The need to reduce and mitigate the risk of global warming necessitates developing new methods or at least improving existing ones while maintaining a continuous, reliable and efficient supply of electricity. The increased share of power production from renewable sources like wind and solar brings challenges to grid stability due to their intermittent nature. Gas-fired power plants currently provide dispatchable generation; however, their continued reliance on fossil fuels presents a pressing environmental challenge. One potential solution to decarbonize gas turbines is the deployment of bioethanol, a renewable liquid fuel, that has the potential to have a closed carbon cycle.
Lean Premixed Pre-vaporized (LPP) technology has emerged as a promising solution, enabling the use of bioethanol in LPM systems by vaporizing the liquid fuel prior to injection. This study evaluates the use of pre-vaporized bioethanol in a combined cycle power plant by specifically examining the design and transient behavior of the fuel conditioning system required for bioethanol deployment. While existing research primarily focused on combustion behavior and steady-state performance, this study aims to fill the gap regarding the system integration, equipment design, transient operation, and the impact on the plant overall performance.
Initially, heat integration is conducted to optimize energy recovery within the combined cycle plant. Subsequently, preliminary design and sizing of major fuel conditioning equipment including a vaporizer, a superheater, and phase separator. The sizing of heat exchangers are performed using ASPEN Exchanger Design and Rating software. In the third phase, 1D dynamic models of these heat exchangers are developed using the finite volume (FV) method, chosen for its effectiveness in capturing complex transient thermal behaviors. Finally, these dynamic models are integrated with real combined cycle plant data to simulate startup.
Due to ethanol’s lower heating value compared to natural gas, a significantly higher mass flow rate was required to achieve the same heat input to the turbine. Intermediate-pressure (IP) steam was identified as the most suitable heating medium, minimizing heat exchanger size, cost, and installation challenges. A horizontally oriented shell-and-tube heat exchanger was selected for ethanol pre-vaporization due to its effectiveness in managing phase-change heat transfer. Results showed significant temperature gradients highlighting potential risks of localized heating, thermal stress and critical heat flux exceedance, necessitating further mechanical assessment.
Transient model results revealed that controlling heat input rates and managing the transition from external heating to IP steam are critical for maintaining stable fuel supply system operation during startup. Additionally, deployment of bioethanol required an external heating input for startup before IP steam became available. The use of IP steam as a heating medium introduced a measurable power penalty to the overall power output.
In conclusion, key recommendations are provided that include conducting detailed mechanical stress evaluations to address thermal stress concerns in the fuel conditioning system. Additionally, experiments on boiling heat transfer for ethanol under gas turbine operating conditions are essential to improve the accuracy of heat exchanger design. Furthermore, improving the transient model by incorporating real-time dynamic control strategies and relaxing simplifying assumptions such as accounting for pressure drop and void fraction will significantly improve the accuracy of transient predictions. Finally, conducting a pilot-scale test is recommended to validate the numerical model and confirm bioethanol’s feasibility, accelerating its deployment and marking an additional step toward decarbonized power generation.

The study delves into the critical necessity of long-duration energy storage to ensure the consistent reliability of electricity derived from renewable sources. Numerous technologies have emerged to address this need, enabling the storage of surplus energy generated by renewables in diverse materials, taking the forms of sensible or latent heat. The comprehensive analysis explores these various techniques in detail, providing a thorough examination of their respective advantages and disadvantages. High-temperature TES emerges as a pivotal component, particularly within CSP. This storage capability becomes imperative for maintaining a seamless and predictable power generation process, especially during periods of limited or intermittent sunshine, coinciding with high electricity demand and costs. Presently, CSP plants predominantly employ sensible energy storage in molten salt, which requires substantial salt volume, two large tanks, or a single tank system. A detailed study is conducted on a similar type of TES system using a PCM as an insulation to minimize the heat losses from the storage tank. The impact of the PCM layer in minimizing the heat loss is analyzed via an analytical model and CFD simulations. The cost incurred for the PCM is further compared with the cost of electrical heating.

