Turbulent plumes play a crucial role in geophysical and industrial processes, yet their behavior in rotating environments remains insufficiently understood. This study experimentally investigates the influence of background rotation on negatively buoyant turbulent plumes using Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF). By varying the source Richardson and Rossby numbers, the effect of rotation on plume self-similarity, width and centerline velocity is examined.
Results reveal that rotation significantly alters plume dynamics, reducing entrainment and modifying self-similarity profiles at the edges of plumes. The evolution of plume half-width is inhibited, and centerline velocity decays faster in rotating environments. The curved area of both buoyancy and velocity self-similarity profiles do however overlap with a Gaussian curve.
These findings contribute to a better understanding of rotating turbulent plumes and provide insights relevant to oceanic convection, atmospheric dynamics, and industrial mixing processes.

This study aims to develop a methodology for upscaling commercially available latent thermal energy storage systems using salt-hydrate HS48 as a phase change material, with a focus on determining optimal design parameters for different sizes and energy demands. The paper begins with a comprehensive review of phase-change materials' properties, theories, and practical applications in various energy storage systems, including integration into hot water storage tanks with diverse configurations. Subsequently, a research method is introduced, based on heat transfer principles and commercial heat battery specifications, providing a scalable simulation model validated through experimental setups. The model emphasizes two different scales and demonstrates non-linear charging time increases with PCM mass. After optimization, the model is scaled to match a commercially available heat battery's power output, evaluating various configurations based on key independent variables. Results favor the configuration, which consist of twice the number of parallel tubes, as the optimal design for reducing PCM mass, system size, and charging time. While the study shows promising outcomes, it also highlights areas for improvement, such as refining assumptions about phase change behavior and addressing specific application requirements like domestic hot water supply

A magnetorheological fluid is a type of smart fluid that can change its rheological properties when a magnetic field is applied to the fluid. The fluid is a suspension of magnetizable particles in a non-magnetic carrier fluid. When a magnetic field is applied to the fluid, the particles will form structures along the magnetic field lines. These structures resist flow, thereby increasing the viscosity of the fluid. In a non-uniform magnetic field that is generated by a permanent magnet, the particles separate from the fluid and aggregate on the surface of the magnet. This phenomenon is of use in hydrodynamic bearings, where textures are used to increase the load capacity of the bearing. Replacing the fluid in the bearing by a magnetorheological fluid and placing permanent magnets at the desired texture locations, results in particles separating from the fluid, aggregating on the permanent magnets and forming textures. These textures are of a self-healing nature, because any particle that is sheared off, returns into the fluid and is replaced by another particle. Currently, not much is known about the formation of these textures and the influence of their formation on the rheological behavior of the magnetorheological fluid. Therefore, several discrete element models are constructed, that can simulate a magnetorheological fluid in non-uniform field. These models use basic physical laws to determine the dynamics of the particles. A single core model is constructed to simulate the behavior of the magnetorheological fluid in small domains using two different methods for the magnetic interaction forces between the particles. A parallel model is made to simulate the magnetorheological fluid in larger domains. Finally, a model that is used to simulate particulate flows, is modified to research whether two-way coupling is required to determine the steady state behavior of the magnetorheological fluid. The models show that the particles do not separate from the fluid due to the attractive force of the magnet by itself, but rather by a combination of the attractive force and the deformation of the structures when a flow is applied. Furthermore, the rheological behavior of the fluid can still be approximated using the standard viscoplastic models. Finally, the modified particulate flow model showed that two-way coupling is not required to determine the steady state behavior of the magnetorheological fluid.

