capture global and local patterns important for accurate prediction in SCO₂ heat transfer. In addition, the multiple kernels per layer enables the model to extract a large range of features from the heat transfer trajectory allowing for higher accuracy. The developed CNN model has shown superior performance, achieving an R² score of 0.970 and an MSE of 1477, outperforming existing ANNs and Nusselt correlations.

Furthermore, a feature importance study is done, to identify the nondimensional parameters that are important for heat transfer prediction The feature importance study highlighted the importance of parameters such as the Reynolds number and the Froude number in improving the model predictions. In addition, the feature importance study led to the development and identification of a new nondimensional parameter, (q_wk)/(T d), as one of the most important features influencing heat transfer. Furthermore, the experiments show that normalization is vital to enhance model stability and performance, countering issues such as exploding or vanishing gradients.

The findings in this research suggest great potential for using machine learning models to design more effective and compact heat transfer equipment. Future work will focus on a feature importance study for normalized, dimensional and nondimensional parameters for improved predictions. In addition, more synthetic data should be generated in the heat transfer detoriation and heat transfer enhancement areas for better prediction of those parts. Finally, a study into the application of machine learning models for designing heat transfer equipment should be done to show the benefits of such models.

Decarbonization is an important step to achieve the goals set by the Paris Agreement. Greenhouse gas emissions should be reduced to zero, and therefore, the reliability of fossil fuels should be reduced. This causes a shift of interest towards more renewable solutions. However, the intermittent nature
of renewable energy sources such as solar and wind energy leaves a gap in the energy supply. Currently, this gap is still filled by natural gas, but biofuels could potentially help in decarbonizing this gap. Biofuels could be prevaporized and used as an alternative fuel within existing natural gas-fired power plants. Ethanol is an interesting biofuel as it has a relatively low boiling point, meaning that it is relatively easy to evaporate. Next to that, ethanol has a similar Wobbe Index (WI) compared to natural gas, meaning that it could be potentially used with only minor adjustments to the gas turbine. This research will focus on implementing ethanol as an alternative fuel for the Killingholme power plant, a
600 MW power plant in an open cycle gas turbine (OCGT) configuration that Uniper operates in the United Kingdom.

This research focused on the effect of ethanol on the process design and combustion characteristics, where the process design was only briefly touched upon. It was found that ethanol should be heated to a temperature of 467 K to be in vapor form at the relevant gas turbine conditions. A process design was
made for the baseload operation, where the required heat for the evaporation process was provided by the flue gas flowing out of the gas turbine. Next to that, the use of ethanol requires slightly higher volume flow rates, meaning that the pipes and fittings should be adjusted to keep the desired fuel pressure. The effect of ethanol on combustion characteristics was researched by a kinetic modeling study and a CFD study focusing on fuel-air mixing, where the results will be compared to methane. A RANS study was performed for the CFD study, which showed that the use of ethanol results in a better quality of mixing. From the kinetic modeling study, it was found that ethanol has a lower autoignition
delay time than methane. This will probably not lead to the autoignition of the fuel-air mixture in the mixing section, but it could lead to periodic flashes in regions close to the recirculation zones within the burner. It was also found that ethanol has a 78% higher laminar flame speed than methane. Next to that, it was found that ethanol has an effective Lewis number of 1.56 at the relevant gas turbine conditions, whereas methane has an effective Lewis number close to unity. From the laminar flame speed and the effective Lewis number, it was concluded that the use of ethanol results in an increase in turbulent flame speed. The increased turbulent flame speed and better quality of mixing of ethanol suggest a decrease in flame length. The decrease in flame length and increase in turbulent flame speed lead to higher flashback risks, but it is expected that this will be within the flashback margin of the burner. Based on the kinetic modeling study and the fuel-air mixing study, it was concluded
that ethanol will have NOx emissions similar to methane and that the driving energy source of the combustion dynamics will probably shift to higher frequencies.

Recommendations for further research are the gas turbine’s start-up, ethanol contaminants, extended CFD study of the burner, and the blending of ethanol and natural gas. Further research could complement the already promising results from this report, and eventually, this could lead to combustion tests where ethanol will be used as fuel.

The world is witnessing unprecedented climate changes, which have worsened over the past couple of years. Despite significant progress in renewable energy, achieving a reduction of global warming remains challenging. Renewable energy sources like solar, wind, and hydroelectric technologies offer promising solutions, but challenges persist due to their intermittency and geographical dependency. Green hydrogen, generated through electrolysis using renewable energy, has come up as a solution for storing electric energy and providing a constant energy supply.

Alkaline water electrolysis has emerged as one of the most promising electrolysis methods due to its large-scale operation, long durability and low costs. This does not come without challenges, one of them being leakage currents, which reduces the efficiency of the electrolyser. Leakage currents occur when not all current is used for hydrogen production, but some of it leaks into, for example, the produced hydrogen stream. To reduce these leakage currents, more research into the origins is needed. The first step is modelling the electrolysis while considering as much as possible of the physics happening inside the system. Current models of alkaline water electrolysers are either modelled using only mathematical equations or neglect the operating parameters, which makes the results highly variable per system. Other models do use analytical methods with experimental results, but these models only comprise one electrolysis cell rather than a full stack.