In recent years, a considerable amount of research has been directed towards making energy generation more efficient to combat global warming. To aid this goal, the use of supercritical fluids (SCFs) is gaining a lot of traction. SCFs have only one phase and experience a sharp variation of thermophysical properties when heated sufficiently. These facts can be exploited to gain several advantages over conventionally used sub-critical fluids. However, the strong property variation in heated SCFs also complicates the flow Physics considerably if the wall heating is strong enough. Further, the flows in such settings are often turbulent and spatially developing. For these flows, a strong variation of properties can lead to a modulation of turbulence, which the conventional turbulence models can not predict well. This thesis is an attempt to better understand how spatially developing heated supercritical turbulent flows behave and use this understanding to improve turbulence model predictions.

In this thesis, we investigate two supercritical heated developing turbulent flows from Nemati et al. (2015). One case is oriented horizontally and the other is oriented vertically. The latter has an additional effect of buoyancy resulting from its vertical orientation and strong density variation near the wall. We analyze how the strong property variation modulates the turbulence in both cases. Then, we assess if Semi-Local Scaling (SLS) and Apparent Reynolds Number (ARN) theories can characterize the modulated turbulence. Further, we propose a methodology to make use of ARN by itself, and in combination with SLS to improve turbulence model predictions.

The results indicate that ARN theory provides a robust way to sensitize conventional turbulence models to the additional effects arising in supercritical heated developing turbulent flows due to property variation. Additional research in turbulent heat flux modeling is deemed necessary to improve the model performance further.

Stratified turbulent flows abound in environmental and industrial settings. Examples are atmospheric boundary layer flows, the transport of nutrients and organisms and the mixing of heat and salinity in the oceans, fluid flow in heat exchangers, and the transport of reactants and products in chemical reactions. These examples and many others consider stratified wall-bounded turbulence, in which the creation of turbulence by mechanical processes contends with its dissipation due to buoyancy effects. These flows are said to be stably stratified as these are inherently stable flows and are averse to mixing. The buoyancy effects alter the structure of the flow, and consequently the dynamics of mass, heat, and momentum transport. As density fluctuations become more severe, the Oberbeck-Boussinesq approximation becomes inaccurate and the resulting dynamics are not correctly predicted. In the current work, we developed and validated a numerical solver for direct numerical simulations (DNS) of turbulent flows featuring strong property variations. More precisely, we solve the Navier-Stokes equations in the limit of vanishing Mach number (so-called low-Mach number limit), with the fluid density given by the ideal gas law, and the dynamic viscosity and thermal conductivity also expressed as functions of temperature.
Our numerical solver is used to study stably-stratified turbulent channel flow under non-Oberbeck-Boussinesq conditions. The simulations are carried out at friction Reynolds number of  180, Prandtl number of 0.71, and friction Richardson number of the O(10). These non-dimensional numbers are the governing parameters and are defined based on the prescribed pressure drop and properties of the fluid at the reference temperature. Stratification is achieved by imposing constant temperature boundary conditions, with a high upper-to-lower wall temperature ratio (larger than 2), resulting in strong density variations in the flow. We will vary the temperature ratios and adjust gravity to maintain a similar Richardson number between cases, thereby isolating the effects of strong property variations in the flow dynamics. We will analyze the dynamics of heat and momentum transport under strong stratification for these conditions, also in light of DNS data of the same system under the Oberbeck-Boussinesq regime.