Prilling is the industrial process in which thin jets of a molten substance break up into drops and the subsequent solidification of the droplets into solids due to cooling when they fall down in the prilling tower. Prilling is widely used for efficient production of a variety of materials such as, e.g., fertilizer grains, laundry detergent, and substances for drugs and foods.
While in most prilling applications the jet can be considered as a pure liquid, in some cases the molten substance may contain suspended particles. The present study is motivated by prilling of fertilizers composed of Urea or Ammonium Nitrate (AN) containing polyhalite particles. To date, the effects of suspended particles on jet breakup, drop formation and crystallization are not well-understood. In the first part of this report a literature review is given on prilling and the current understanding of the effect of suspended particles on this. It has been found in literature that a large concentration of suspended particles will increase the viscosity of the suspension but also promote breakup of the jet. Besides, the size of the particles also influences the viscosity and jet breakup, resulting in a decrease in the amount of smaller formed secondary droplets at larger suspended particle sizes, hence increasing monodispersity. The objectives of this MSc research project are to assess under what conditions high-quality fertilizer grains can be optimally prilled from polyhalite suspensions in molten urea or ammonium nitrate. An experimental setup was designed to assess how molten liquid jets of Urea or AN with suspended polyhalite particles can be optimally prilled for producing high-quality fertilizer grains. This design consists of melting and mixing the batch of materials, from where it will pump the particulate suspension through a vertical tube with a screw pump to a nozzle. Here, a jet will be formed. This will breakup into droplets, which will fall in an oil batch to increase cooling and hence fasten the solidification process. Unfortunately, the current design was always dripping and not able to form a jet due clogging and the formation of hard lumps, which decreased the flow rate. The droplets were however collected and analyzed. It was found in this thesis that a larger concentration of P4 particles in urea will increase the viscosity. When the particles are grinded more, the viscosity increased as well, however a direct relationship between particle size and viscosity has yet to be determined. The particles were also unequally distributed over the prills, because P4 tends to stick together. To obtain the best quality prills, it is recommended to use relatively large P4 particles in combination with larger nozzles of >1mm to decrease the desired prilling force and prevent clogging. Also melting

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.

Zero Emission Fuels B.V (ZEF) is currently in the process of designing and prototyping a sustainable methanol micro-plant, which captures water vapor and carbon dioxide from the atmosphere and uses those components to eventually produce methanol. In order to produce methanol, hydrogen is required which is produced from the captured water by electrolysis in a zero gap design, high pressure alkaline water electrolyser. The pressure in the electrolysis setup is regulated at a desired operating pressure of 50 bar by a pressure control system, designed by ZEF.

Insight is required in how the flow of electrolyte and bubbles behaves in the electrolysis setup and the electrolyser cells, in order to implement design or operation condition changes to allow for an efficient operation in the eventual micro-plant.

This study focuses on the experimental characterization of the flow and bubble phenomena in the ZEF electrolyser design, during operation at a desired pressure of 50 bar, with the use of visualization by video footage inside the electrolyser cells. Camera equipment used for the visualization consists of a microscope camera and a GoPro camera. An ultrasound Doppler velocimetry device (UDV) was used to measure the electrolyte flow velocity and direction in different parts of the electrolysis setup, in order to substantiate findings made by the camera equipment.

Experiments were conducted with an electrolysis setup based on the current electrolyser design made by ZEF, with the addition of a transparent add-on to allow for visualization of the flow and bubbles inside an electrolyser cell on the oxygen side. First the operation of the electrolyser was characterized by means of IV curves and temperatures, followed by visualization experiments for varying operating pressures and current densities at different parts of the electrolyser cell.

One of the main observations made during the visualization experiments is the periodic occurrence of stagnation of electrolyte and gas flow out of the electrolyser. Stagnation of the flow occurs inside the electrolyser cells on the oxygen side, at the top channels leading the flow out of the electrolyser cells, to eventually a flash tank. Different multiphase flow regimes were observed during active flow through the top channels, which vary based on the operating current density, pressure and temperature.
Bubble behavior was observed at different positions on the electrode in the electrolyser cell and possible enhancement of gas crossover was detected in the electrolysis setup and investigated.

The effect of the pressure control system on the operation of the electrolyser was also investigated and showed to affect the electrolyte flow direction in the electrolysis system.
Models were made to validate and explain the findings from the visualization video footage and characterization of the electrolyser. A previous made model that characterizes the flow in the electrolysis system is used for comparison.

Due to the lack of a temperature regulation system, a finned tube heat exchanger was modelled to be implemented in the experimental setup as for the eventual electrolysis setup.

Based on the findings from the experiments, recommendations are proposed regarding the experimental and eventual design, and for further development of the high pressure alkaline water electrolyser and the electrolysis system.