This work consists of developing two three-dimensional models, validating them using experiments, and using them to predict the effect of changes in geometry. The first model was made using COMSOL Multiphysics software and used to research the water electrolysis stack, which comprised one or eight cells using electrochemical relations and physical data. It was found that a model could be made that fitted the experiments within the error margin of the experiments (<2.5%). It lacked flexibility but overall showed good results for an electrolyser stack of one or eight cells. The second model was made using an equivalent electrical circuit (EEC) of the electrolyser in Python via the PySpice module. A steady-state model, including the leakage currents, could be developed by calculating all system resistances, namely the cell, inlet/outlet, and manifold resistances. This model overestimated the performance of the electrolyser by 10-15% for low current densities and 2-4% for high current densities. Nevertheless, it was highly adaptable for different scenarios, making it valuable for research into optimising the electrolysis stack. Both models were used to predict the effect of changes in geometry; the effect of the length of the inlets, and the number of cells. This showed that the EEC model was better suited for this research.

Ascending thoracic aortic aneurysm is a dangerous condition which is hard to locate in patients. CFD can be used to assess someone’s risk of suffering from such an aneurysm and also to assess any possible relation of an aneurysm with relevant parameters, like the shape of the aorta or how blood flows into the aorta.

Existing research on ascending aortic CFD flow generally focuses on assessing relatively high wall shear stress, seen as the cause for aortic wall damage and aneurysms, in patients and not much on any possible relationships between aortic shape and inflow with ascending aortic aneurysm. Work that does exist often uses 4D flow MRI, shown to produce results regarding peak wall shear stresses with less accuracy compared to CFD.

This thesis aims to supply that information using an aortic geometry from a large dataset of synthetically generated aortas together with synthetically generated inlet velocity profiles, showing a possibility to work with-out the need for patient measurements. The main work in this thesis focuses on finding any possible relationship between aortic inflow angle and relatively high wall shear stress, such that the results may better explain how aortic inflow can influence the appearance of aneurysms.

For this, a workflow has been established that allows working with large aortic geometry and inflow profile datasets with relative ease. This workflow process uses a mix of OpenFoam and Ansys Fluent usage.

Results have shown that the flow jet angle in the core of the flow has a significant negative correlation with peak WSS within the ascending aorta for the chosen aortic geometry. This is in contrast with other work showing positive correlations with WSS. This may suggest aortic geometry dependence together with the need to look at other flow variables, like jet impingement angle, to explain how aortic inflow can influence high WSS within the ascending aorta.

Recommendations for future work include: The same study but with a focus on aortas of healthy young people to see if aortas at risk can be found, as existing work mainly focuses on old people with ascending thoracic aortic aneurysm, a similar study with a focus on the inclusion of various aortic geometries to find how aortic geometry may influence the effect inflow variables can have on high WSS, the creation of a tool to easily calculate the impingement angle from aortic flow data and research into if aortic jet flow disruption could reduce peak WSS in the ascending aorta for use in aortic reconstruction.

Silicon based power semiconductors have long been used as the standard in ‘semiconductor technology in power conversion applications’. Recent developments replaces the Silicon with Silicon Carbide as it results in superior performance of the power conversion applications. However, due to the increased performance, challenges regarding heat dissipation emerge and the lifetime of the power semiconductor packaging or power module is compromised. Since this leads to an increase power density, the cooling of the power module is becoming of more importance and the heat sink becomes an interesting component to optimize. The best performance of a heat sink can be obtained when the flow through the device is turbulent. Developing turbulent flow heat sinks by using topology optimization methods can significantly improve the cooling performance compared to the current designs. This work is thus aimed towards improving methods for topology optimization of turbulent flow cooling devices. However, this work focuses on turbulent flow topology optimization only and aims to improve the accuracy of current methods. It is important that the flow physics are accurate since the thermal energy transfer is dependent on the flow field. The current state-of-the-art method based on the 𝑘−𝜔 turbulence model developed by Dilgen et al. is investigated. A design domain is subdivided into elements since the finite element method (FEM) is used, such that an optimization algorithm is able to turn every element into either fluid or solid with the goal of finding the best performing structure. This density based approach, models the solid domain as a highly impermeably porous material. To inhibit flow in the solid domain a Darcy penalization is added to the momentum equation. Moreover, in the method by Dilgen et al. boundary conditions in the other turbulent fields are also enforced using a similar penalization approach. Weaknesses and errors in the density based method are investigated by comparing solutions to ones computed on a body fitted mesh. It has been found that the largest errors in the solution, by using the state-of-the-art method, appear at the solid/fluid interface in the design. In these regions the penalizations are not applied correctly for the desired boundary conditions. Therefore, in this work it is improved on by the enforcement of the boundary condition by using the Dilation method. The Dilation method focuses on the solid/fluid region where it shifts the boundary conditions for the specific dissipation rate (𝜔) and ensures it reaches the desired value at the solid/fluid interface. Secondly, severe flow leakage is found in the “porous” solid domains using the state-of-the-art method. Flow leakage is reduced by using an improved formulation of the maximum Darcy penalization in the solid domain. Finally, the improved approach is investigated in several topology optimization cases and compared to the state-of-the-art Dilgen method. It is shown that by using the new approach, different designs with a better accuracy can be obtained. In an extreme test case, the Dilgen method resulted in an infeasible design which disconnects the flow inlets from the outlets while the new and improved method resulted in a feasible design.