Currently, the energy transition is a topic for discussion and many organizations are taking measures to reduce the effects of climate change. Armed forces are exempt of these new regulations on the grounds of national security, but some have already started taking steps to reduce the amount of harmful emissions. Due to the high efficiency and advantages that fuel cells present, the aim of this master's thesis is to study their feasibility on board of a naval surface vessel, testing it in a concept design.
This report consists of two main parts. The first part tries to find the most suitable combination of naval surface vessel, type of fuel cell, and type of hydrogen carrier. It is concluded that the types of naval surface vessels that could benefit the most from the use of fuel cells are frigates and mine warfare vessels, in order to reduce their acoustic signatures; and offshore patrol vessels (OPVs), due to their constant base load, because it is a type of ship that is mostly cruising or patrolling.
Solid oxide fuel cells (SOFCs) are here considered to be the most promising type of fuel cells for naval surface vessels. This is not only because of their high efficiency, but also due to their ability to internally reform hydrogen carriers, making them more versatile. Other fuels than pure hydrogen can therefore be selected, which leads to the choice of ethanol and methanol as best candidates. The latter has already shown good performance when combined with SOFCs and is largely available worldwide.
Next, in the second part of this report, the concept design of an OPV using SOFCs fuelled by methanol is tested. On a systems level, the integration of SOFCs adds multiple elements and complexity to the power plant of the concept design. Moreover, since the SOFCs are only used to generate the base load needed to reach cruising speed and its auxiliary power, a peak power source is needed to reach higher speeds. After having considered batteries, diesel engines, and methanol engines, it was concluded that methanol engines provide the highest operational flexibility, on top of being the lightest combination.
When comparing the concept design to the reference design, they both score similarly in terms of redundancy and they both fulfil the same design requirements. In terms of operational capabilities, the concept design can sprint for less time due to the difference in efficiency of the power source used to cruise (fuel cells) and to sprint (methanol engines) and does not have the ability to extend its range above the design requirement, opposite to what the reference design can.
The use of a SOFC-based power plant shows a reduction in CO2 emissions between 13-23% depending on the type of mission, a 100% reduction of SOx and PM due to the use of methanol as a fuel, and a reduction of at least 68% in NOx emissions.

Recent advancements in the field of nanotechnology have proven to offer viable alternatives for energy production, transport, and storage. As far as thermal energy is concerned, nanofluids have emerged as a novel method to enhance heat transfer. Indeed, nanofluids exhibit superior thermal capabilities, which may be able to meet the requirement of high heat dissipation rate in limited space advanced by various high-tech industries.
In particular, boiling heat transfer is an efficient heat removal mechanism that may be further improved by using nanofluids. Indeed, it has been reported that nanoparticles play a crucial role in affecting the parameters which have major impact on the boiling process (i.e. thermophysical properties of the fluid, heating surface morphology, near-surface hydrodynamics). Being boiling very sensitive to surface characteristics, the latter factors have been found to have a significant influence on the boiling heat transfer coefficient. Hence, the aim of the present research is to elucidate the physical mechanisms underlying pool boiling of nanofluids.
Based on this framework, a pool boiling test facility has been designed and validated, thus enabling to conduct a comparative study on boiling of a pure fluid (water) and a water-alumina 0.1% da nanofluid. The pool boiling experiments were performed on six aluminium samples, which were characterized by SEM (scanning electron microscopy) and WLI (white light interferometry) before and after boiling in order to highlight the change in surface topography.
The research efforts were targeted at correlating the trend of the boiling curves and the surface parameters of the corresponding sample. Nonetheless, due to the limited dataset and the inconsistencies in the behaviour of the tested nanofluid, further investigation is required to assess the potential of nanofluids as more efficient heat transfer media.

Mobile electronics or remote devices like sensors for the Internet of Things (IoT) require energy storage with increasing energy densities, approaching the limits of the widespread lithium batteries. At the same time, lithium batteries are subject to growing criticism about their environmental impact due to their raw materials and their mining methods. Chemical energy seems to be a promising alternative and the development of micro combustion to make use of it is already for almost 30 years the topic of ongoing research. Recent studies show that compliant combustion engines are a good solution to mitigate some problems like leakage and friction and thus, achieve higher efficiencies. However, the most prevalent problem is heat losses through the walls because of the high surface-to-volume ratio in small-scale combustion. In this work, a design concept was proposed that not only insulates the compliant combustion chamber of the actuator for Flapping Wing Micro Air Vehicles (FWMAV) but also uses the waste heat to generate electricity. That was done by attaching Thermoelectric (TE) modules to a support structure and thermally connecting them with porous media to the oscillating combustor wall. The actuator uses the heat and pressure generated by the catalytic reaction of hydrogen peroxide and therewith, only emits steam and oxygen. With the help of a one-dimensional steady-state heat resistance model and a one-dimensional transient heat resistance-capacitance model, an optimum design was found. With that, it is possible to create a temperature difference sufficient enough to enable energy harvesting by TE modules. That was achieved by implementing an exhaust gas recirculation which
transfers heat to the modules and simultaneously serves as a heating blanket for the combustor. A downside of the design is the relatively long time it takes to reach its steady state and especially for FWMAV, the increase in weight.