To meet energy demand towards a low-carbon future, the global market demand is growing for metals such as cobalt and nickel which are major elements in batteries. Polymetallic nodules, which are formed on abyssal plains at depths ranging from 4 to 6 kilometres and are distributed in high abundance on the top of the seabed, contains several times more cobalt and nickel than the entire global terrestrial reserves. This has raised the interest to exploit these resources from the deep ocean. The seafloor mining tool (SMT) can move along the soft sea bottom and can collect polymetallic nodules. While doing so, it will also entrain sediments and water. The excess of water and sediment entrained is discharged at the back of the SMT, forming a sediment plume. The sediment plume dispersion has strong adverse impacts on deep-sea environment.Thus, it is essential to study the sediment plume behavior in order to limit plume dispersion and thus to reduce its environmental impact.
Experimental research is a powerful technique to study the plume behaviour. However, experiments sometimes take a long time due to complex set-up. In comparision, numerical analysis can save time and costs when solving complex problems. Furthermore, numerical modelling can provide deeper understanding and flexibility for boundary conditions and sediment types, which is applicable on both model and prototype scale. Previous numerical studies have noted the significant role of flocculation in limiting plume dispersion, but flocculation process has not been modelled explicitly. This study aims to establish a numerical model to study flocculation process and its effect on sediment transport.
Previous flocculation-fluid dynamics modelling has applied a Euler-Euler method with additional population balance equations. The disadvantage is that many equations need to be solved. To avoid excessive computational costs, the sediment transport is described by a multiphase drift-flux model in this study. The flocculation process is modelled by a discretized form of population balance equations. The author has found that, by multiplying the particle volume, the population balance can be efficiently incorporated in the phase continuity equations in the drift-flux model. The population dynamics of particle aggregation and breakup can thus be characterized by the phase transition terms in the phase continuity equations. Hence, no additional equation needs to be introduced and solved.
Verification is carried out to check conservation relationships and iterative convergence of numerical results. Then, an initial numerical investigation has shown the results can qualitatively show the three settling stages (i.e., flocculent settling, hindered settling and compression settling) found in the experimental studies. Afterwards, the collision efficiency is calibrated using the settling column tests conducted by Enthoven (2021). The results of calibration show a good fit to the experimental data. Another advantage is that numerical simulations can provide the particle size distribution over time, which is not measured in the experiments.
The major novelty of this study is the coupling of the drift-flux model and the population balance equations, which inherits both the characteristics of population balance and the merits of drift-flux model in reducing computational costs. The flocculation modeling technique as proposed in this study can be incorporated as a module into an extended drift-flux model to predict the dispersion of deep-sea mining plumes.

Zero Emission Fuels (ZEF B.V.) is a start-up working to build a sustainable methanol micro-plant. Carbon dioxide and water are obtained from atmosphere. Hydrogen is obtained from captured water through alkaline electrolysis and the hydrogen is used for methanol production. The alkaline electrolyser should run at 50 bar and 90°C with 30% KOH as the electrolyte. It is a multi-cell design and the electricity supplied to the electrolyser leaks within the system through the electrolyte. This is the energy supplied to the system not being used for splitting water into hydrogen and oxygen. Leaking current and flow scale with the length and the radius of any channel in the system and contradict each other in terms of requirements.
This study focuses on characterising the electrolyser to validate a modified version of the existing Matlab model at a channel in the system for two different dimensions of the channel. The Matlab model predicts
the flow and the leaking currents in the system. Estimating the flow in the system is essential to validate the model and using the model, the leaking currents are estimated. Experiments were performed on a single cell version of the electrolyser at atmospheric pressure to obtain the flow and thermal characteristics. Comsol simulations were ran on the single cell system to support the Matlab results.
The heating curve of the system was obtained at four different points in the system to check for the necessity of insulation. The system went up to 56°C at the hottest point in the system after 2 hours. This confirmed for the necessity of insulation. The diameter of the bubbles at the electrode were measured and compared to the estimated diameter in Matlab. They were 34% off. The flow part of the experiments were done at a lower current density to simulate 50 bar flow at 1 bar. The flow rate of the electrolyte was estimated using high speed cameras and tracking fluorescent particles at one of the channels in the system. The values of the flow rate from Comsol, Matlab and Comsol were 25% away from each other whereas Matlab and experiments were 65% away from each other. The mass flow rate predicted by Comsol was in between Matlab and experiments. The absence of a temperature network to estimate temperature at every part of the system, the approximate geometry and ignoring smaller resistances to flow in the Matlab model are responsible for the offset.
Leaking currents were estimated in the single and multi-cell system using Matlab. The power lost due to leaking currents were higher in the multi-cell system as compared to the single cell system. The reduction in leaking currents by using gas slugs was estimated and the current design achieves the electrical efficiency of 99%. Recommendations were to make the channel diameter larger and remove the part of the channel that is in the cell. Further modifications to the Matlab and Comsol model were suggested to improve the prediction. Experiments related to multi-cell setup have to be done to estimate the actual electrical efficiency of the system and validate the Matlab model.