The majority of Dutch homes currently use natural gas boilers to meet their space heating demand. Changes in this significantly large sector are required to achieve the sustainability goals established by the Paris Agreement. Since most renewable energy sources produce electricity, transitioning residential heating systems might cause problems managing national electricity grids. Furthermore, the intermittent nature of renewable energy sources leads to an imbalance between supply and demand.

These challenges can be overcome by combining different storage techniques. An example of such a technique is the thermal energy buffer developed by Borg, which focuses on single-household use. This thesis looks into the feasibility of such a system by comparing different scenarios for residential heating systems.

Three scenarios were modelled using Simscape. In scenario I, a natural gas boiler provided all the heating demand of the house. In scenario II, the heating system consisted of Photovoltaic Thermal (PVT) panels and the thermal energy buffer. In scenario III, the house was heated by PVT panels, the thermal energy buffer, and a heat pump. In all scenarios, the same house was connected to the heating system.

In scenario II, the system sizes were 0, 1, 2, and 3 PVT panels, combined with buffer capacities of 0, 2, 4, and 6 m3. In scenario III, the system sizes were: 3 PVT panels with 6 m3 buffer capacity, 1 PVT panel with 4 m3 buffer capacity, and 3 PVT panels with 2 m3 buffer capacity.

The total yearly heat consumption of the modelled house was 5109 kWh. In scenario I, 378 kg of CO2 was emitted. In scenario II, CO\textsubscript{2} emissions were highly dependent on the sizing of the PVT system and the TESS and ranged from 20 to 405 kg, based on a heating demand of 9444 kWh.

Scenarios I and III maintained a comfortable temperature during the entire year. The heating system of scenario II was insufficient to heat the house throughout the year for all modelled system sizes; however, the scenario could be acceptable with larger PVT and TESS sizing. By comparing CO2 emissions and payback time, the optimal capacity of the thermal energy buffer was found to be 6 m3.

The uncertainty of the future gas price causes a challenge in comparing the cost of electrified heating systems to traditional heating systems. Additionally, it underlines the necessity of decarbonizing residential heating systems to secure comfortable, affordable housing.

The Intergovernmental Panel on Climate Change (IPCC) believes that multiple solutions must be deployed simultaneously to reduce the emission of greenhouse gases into the atmosphere. Carbon, Capture & Storage (CCS) is an unavoidable technique within this portfolio as an intermediate solution. CCS requires transport of CO₂ through pipeline systems and into wells. There are still large uncertainties on the thermodynamics of the CO₂ in flow through valves. The CO₂ flow undergoes significant changes in pressure, temperature and phase distribution when it passes this control valve. Therefore, the behaviour of the CO₂ flow flowing through a control valve is examined in this study.

The simulation of CO₂ in a 3D valve including phase transitions is complex. Furthermore, few validation experiments are available. As a first step, more simple nozzles are simulated. In these devices, the same processes occur and validation data is available. These simulations are validated with experimental data by Nakagawa et al. to examine the accuracy. Three types of models (isenthalpic, Euler and Enhanced Mass Transfer (EMT)) are implemented in increasing levels of complexity to investigate the differences between these models and to consider when complexity is needed or simplifications are valid.

The validation cases show experimental pressure data of high-pressure CO₂ flow through converging-diverging nozzles with phase transitions. The results showed that the EMT model matched the experimental data best. A substantial similarity was achieved regarding the pressure data. The mass transfer mechanism, however, needed adjustments in its coefficients to match the experimental data. Finally, after finding the right values, the EMT shows the best technique for modelling flashing or cavitation.

In short, the overall consequences of the transition in a valve are substantial and must be considered. The behaviour of the high-pressure CO₂ flow is heavily influenced by flowing through a valve. Substantial amounts of vapour are formed, but only after the throat. This is the same for choking condition, which is achieved in the diverging section of the nozzle. The large expected drop in temperature due to pressure reduction has also been noted.

Although a high degree of similarity between the results of the model with experimental data is obtained, there is room for improvement regarding the model. A flaw was discovered in the handling of the thermodynamic properties of the fluid near critical points. Also, the surface tension has not been considered, but might have a substantial influence. Next steps in the research are 2D and 3D simulations of actual valves, but require experimental validation data.