Due to the global energy crisis, there is a growing need to make industrial processes more efficient and clean. One such method is to use effective and renewable working fluids in power and refrigeration cycles. Research in this field during the past decade has placed supercritical CO2 (SCO2) in the forefront due to its non-ideal properties and abundance in nature but, to fully understand their heat transfer properties, insight into boundary layer flow and hydrodynamic instabilities are crucial. Experimental setups such as closed loop wind tunnel contraction facilities are needed which provide a laminar steady flow to take boundary layer measurements. However, using a contraction causes various problems such as formation of secondary flows like boundary layer separation, Gortler vortices, cross-flows and non-uniformity’s. In this study a novel 1D contraction shape with curvature on only one side is pro- posed. This will completely eliminate the risk of Gortler vortices on the bottom plate and make it a suitable candidate for boundary layer experiments. Nevertheless, the risk of the other secondary flows and Gortler vortices on the top wall disturbing this laminar flow remains. Hence, the present research aims to optimise this 1D contraction to reduce the risk of such secondary flows and analyse the growth of Gortler vortices on the top wall using numerical simulations.

Steady state laminar simulations were performed to optimise the design of the 1D wind tunnel contraction. Optimum contraction and settling chamber lengths were found by identifying the risk of flow separation and boundary condition effects respectively. For the optimisation of the contraction wall shape, a family of transformed fifth order polynomial curves with varying inflection point distances were selected. To identify the curve with least risk of boundary layer separation and lowest flow non-uniformity, a multi- objective optimisation procedure was implemented. The procedure found that a curve with inflection point of 101mm downstream of contraction inlet gives the best performance.

To analyse the effect of Gortler vortices on outlet uniformity, unsteady laminar simulations were performed on the contraction by forcing sinusoidal perturbations of different wavenumbers. This study found that perturbations form symmetrical steady steam-wise Gortler vortices in the contraction. The most unstable wavenumber was found to be 𝜆=83.33 𝑚^-1 which formed secondary vortices of high vorticity. However it was also seen that for all wavenumbers, the vortices lose energy as they exit the contraction and do not affect the bottom boundary layer. Further, the effects of a side wall on the formation of Gortler vortices were also investigated. The results showed that the Gortler vortex closest to the wall is absorbed into high vorticity corner vortices, while those close to the centreline develop into steady Gortler vortices. No effect on the centreline velocity profile was seen due to the vortices making the 1D contraction suitable for boundary layer experiments. Finally, it was also seen that the 1D contraction shape produces asymmetrical Gortler vortices due to alternative perturbation methods such as random inlet perturbations.

Nasal airway obstruction (NAO) is one of the most common symptoms in the human respiratory system and causes a considerable financial burden to both individuals and society. Currently, NAO detection is troublesome to achieve, which can influence the accuracy and effectiveness of clinical surgery. Although several objective measurement techniques are currently available, they are found to be inconsistent with patients’ sensations. In recent years, computational fluid dynamics (CFD) has become a novel technique to objectively assess nasal airflow by simulating the nasal airflow of NAO patients. Existing literature has also shown the potential to correlate relevant CFD parameters with patients’ sensations. Nevertheless, there is still debate on the numerical setup for an accurate solution and the CFD parameter to correlate with the subjective measurement. Based on these existing issues, we mainly investigated the boundary configuration (with and without the external nose), the usage of turbulence/laminar models, the usage of steady-state/transient solvers, and briefly discussed the potential of using unilateral pressure drop ratio as a parameter for NAO detection. 