In the oil and gas industry the occurrence of slug flow in flowlines and risers can cause operational problems. Such flowline-riser systems are used to transport the oil and/or gas from the location of the wells to a production platform, where the fluids might be separated into single phases. Slug flow conditions imposefluctuations in the production rate, which may lead to the flooding of the separators, trips of compressors or pumps, and to increased loads on the supports of the pipeline and piping sections at the production platform.Therefore, measures have to be taken to mitigate the slugging, which can be a reduction of the production rate, making adjustments to the pipeline system, or adding surfactants to the flow. For vertical flow in production wells, the use of surfactants is a proven technology. Here the creation of a foam through adding asurfactant can increase the production life time, as the accumulation of liquid, which typically occurs when the reservoir pressure has decreased at the end of field life, is prevented. Far less is known yet about the effect of using surfactants for the mitigation of hydrodynamic slugs in a nearly horizontal flowline or the mitigation of severe slugs in risers. From a previous Master’s Thesis project by Pronk [1], in which lab experiments were carried out in the same flowline-riser facility as used in the present study, it was concluded that growing slugs could be suppressed by adding a surfactant to create foam. The surfactants also influence the characteristics of the severe slugging cycle, but they are not able to fully suppress it. Re-analysis of the measurement data by Pronk hasturned out that instead of growing slugs, severe sluggingwas measured and therefore that severe slugging can be suppressed using surfactants and no conclusions can be drawn on growing slugs or hydrodynamic slugsfrom her research. There is limited literature on the effect of adding a surfactant to hydrodynamic slug flow in horizontal flowlines, but there are some indications that hydrodynamic slugs can be suppressed, though requiring a higher concentration of the surfactant than vertical flow. Suppressing slug flow using surfactants should eliminate the fluctuations in the flow and can lead to a lower pressure drop. For this research experiments have been carried out in the Severe Slugging Loop at the Shell Technology Centre Amsterdam. This flow loop consists of a horizontal flowline of 50 m, a U-turn and a 15 m section. From there onward the pipe starts declining at an angle of -2.54° over a length of 35 m. Thereafter there is a short horizontal section of 5 m before arriving at the base of the riser, which has a height of 16.8 m and an internal diameter of 32 mm. The other sections have an internal diameter of 50.8 mm. For this research the configuration was used where air and water are connected through a Y-sprout at the start of the flow line. The air is supplied as pressurized air of 6 barg. The water is supplied by a pump, and it is separated from the airat a section at the top of the riser and recirculated through the system. The used surfactant is the household detergent Dreft. At several points along the flowline and riser the flow conditions have been measured using pressure indicators and at two locations along the flowline the pressure difference over 3 m has been measured using differential pressure indicators. Through the inspection windowat the end of the flowline the flowis recorded by using a GoPro camera. Furthermore, over a distance of 40mthe acoustic energy of the flow has been measured by a Distributed Acoustic Sensing (DAS) system. The DASmeasurements have been used to determine the flow behaviour of the flow. In the case of slug flow, the velocity of the slugs was deduced from the DAS data. Adding a 2000 ppm concentration of the surfactant Dreft Original has resulted in the full suppression of hydrodynamic slug flow. This has been verified using the pressure data, the video data and the DAS data in combination with the velocity tracking tool. The waves that were present without surfactant are decreased to a much smaller wave height when the surfactant is added; therefore, the flow pattern is changed from slug flow to wavy stratified flow. The severe slugging cycle can also be suppressed using 2000 ppm concentration of the surfactant Dreft Original. At the same flow conditions, the gas is able to lift more liquid through the riser. The measurements by the DAS system were quantified using a velocity tracking algorithm. The measurement data can now be used to accurately measure the slug velocity and to determine whether slug flow or stratified flow is present in the system. This measurement method is ready for scale-up. When the concentration of surfactant is sufficient to suppress slug flow, the flow can be described from visual observations as a threephase flow, containing gas, foam and liquid. The foam absorbs the stresses imposed by the other phases, because of its non-Newtonian fluid properties. The waves are no longer able to grow until they reach the top of the pipe and are diminished into slow moving dampened waves of the foam layer.