The transportation of high viscosity oil is an ongoing research topic for quite some years because of its importance in industrial applications and the increasing energy requirements of the world population. The water lubricated oil core with a core-annular flow configuration is a viable option for the transport of high viscosity oil. It was observed that there are significant differences between the predictions from the RANS simulations and the experimental data for turbulent oil-water core-annular flow obtained in the flow loop in the lab at TU Delft. Thus, this study is performed to understand the difference between the RANS simulations and the experimental data. 1D and 2D OpenFOAM simulations for turbulent vertical core-annular flow are performed using the Launder-Sharma low-Re k-ε turbulence model. The pressure drop and the oil holdup are imposed while the total flow rate and the water-cut is obtained as the output from the simulations. The results from the OpenFOAM simulations are compared with the DNS data available in literature to understand the impact of the turbulence characteristics and the interfacial waves. Both 1D and 2D simulations were carried out. The 1D model does not include the interface waves. The 2D axisymmetric model is able to capture the travelling interface waves. The 1D OpenFOAM simulations are carried out for five cases with different oil holdups and the results are compared with the asymptotic wall laws. It is concluded that the turbulence is sufficiently resolved by the 1D simulations. The 2D simulations are also performed for five different oil holdups. The 2D simulations are first validated by performing grid independence tests. The dependence of the simulation results on the streamwise domain length, presence of gravity and oil viscosity is also analysed. It was found that the streamwise domain length and the oil viscosity used does not have a significant influence on the prediction of the results. The comparison of the RANS simulations with the DNS data shows that the difference depends on the turbulence levels inside the simulation domain. The difference for the total flow rate between the 2D OpenFOAMsimulations and the DNS data is found to be 10% for fully turbulent core-annular flow while the difference increases to 29% for core-annular flow with no turbulence. The friction factor for the 2D Open- FOAM simulations is found to be about 17% lower for fully turbulent flow while it is 39% lower than the DNS predictions for the simulation with no turbulence. The difference for the holdup ratio (when comparing 2D OpenFOAM simulations with respect to DNS) decreases first from 15% higher for the low oil holdup cases to 8% higher for simulation with oil holdup fraction 0.71 and then increases to 14% higher for the highest oil holdup case. It is also observed from the comparison of the mean streamwise velocities and the mean shear stresses that the turbulence level in the simulation domain impact the extent of the difference. The agreement in the wave characteristics between the 2D OpenFOAM simulations and the DNS data depends on the mixture of wavelengths present in the simulation domain. The RANS simulations predict a specific dominant wavelength while the DNS data gives a mixture of wavelengths for lower oil holdups. The dominance of a specific wavelength increases with increasing oil holdup for the DNS predictions.

The human body is the subject of several interesting phenomena and blood flow comes under that category. The main motivator for this thesis is the flow of blood in aneurysms. An aneurysm is a sudden expansion of an artery, with large expansion angles causing an adverse pressure gradient and leading to flow separation. Several studies have shown the transitional nature of the flow in aneurysms, and it has been seen that the variation of the wall shear stress from cycle-to-cycle is one of the major reasons for the growth of aneurysms, and possibly leading to their rupture at a later stage. It has also been noticed that the turbulent kinetic energy (TKE) does not decay with the mean flow kinetic energy and the periodic kinetic energy of the cardiac cycle. Therefore, it is intersting to research the turbulent decay to see how the flow in an aneurysm can be affected. However, the number of variables in an aneurysm are too high to effectively characterize this, and therefore, a simplified geometry was chosen. Pipe flow is a good choice to start with, as it can mimic the wall-bounded nature of an aneurysm. It also has a well-defined statistically steady turbulent state from where the decay of turbulence can be studied. Additionally, blood being a non-Newtonian fluid, makes it interesting to study the effects of shear-thinning.

To this extent, a Direct Numerical Simulation (DNS) study has been carried out using a higher-order spectral element method code. First, statistics for fully-developed pipe flow are compared with existing results. To a fully-developed turbulent state, a deceleration is applied to bring the flow to a steady, laminar state. The decay of the turbulent quantities is monitored during this process. Comparison studies are undertaken to study the influence of the ramp rate, the dependence of the decay on the initial Reynolds number, and the variation of the results between Newtonian and generalized Newtonian fluids. A modelling approach using RANS has also been undertaken to see if only studying the mean flow is sufficient to characterize the decay.

It is seen that two regimes of decay exist -- a power-law decay based on turbulent scaling, and an exponential viscous decay. The power-law decay is further divided into two stages -- one before the saturation of the integral length scale, and one after the saturation of the length scale -- with the maximum length scale being set by the diameter of the pipe. The exponential model has been validated using the hypothesis of Skrbek (2008). The point of divergence from the power-law to the exponential decay has been hypothesized here. It is seen that for all the cases studied, the point of divergence occurred at $Re_\tau = 60$. It is noticed that the decay is independent of the ramp rates when they are applied at time on the order of magnitude of 1 Eddy Turnover Time (ETT). The decay does show a dependence on the initial Reynolds number and the reasons for this are hypothesized. The RANS modelling used was found to be insufficient due to the inability of the RANS model to gauge the size of the domain. For generalized Newtonian fluids, it is noticed that the decay rate increases with shear-thinning. The results obtained are discussed in the context of an aneurysm. Based on the diameter, length and flow rate of the aneurysm, it can be hypothesized at which stage of decay the flow is, and based on this, it has been discussed whether using a non-Newtonian modelling approach is more beneficial than using a Newtonian approach for the decay.