To begin with, including the external nose in the nasal airflow simulation is recommended in the nasal airflow simulations. The external nose configuration can affect the flow direction through the nostrils and downstream flow distributions. However, the static (and total) pressure drop only shows a 4% difference compared to the commonly-used plane-truncated boundary configuration. Furthermore, using the laminar model is sufficient for the nasal airflow simulations concerning the static pressure drop prediction. The laminar model shows a difference lower than 15% in static pressure drop compared to the experimental values on 3D-printed nasal airway models. We stress the caution of using the 𝑘 − 𝜔 model in the nasal airflow simulations because it tends to overpredict the turbulent viscosity ratio near the inlet unphysically. Moreover, steady-state simulations can also reasonably predict nasal airflow. We observed unsteady effects when comparing the steady-state simulation with the transient simulation with a constant flow rate and the transient simulation with a sinusoidal flow-rate-versus-time profile representing the real-life breathing cycle. Nevertheless, the steady-state simulation achieves an accurate prediction in static pressure drop, with a difference lower than 6% compared to the tested transient simulations. The steady-state simulation can also perfectly match transient simulations in the velocity profile of the recirculation zones and require a much lower computational cost. Last but not least, we also tested the possibility of using the unilateral static pressure drop ratio for NAO detection using CFD. However, we note that future studies should make corrections to account for the nasal cycle effect for NAO detection. 

Overall, we conclude that including the external nose and using the laminar simulations with the steady-state solver can give an acceptable prediction for the nasal airflow, especially concerning the static pressure drop prediction. We also state that applying CFD in the nasal airflow shows the potential for NAO detection, although future studies may consider making some corrections to include the nasal cycle effect.

The continued development of improved algorithms and architecture for numerical simulations is at the core of increased computational performance and, therefore, the ability to perform more complex and precise numerical simulations in less time in areas such as Computational Fluid Dynamics. Employing faster algorithms on more efficient processing units, such as Graphics Processing Units (GPUs), can reduce not only the time spend per simulation but also the energy required to perform these computations as well. This will be of significant benefit to different areas of research and engineering and through improvement achieved in these to society at large as well.

As simulations grow in dimensions and accuracy the wall clock time is bound to increase substantially, reaching days and potentially months depending on the parameters and geometry of the chosen simulation. The performance of different finite difference solvers with different degrees of optimization on different types of compute hardware was investigated and the achieved speedups assessed. Specifically, one serial CPU-based solver was presented as a baseline, which was then transitioned to a GPU-based solver. This in turn was then optimized further with regards to improved memory redundancy. To make comparisons fairer all of the solvers used the same temporal and spatial discretization techniques. Further, a benchmarking scenario was proposed to be used for the different solvers across used hardware, including the relevant geometry, gird, and initial and boundary conditions.

The speedups between the different solvers were observed and contextualized with regard to the effort that went into implementing the solvers and the capability and cost of the used hardware. The speedups at different problem sizes were investigated with the aim to establish how the performance gain from parallelizing and optimizing solvers scales with the chosen number of grid points and therefore the computational load.

Very significant speedups were achieved between the regular CPU solver and its GPU implementation, clearly showing the possible performance gains when moving from a serial to a parallel implementation running on an accelerator. The speedups between the two GPU-based solvers were more modest but still significant when considering the possible time spend on one simulation, depending on the chosen number of grid points.

Finally, an additional performance analysis was performed on two Navier-Stokes, one of which was the optimized version of the other, to investigate whether the performance increases were in line with prior findings and what magnitude of a reduction of wall clock time was possible for a state-of-the-art finite difference Navier-Stokes solver.

Convective heat transfer finds applications in several domains of industry like heat exchangers, gas turbine blades, IC engine surfaces etc. The surfaces of these heat transfer applications are either naturally rough owing to manufacturing techniques or become rough over a period of time during operation. These rough surfaces usually contain multiple length scales and exhibit random and heterogenous properties. Fractal roughness, in principal, is characterized by self-similar detail on smaller and smaller length scales and hence fractal dimension which is independent of any length scale can become a very viable option to characterize these multi-scale random surfaces. Therefore in the current thesis, rough surfaces characterized by fractal dimensions were designed and their role in the heat transfer enhancement relative to the pressure drop was studied using DNS.