In solving two-phase flows, the location of the interface between the phases is necessary to handle interface jump conditions when solving the Navier-Stokes equation. Current interface capturing and advection methods, however, suffer from various issues. The level set method uses the signed distance to the interface and the interface being the zero level set of this function allows the evolution of the level set field to be described with a simple advection equation. This means that no additional steps are required, but solving the advection equation generally does not conserve mass. On the other hand, Volume of Fluid methods utilise the local fluid fractions to represent the phase interface. For these methods, a mass conserving advection algorithm exists, but the absence of an explicit interface requires expensive reconstruction methods to be used instead. Additionally, this Volume of Fluid advection method is subject to a restrictive CFL condition on the time-step and, being dimensionally split, requires a structured grid to be used. Other Volume of Fluid or Moment of Fluid advection methods that do not have these conditions are not mass conserving. Dual interface methods that combine information from the level set and volume fractions are able to achieve higher accuracy, but these method still use Volume of Fluid advection to remain mass conserving and are thus subject to the same conditions. These methods use the level set field to create a cheaper and better justified reconstruction method, which negates one drawback of VoF methods. In this thesis, a method is formulated to allow interface advection on unstructured grids without such a strict CFL condition compared to the MCLS method. To do this, the finite volume level set advection method of the MCLS method is replaced with a nodal-modal discontinuous Galerkin method. The DG method is analogous to the Galerkin finite element methods, but the basis functions are now only valid on one element, and the solution can be multiply defined on the cell boundaries. This allows for level set advection on polygonal cells, and has the added benefit that since cells are semi-independent, level set correction now only has to be applied locally. This, like the finite volume level set advection method, is not necessarily mass conserving though, so the volume of fluid will need to solve this issue. The difference with MCLS is that no volume of fluid advection method is used, since these methods are the parts that introduce the aforementioned problems that are being avoided. Instead, a minimisation method is done on the VOF field in order to obtain a correction which can be applied to the level set and keep it mass conserving. For this minimisation method, a flux condition is used to impose additional constraints on the solution. This condition effectively attempts to match the interface intersections on a cell edge for both adjacent cells, for all cell edges that have a flux going through. The resulting method is tested for four test cases, with solid body translation and rotation, the corner flow test, and the vortex deformation test. In the translation test, there is only vertical velocity, so the flux condition does not take vertical edges into account. To stabilise this test case, level set reinitialisation is applied, which is a technique used to maintain the signed distance property of the level set function to use in the Navier-Stokes evolution method. This keeps the solution in the translation test somewhat representative of the exact interface, but makes the obtained results no longer representative of the actual method. For the other test cases, the method appears to converge when the grid is refined, except for the solid body rotation test using Euler Forward, as this result becomes unstable. However, no apparent order of convergence is present, and the effect of a more accurate time integration method is very inconsistent. To solve this, the method is altered to allow multiple level set advection steps for every optimisation routine. This makes the method better follow the order of convergence of the discontinuous Galerkin method, although this order is slightly lower for the vortex deformation test. The obtained results show that in it's current form, this optimisation based method is likely not usable for general applications, but that with additional work, this kind of method may prove to be more applicable than MCLS.