Experiments to characterise vortices in hyperbolic shaped funnels are being conducted at the Water Application Centre (WAC) in Wetsus. These have demonstrated their higher gas transfer rates in comparison to the conventional aeration systems presently in use. Depending on the imposed flow conditions, different regimes of vortices are formed, among which the Twisted vortical structure is observed to have the highest gas dissolution rates. This has probed several questions on the physical mechanisms responsible on both micro-and macroscopic scales. The present research aims to numerically analyse the flow field organisation in these vortices to reason the observed high gas transfer rates using Computational Fluid Dynamics (CFD).  Transient simulations were performed on a three-dimensional radially structured hexahedral mesh. Multi-phase modelling was done using the Euler-Euler approach-based Volume-of-Fluid (VOF) method, while the turbulent flow was modelled using Shear Stress Transport (SST) based on k - ω equations with curvature correction. The choice of boundary conditions and their location is crucial for forming a stable vortex in the hyperbolic funnels. The position of the air-water interface from experimental results was used to validate the obtained numerical results. Two regimes of the vortex, namely the Twisted and Straight vortical structures, were evaluated for their gas transfer capabilities in terms of Hydraulic Retention Time (HRT), interfacial area and mixing in the bulk.  Instabilities arise in the secondary flow field of these vortical structures analogous to the Taylor-vortices that develop in the well known Taylor-Couette flow systems. In hyperbolic funnels, these instabilities aid in advecting the bulk of liquid to the air-water interfacial region and also enhance mixing within the bulk of water. The former enhances the gas transfer rates while the latter promotes uniform mixing. The strength of these instabilities is qualitatively analysed in terms of average vorticity per unit mass of water. This is found to be higher in the Twisted regime in comparison to other regimes. This is augmented by high air-water interfacial area making this regime possess superior gas transfer rates.  Although the gas transfer rates are high, water exiting the funnel is undersaturated at the given operating conditions. In order to further enhance the amount of gas dissolved few possibilities are qualitatively discussed at the end of this study.     

In order to reach the goals of the Paris Agreement, global emissions must decrease at a high rate. Looking at the industrial sector, the largest share of energy is used for process heating, with fossil fuels as its primary source. A way to improve the energy efficiency and reduce the emissions of process heating is by the integration of heat pump technology.

However, operational costs are still a limiting factor in the widespread uptake of this technology. A potential of industrial heat pumps that will have a positive effect on the operational costs, as well as its general applicability, is the possibility to operate the equipment in a flexible manner. At this point, the potential to respond with high temperature industrial heat pumps to heat demand, the electricity market or to grid congestion, is not yet unlocked.

The focus in this study is on gaining more insight into the dynamic characteristics of industrial heat pump operation. The approach is based on the modeling of a 2 MW high temperature industrial heat pump in the Dymola simulation environment. Based on the system dynamics, a suitable control method is selected and the controller settings are optimized. With this control system in place, the dynamic limitations are studied.

The limiting factor in the considered economizer based heat pump cycle is the ability of the control system to keep the superheat at the screw compressor injection port within acceptable limits during load level changes. The heat pump is found to be able to ramp up and down at a maximum rate of approximately 20%/min. This level of flexibility allows use of the heat pump for both heat demand and electricity price response, as well as for participation in grid load balancing pools. On a higher level, based on these results flexible heat pump operation can be considered possible.

Throughout the last century, high energy-density fossil fuels have been the main energy source for merchant shipping. As the world tries to minimize the effects of global warming, shipowners are forced to significantly reduce their $CO_2$ emissions. Because alternative fuels are expensive, the interest in wind energy, which has been the single energy source for centuries, is renewed. There is a desire for passive and inexpensive wind-assisted ship propulsion (WASP) systems. The two-element wingsail is a promising option. This combination of a wing with a slotted flap can generate more than one and a half times more lift compared to a one-element alternative. It is used in the aviation industry as a high-lift device and has been applied on board yachts in the America's Cup and other races. Used as WASP system places new requirements as limited deck space and the ability to tack. This report describes the effects of the geometry on the performance of the wingsail and advises an ideal configuration that maximizes the overall thrust contribution. Both fixed design parameters such as the profiles and chord lengths and adjustable settings as the angle of attack and flap angle are examined. This is achieved by doing systematical geometry variations at a Reynolds number of 1.86 $\cdot 10^6$ using 2D CFD simulations. The design goal was to maximize the lift coefficient. The variations show that the lift coefficient is very sensitive to the flap angle. Increasing this angle increases the lift, but requires a well-positioned slot to ensure attachment of the flow along the suction side of the flap. The influence of the profile thickness is small. However, a minimum flap thickness was required to avoid flow separation at low angles of attack. Stall was always observed around the leading edge of the flap. The maximum lift coefficient of 2.73 at an angle of attack of 10 degrees was found for a configuration with a NACA0024 wing and NACA0012 flap profile, equal chord lengths, and a flap angle of 25 degrees. The corresponding slot had an offset of 0.5 and a side placement of 2.5 percent of the total chord length. Using the same method, the effects of adding a steering flap downstream of the wingsail were investigated. This third flap should keep the wingsail stable at a specific angle of attack. This ability is interesting for WASP purposes because it eliminates the need for fast-responding software-based control systems. The simulations show that the steering flap, which had a fixed chord length of 30 percent of the wingsail, can stabilize the system. Compared to the standard configuration the decrease of the lift was approximately 10 percent. The steering flap operates in the wake of the wingsail, which causes a decrease of drag of approximately 5 percent. This is an interesting feature because it reduces the negative effects of the steering flap. Depending on their positions, the interaction between all three elements can further enhance the performance of the whole system.

Marine structures can sustain damage due to violent wave impacts that are characterized by complex dynamics involving both water and air. Methods that predict impacts loads for structural design often assume the water as incompressible. These predictions can be inaccurate during impacts where air is entrained in water, as that greatly increases its compressibility. Initial impact forces may decrease due to a cushioning effect. However, the total severity of the impact may increase due to temporal spreading and oscillations of impact pressure. Previous research on aerated water impacts covers breaking waves against fixed structures, flat water slamming or green water events, but not the interactive motion of rigid marine structures in free surface waves. This work aims to evaluate the effect of aerated water impacts on the dynamics of floating rigid bodies in irregular waves. As a part of this work, a state-of-the-art numerical simulation method for aerated water impacts is extended with a monolithic coupling for rigid body motion. Additionally, boundary conditions for wave generation were implemented for a validation of the method during breaking wave impact. Results of the verification of both extensions compare well to other literature. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.