The fractal surfaces were designed by randomly placing five generations of self-similar cuboids with decreasing sizes from higher to lower generations. The quantity of cuboids sprinkled randomly for each generation followed a fractal dimension. Two principal fractal dimensions were selected, i.e 퐷 = 1 and 퐷 = 2 and eight random realizations of each were generated in order to study the averaged effect of the heat transfer performance. Since the cuboids were randomly placed, different realizations within
the same fractal dimension experienced varied sheltering effects by larger generation cuboids. This ultimately produced a fluctuation in the ”sheltered” solidity of the fractal surface for different realizations whose effect was also studied. The cuboids were resolved in the simulation using an immersed boundary method.

To quantify the heat transfer performance of the rough surfaces, two performance factors namely the aero-thermal efficiency and Reynolds analogy factor were defined. It was found that the two types of fractal surfaces performed approximately similarly with a slight higher mean for 퐷 = 2 based on the above heat performance parameters even though the two surfaces looked very different. However, the above performance factors showed a very strong correlation with the ”sheltered” solidity of the fluctuating realizations displaying an increasing trend.

Lean premixed flames are being used in gas turbines to reduce the emission of NOx. Unfortunately, it results often in thermoacoustic instabilities. These instabilities are high amplitude pressure oscillations which cause damage to the combustion system, reduce efficiency, flame blow-off or flashback. While the cause for instabilities due to longitudinal modes is known, the cause for transverse thermoacoustic instabilities is still not fully understood. Currently, transverse thermoacoustic instabilities are avoided by damping devices that consume considerate amounts of cooling air, thus raising the flame temperature and NOx emissions. Therefore, a good understanding of the feedback cycle that cause transverse thermoacoustic instabilities is crucial in predicting and avoiding combustion instabilities. In this work, Large Eddy Simulations (LES) is used to study processes involved by the driving of instabilities in a small scale burner. Additionally, different dampening alternatives are designed and validated. Experiments have shown that a small scale burner becomes unstable with decreasing flame lengths, due to the addition of hydrogen to the fuel. To research the impact compact flames have on combustion instabilities, simulations of methane flames with decreasing lengths is performed. It is found that decreasing the flame length, increased the amplitude of the pressure oscillations. Similar as to the experiment, both longitudinal, as well as transverse instabilities, is observed in the LES. Unfortunately, to study the transverse thermoacoustic instabilities, the longitudinal modes must be avoided. Hence a modification is designed to dampen the longitudinal mode without affecting the transverse modes. Ultimately an orifice plate was designed that reduced the pressure amplitude of the longitudinal mode by 92 %, while hardly influencing the transverse modes. The sufficient suppression of the longitudinal modes enabled the research into the transverse thermoacoustic instabilities feedback cycle. The research found two driving mechanisms for transverse instabilities. First of all, flames located at pressure anti-nodes are excited by pressure-induced flow oscillations from the premixing ducts. Consequently, the flame’s heat release positively coupled with the acoustic pressure. Additionally, the lateral movement of the flame towards regions of higher pressure, due to transverse acoustic velocity, is observed. Especially at pressure nodes, where acoustic velocities are most potent, the flame movement was substantial. While both mechanisms are found and agree to the driving criteria, it is difficult to say what their actual contribution to the instabilities are. Finally, the actual driving of transverse thermoacoustic instability by flow oscillations inside the premixing duct is studied. At first, a modification is designed to deliberately cause a quarter-wave resonance that forces the pressure anti-node away from the flames. This way, the unsteady heat release is not able to couple with acoustic pressure oscillations. Secondly, a modification inspired by a muffler is aimed at dampening all acoustic waves travelling through the premixing duct. Unfortunately, the quarter-wave resonance created new unstable modes as well as increased the strength of the other instabilities. Also, the muffler modification is not able to dampen the transverse instabilities. Moreover, a resonant mode arose inside one of the muffler elements causing a new instability inside the combustion chamber.