Laminar-turbulent transition (LTT) is the process through which smooth laminar flow transits into chaotic turbulent flow. Investigation of the paths taken to transit into turbulence is a front-runner among other methods followed to characterise turbulent flows. This is of particular importance in aerospace and energy industries for the design of wings and gas turbines. Early research used Linear Stability Theory (LST) to analyse the stability of the flow; with the increase in computational power, Direct Numerical Simulation (DNS) has been developed to solve the flow field entirely. Most of the research on LTT has been centered on ideal fluids with limited focus on the effects of high temperature. The impact of other strong non-ideal effects on LTT such as dense gas effects have not been investigated. This work aims to study the effects of dense gas on LTT for boundary layer flows of toluene over a flat plate.Flows over a flat plate boundary layer are investigated in 3 stages. First, the base flow is solved for ideal air and non-ideal toluene for 6 different Eckert numbers (Ec). Second, the base flow is provided as an input to solve the eigenvalues of the stability equations for both fluids and each Ec using an in-house MATLAB code. The unstable eigenmode is identified and tracked. The growth rates and phase velocities are calculated and compared between ideal air and toluene. Third, DNS simulations are performed using a FORTRAN code, to solve the governing equations of compressible flows for different Ec. The simulations are performed on a pre-processed base flow solution, subjected to 2D sinusoidal perturbations forced into the computation domain at the wall. A no-slip and adiabatic wall boundary conditions are applied to the flat plate with a sponge region at the outlet and the top of the computation domain. The growth rates and phase velocities of these perturbations are calculated and validated with the predictions made by LST. Finally, a perturbation energy budget analysis is conducted to study the nature of the unique results obtained.The LST results show that as Ec number increases, all flows over a flat plate become more stable. For toluene flows, the stabilising effect of increasing Ec is more pronounced and all flows with Ec > 0.15 are stable and have no modal instabilities. The results from the DNS simulations validate these predictions from LST and match perfectly in growing conditions, but deviate from one another in stable conditions. The deviation in results are hypothesised to be the contribution of multiple decaying modes to stable behaviour of the flow. Furthermore, perturbation energy budget analysis showed that for Ec = 0.05 and 0.10, the spatial growth of perturbations are positive due to a positive production term and negative for Ec = 0.15, with a negative production term. The negative production term is attributed to the negative integral of the perturbation profile.

Heat transfer in multiphase flow plays an important role in nature and in numerous industries such as petrochemical, automotive, food processing, ocean engineering etc. It is becoming increasingly crucial to design more efficient industrial systems to reduce the environmental impact of industrial activities because of global warming and growing awareness about sustainability. To design these efficient systems it is important to thoroughly understand the details of heat transfer in multiphase flow.

This thesis follows the method of Direct Numerical Simulation (DNS) to provide a detailed physical insight of the fluid motion and heat transfer in the model. To accurately model the two phase flow the Coupled Level Set Volume Of Fluid (CLSVOF) method is used. The main advantage of using the CLSVOF method is that it can accurately capture the interface geometry and it has excellent volume conservation capability. The work presented here is an extension to an in-house code developed at TU Delft for Direct Numerical Simulation of two-phase flows using the CLSVOF method. The existing code has been thoroughly validated for the fluid and interface motion but the validation of the heat transfer model remains an unaccomplished task till now. Therefore, the main objective of this thesis is to validate the heat transfer model and then to study the heat transfer in droplets coalescence using this model.

The validation of the heat transfer model was accomplished by calculating the Nusselt number distribution over a bubble surface and comparing it to the available literature. Both the model results and the information from the literature showed very good agreement with each other. After completing the model validation, this model was used to study the heat transfer phenomenon between two coalescing droplets and the surrounding fluid. The results of the droplets coalescence are as per the expectations and are discussed in detail in this thesis. In the process of doing these analyses different ways of calculating the local Nusselt number and the global Nusselt number have been discussed. The validation of the heat transfer model and the analysis done for the case of coalescing droplets paves the way for conducting more complex heat transfer analysis in two-phase flows using this model.