Power electronic systems are reaching higher efficiencies as their technology advances, which often results in components of smaller size with higher power densities. Cooling these components becomes increasingly challenging as high power densities require cooling with large heat fluxes. Topology optimization (TO) of thermo-fluids can be used to find cooling interface geometries which achieve high heat transfer with realistic pumping power. However, current methods for thermo-fluid TO show several issues. Firstly, the fluid models used for thermo-fluid TO show weaknesses that cause deviating behavior compared to conventional fluid models, which can result in large under- or overestimation of heat transfer especially when applied to turbulent flow. Secondly, although this deviating behavior is known to result in inferior modeling accuracy, the achievable accuracy of thermo-fluid TO has never been quantified. Lastly, TO currently requires many modeling parameters to be specified manually. Since these parameters largely affect the accuracy of the thermo-fluid solver, tedious parameter tuning is part of the TO design process. This thesis firstly presents a framework which allows quantitative analysis of the modeling accuracy achievable with density-based thermo-fluid models in 2D. The framework reveals several effects causing errors in turbulent flow, as well as a predictability of density-based boundary layer flows. Secondly, a method is tested which minimizes errors of the density-based thermo-fluid models by meta-optimizing the modeling parameters. Applied to a test-case with turbulent flow, this method achieves up to 27% reduction of the modeling error compared to a parameter sweep. When applied to a laminar TO, it achieves similar accuracy as a manually tuned TO without needing any tuning. A second mitigation method which adjusts the thermal conductivity of porous solid material to compensate for erroneous convective heat transfer is infeasible, as it provides less accurate results than the first method. Lastly, a post-processing method which uses the meta-optimized data is tested and found to provide better accuracy than a conventional post-processing method.

Although the Dutch energy supply is gradually progressing from fossil fuels to renewable energy sources, the consequences for the grid are becoming increasingly evident. Meanwhile, the gas extraction from the Groningen natural gas reserve is de- clining as the induced earthquakes in the northern Netherlands persist. Hydrogen, as a flexible carbon-free molecule, offers a potential solution to the overcapacity of the electricity grid and thus has the potential to fulfill an important role in the fu- ture energy supply. It has recently been proved that by making few adjustments in the grid assets, the existing gas grid can be made compatible with hydrogen. A relating issue to hydrogen distribution through the gas grid is the resulting hydrogen qual- ity. Similarly to natural gas, the roll-out of a national hydrogen grid needs a national quality standard. There will be a variety of end-use applications, such as hydrogen boilers and fuel cell technologies that have different quality requirements.
This report researches the influences of the distribution of hydrogen through the existing gas grid. There are five sources of contamination: odorant, inward permeation of air through polymer pipelines, particles circulating in the grid, leaks causing an entrance for contaminants and byproducts from hydrogen production technologies. All five sources are considered, but the focus of this report is on inward permeation of air through low pressure polymer pipelines. There are three significant risks which are linked to the permeation of air: feed dilution, explosion risk and damage to fuel cells. Fick’s laws for diffusion were used to create a computation model, from which relationships were found between seven variables and the amount of contamination. Correlations were found between the amount of permeated air and the pipeline material, pressure, inner diameter, wall thickness, flow velocity, temperature and soil type. Pure hydrogen was modelled to be distributed through the low pressure grid at different conditions. After traveling 100 meters through an MDPE pipeline with 26 mm inner diameter and wall thickness of 3 mm at a flow velocity of 1 m/s, 1.4 mg oxygen and 1.9 mg of nitrogen per m3 hydrogen will have diffused into the pipeline. The results have been implemented in a case study in Stad aan ’t Haringvliet in Goeree-Overvlakkee, and the contamination for the farthest distance in the grid was found to be 0.057 ppm oxygen and 0.085 ppm nitrogen at a flow velocity of 1 m/s. Following the research set out above, no issues were found in connecting hydrogen boilers to the existing grid. Low temperature PEM fuel cells are more sensitive to impurities than boilers and some concerns were found under certain specific conditions with exceeding the current hydrogen fuel contamination limit for oxygen stated in ISO 14687-2. After traveling 529 m through MDPE and 5.8 km through HDPE the oxygen limit was exceeded. This is not considered as a constraint for the development of a future hydrogen grid, as this upper limit was set for the fuel requirement of metal hydride storage, and existing hydrogen road vehicles have another means of storage. A positive side effect of the presence of oxygen in the hydrogen feed is that it reacts with carbon monoxide, thereby decreasing fuel cell poisoning effects. Nitrogen contamination of the hydrogen feed can dilute the fuel and only at high concentrations increase fuel cell cathode poisoning caused by carbon monoxide. These high concentrations are not expected to be achieved as a result of inward permeation of nitrogen. Consequently, through the obtained results in this research it is believed that a sufficiently high purity hydrogen can be achieved in the existing distribution grid.

Tip vortex cavitation was previously identified as a source of the broadband noise emitted by ships underwater. This leads to unwanted noise and vibrations on board, which reduces passenger comfort, and increases the underwater signature of naval ships. The influence of different cavitation regimes on propellers and boundary layer transition on wings, has led to studies that isolate the cavitating tip vortex. This is done by using a converging-diverging nozzle (Venturi tube) geometry, combined with a suitable inflow condition to simulate a cavitating line vortex. The currently available computational power allowed for a comparative study of SRS methods (i.e. the IDDES model) and RANS models (i.e. the k − ω SST and EARSM) in wetted and cavitating vortex conditions. Both the general flow field, as well as the dynamics of the cavity deformations in the vortex were analyzed in this thesis. The geometry was selected based on the available experimental validation data for the wetted flow case, whereby preliminary studies determined the inflow and outflow lengths of the domain. A Lamb-Oseen tangential velocity profile was specified at the inlet. The vortex strength and viscous core size were tuned to obtain similar inflow conditions as in the experimental measurements, which used a fixed-blade swirl generator to generate the line vortex. The flow field in the wetted vortex case demonstrated an excessive amount of viscous diffusion of the vortex for both RANS models.This was caused by the overproduction of modeled turbulent kinetic energy at the viscous core edge. EARSM resuls were verified in a grid- and time step refinement study, however the large modeling error in the viscous core prevented the validation of the results. The IDDES model results were closer to the experimental reference, but were obtained with an almost laminar flow field. Modeled turbulence was mostly dissipated and no resolved velocity fluctuations were present in the flow at the measurement section. In the cavitating vortex simulations, numerical diffusion of the EARSM led to a more diffuse vapor core interface and a more downstream development of the vortex cavity compared to IDDES, where the flow field remained predominantly laminar at the measurement section. Both EARSM and IDDES predicted a solid-body rotation of the vapor core, combined with an increase of radial velocity towards the vapor core edge. The non-negligible radial velocity resulted in a conical cavity shape. The velocity profile of the simulated line vortex therefore did not correspond to that of a cavitating Lamb-Oseen vortex (which assumes no radial velocity components), such that the simulated line vortex was not representative of a cavitating tip vortex. he sheet cavities originating from sharp edges at the front and end of the Venturi throat strongly influenced the flow field and cavity dynamics. The developed front sheet cavity determined the streamwise inception of the vapor core of the vortex. Periodic shedding and collapse of the irregular downstream sheet cavity caused axial and radial contractions of the vapor core
as well as a noncircular deformation of the cavity cross-section. No traveling or standing Kelvin waves could be identified on the cavity interface using the developed post-processing and spectral analysis tool. The identified cross-sectional deformation did not correspond to cavity deformation modes defined in previous research and none of the other deformation modes were found to occur. Grid-dependent solutions, numerical noise and large wavenumber resolution indicated that a finer grid is required and that the analyzed length of the cavity should increase to improve the quality of numerical analyses of simulated cavity dynamics.

Tata Steel employs physical vapour deposition (PVD) as a novel galvanization technique in their line production to prevent corrosion. In this process, zinc is evaporated in a vapour distribution box (VDB) and directed via nozzles into a vacuum where it is deposited on a steel substrate. In order to reduce stray deposition, achieve high and uniform mass flow rate from the nozzles and avoid condensation, understanding of the flow phenomena in the VDB and nozzles are necessary. Seeing that the PVD process is carried out at high temperatures in vacuum which makes it infeasible to experimentally characterise the flow for optimization purposes, thus, this research utilizes computational fluid dynamics (CFD).

In this study, the flow behaviour of the zinc vapour in the VDB and nozzles are investigated by solving the fluid governing equations numerically using finite volume method (FVM) in openFOAM. This analysis is done using the pressure-based sonicFoam solver to cope with the subsonic flow in the VDB and supersonic flow in the nozzles. The mass flow rate was compared to the ideal isentropic expression and experimental results. Regions in the set-up with high probability of condensation was investigated using a four coefficient Antoine equation.

A uniform pressure build-up in the VDB enabled uniform deposition. Around the corner of the inlet channel, a stable recirculation pattern affected the nozzle flow, however, this effect reduced as the outlet pressure decreased. The simulated total mass flow rate was greater than the experimental data by a factor of four possibly as a result of numerical errors and experimental stray depositions. And the simulated nozzle mass flow rate was less than the analytical isentropic expression by a factor of five due to the nozzle viscous boundary effect, and VDB wall heating. The mass flow rate from the nozzles decreased with increase in distance from the inlet channel. However, the nozzles far away are less likely to form zinc droplets as compared to those close to the inlet stream. For scale-up under the same conditions, it is expected that non-uniformity would be pronounced and the effect of the small eddies would become more significant. Thus, the simulation of the PVD process gives a qualitative description of the flow and a first approximation for the mass flow rate. To further improve the mass flow prediction, the sensitivity of the mass flow rate depending on the melt temperature and inlet velocity boundary condition should be studied in future works.