Pipeline networks are extensively used in the oil and gas industry to transport fluids under multiphase flow conditions from the wells to the production platforms or plants. Over time, the flow rate of the production fluid decreases as the reservoir pressure decreases. When the flow rate decreases to below a certain value, unstable flow with liquid slugs occurs and the transportation of the fluids becomes more difficult. This is because of the high amount of liquid accumulation (holdup) in the pipeline. To extend the operation life of the production system, it is important to maintain the continuous production at low flow rates. Regular pigging, which refers to transporting a “pig” (Pipeline Inspection Gauge) through the pipeline, has been a conventional way of managing the liquid holdup in the oil and gas industry. Also "by-pass" pigs have been actively used to manage the liquid holdup since they generate smaller slug volumes compared to the traditional non-bypass pigs. However, since the pipeline can be as long as a hundred (or more) kilometers, the volume of the pig-generated slug at low flow rates can be very large. Even this reduced slug volume produced by using a by-pass pig may exceed the capacity of the downstream separator or slug catcher. Moreover, by-pass pigs with a large by-pass area have a high chance to get stuck in the pipeline at low flow rates. To enable pigging operation at those conditions, the use of an undersized ball pig was proposed. Here the ball pig is transversed through the pipeline prior to the by-pass pigging operation. A few test runs with undersized ball pigs were carried out in the F14 multiphase pipeline in Sarawak. To improve its performance, it is of much interest to study the detailed flow around an undersized ball pig. Although in actual operation the ball pig will be used under multiphase conditions (i.e. gas and liquid flow), in this study, as a starting point, single-phase flow was considered. This study investigated the detailed flow around an undersized ball pig in a horizontal pipe with the help of Ansys Fluent version 18.2. First, benchmark data from experiments are used to find the best numerical model which can be used to simulate the flow around a sphere. The comparison between experimental data and numerical results shows that the 2D laminar simulation can accurately capture the flow when Re < 300. The 2D simulation with the SST k-ω turbulence model (low-Reynolds number correction) gives an accurate prediction of the drag coefficient when 300 < Re < 5E5. The selected numerical models are then used to study the flow around an undersized ball pig in a pipe. For each set of conditions, this requires to carry out a number of simulations at different ball pig velocities, and find the velocity that gives zero drag force on the ball pig. The undersized ball pig is first moved to the centre, and 2D simulations were performed. The force analysis around the undersized ball pig at low Reynolds number shows that at equilibrium state, the undersized ball pig experiences a positive pressure force and a negative viscous shear force. Moreover, the computed profile of the normalized terminal velocity of the pig with a diameter ratio of 0.5 at various Reynolds numbers shows that the normalized terminal pig velocity profile experiences three stages when the Reynolds number is increased. In the first stage where Re < 365, the motion of the pig is dominated by the viscous shear force and the normalized terminal velocity of the pig is constant. When Re increases, the flow enters the second stage where the inertia force starts to affect the motion of the pig and the normalized terminal velocity of the pig decreases dramatically. When Re is further increased to above 5340, the motion of the pig is completely affected by the inertia force and the normalized terminal velocity of the pig stays in a stable range when Re increases. After this detailed study of the flow around an undersized ball pig with a fixed diameter ratio, simulations of the flow around undersized ball pigs with various diameter ratios are studied to find the influence of the pig diameter on the terminal velocity of the pig. The results show that the obtained profiles of the normalized pig terminal velocity against Re at various diameter ratios have a similar shape as when the diameter ratio was 0.5; the terminal pig velocity decreases when the pig diameter increases at a given Re. A further data regression analysis shows that the normalized pig terminal velocity in the region when Re < 300 and Re > 10000 has a second-order polynomial relationship with the diameter ratio. After the simulations for the flow around an undersized ball pig that moves along the axis of the pipe, the pig is moved to the bottom of the pipe and the influence of this position change is studied at a diameter ratio of 0.9. Due to this position change, a 3D simulation is required. First a 3D simulation for the previous configuration with the pig at the centre is performed. This simulation shows that the flow is actually unsteady and asymmetric. The terminal pig velocity at a given Reynolds number obtained from the 3D simulation is slightly higher than the one obtained from the 2D simulation. As the 2D simulation cannot capture the asymmetry of the flow, it provides less accurate results. The 3D simulation results of the flow past an undersized ball pig moving along the axis of the pipe is then compared with the one moving along the bottom of the pipe. The comparison shows that the ball pig terminal velocity is decreased when the pig is moved from the centre to the bottom of the pipe.