Horizontal Directional Drilling permits the creation of tunnels which pass beneath rivers and canals to allow the passing of services from one side to the other. The final stage of this process involves lining the tunnel with a plastic or steel pipe. The pipe is pulled inside into the borehole using the same drilling rig that was used to bore the tunnel. For the case of a plastic High Density Polyethylene (HDPE) pipe, issues of buoyancy may arise during this pullback process since the tunnel is pre-filled with drilling mud which now primarily acts as a lubricant. This presents a problem since with this buoyancy, the pipe is lifted up to brush against the tunnel wall, creating issues with the pullback process because of the added pulling resistance. There is the possibility of cutting slots in the pipe wall at the front end of the pipe in order to allow in drilling mud so as to ballast and offset this buoyancy. The drilling fluid is a suspension of bentonite in water and is characterised as a non-Newtonian shear-thinning Herschel-Bulkley fluid, which possesses a finite yield stress. This study aimed to find whether the current slot proportions used for a given borehole diameter, pipe diameter and pipe thickness are sufficient for allowing in drilling mud to ballast the pipe. This was a case where the multiphase flow given by the interaction of air and drilling mud can be simulated using CFD. OpenFOAM is used for this purpose to first simulate the current practice. The multiphase solver interFoam together with the non-Newtonian Herschel-Bulkley and air model was validated for a series of cases before the main simulations were run. These validations included the Marsh Funnel test and the Slump test. These are two kinds of workability tests used for cement pastes and drilling muds. Cement pastes and drilling muds are characterized as three-parameter Herschel-Bulkley fluids and the physical setups of the Marsh Funnel and Slump tests were replicated in computational space. The flow time of theMarsh funnel test and the slump diameter from the CFD simulations were compared with experimental data from literature thus validating the model in OpenFOAM. The main simulation setup recreates the situation of an HDPE pipe concentric with the borehole, with the axis of the domain at an angle to the horizontal. There is drilling mud above the slot at time t = 0s. The drilling mud comes into the domain from the annulus from below the slot. The simulations showed that the flow into the slot initially came in from both the drilling mud above the slot and from the inlet. After the volume of drilling mud above the slot is almost drained completely through into the inner pipe, the mud level inside the pipe starts to become comparable that outside the pipe and both interfaces rise upwards at approximately the same pace. Subsequently, certain parameters were changed from the first benchmark case in order to see what is the effect of these individual variables. The effects of a lower drilling mud yield stress, a longer pipe slot, lower drilling mud density, a different slot aperture shape, increased flow and a steeper angle of pipe penetration were all tested in the simulation campaign. The idea is that with better and faster filling, less pipe buoyancy results. An extension of this idea is that the difference between mud levels inside and outside the pipe should be kept to a minimum. That is why the aim is to increase the flow rate through the slot in the pipe. The results show that increasing the slot length by 30% from the current practice increases the throughput of drilling mud by 10%. The results also show that an elliptical slot profile has a neutral effect and decreasing the yield stress of the drilling mud has a slightly beneficial effect.

Miniaturization of spacecraft has been gaining wide interest in the space industry, given its potential for reducing space missions' costs and providing a novel approach to enhancing and facilitating spaceflight. Recently, a lot of research has been successfully put into this field along with the advancements that make it more feasible, though a major obstacle to achieving the new generation of spacecraft is the technical challenge of fitting a suitable propulsion system. Useful chemical propellants are usually corrosive, flammable, and/or toxic, so alternatives need to be found. The aerospace industry is shifting towards green and nontoxic propulsion systems, so water could be used as an effective propellant, considering its relatively high mass density and low molecular mass. New microelectromechanical systems (MEMS) technologies show promising opportunities for the integration of miniaturized propulsion systems, due to their versatility and robustness. In this thesis, a comparative study of nozzle flow, heat transfer, and thermodynamics in two different thrusters is conducted. One thruster is based on MEMS, with a typically quasi-2D geometry, while the second thruster is based on more conventional technologies and manufacturing techniques, with an axially symmetrical 3D shape. After briefly introducing micropropulsion and discussing the propellant selection and nozzle fabrication along with the background theory related to micropropulsion as well as the analytical and OpenFOAM numerical (DSMC, continuum, and a hybrid approach containing both to accommodate to the variation in Knudsen number throughout the computational domain) modeling methods, the used methodology is based on using OpenFOAM's DSMC solver (dsmcFoam+) following the mesh creation using blockMesh and snappyHexMesh and developed analytical model (using MATLAB and CoolProp) along with an additional VLM ANSYS Fluent CFD model prepared in advance at TU Delft, where their (steady state as well as transient for DSMC) results (including the same conventional and MEMS nozzles) are processed and discussed. The nozzles are simulated for inlet pressures of 5 and 7 bar at inlet temperatures of 550 and 773 K for a total of four cases for each nozzle. To note, many of dsmcFoam+'s functionalities (mass flow rate measurements, inlet pressure boundary condition, axisymmetric capabilities, statistical error measurements, and dynamic load balancing) are implemented and described along with the full methodology, as Blender (with add-ons) and ParaView with a Python script to extract averaged data (along the nozzle and plume region) along with sampleDict are also used in pre and post-processing respectively and the simulations are carried out on a computer cluster. Furthermore, a quite interesting theoretical project on the side has been independently worked on in parallel. It started as a noticed idea that was decided to be explored using equations, which led to extended continuum/kinetic dimensionless numbers for diffusivity and rarefaction intensity relative to the studied object’s timescale. Ultimately, there are tradeoffs to choosing either thruster, where it is impractical to fault one nozzle for not performing better than the other, as it comes back to the desired features and nature of the mission each is undertaking, where compromises have to be made.