The problem of global warming is becoming every day more and more pressing, leading to the necessity of reducing harmful emissions especially in the shipping sector. Most ships sail with Internal Combustion Engines, reason why it is beneficial to keep using this system, with the prospect of being able, though, to cut down on the emissions. This can be done implementing new fuels, such as PODE and methanol. These two fuels can largely reduce the emissions without the need for major changes to the engine system. Also, they can be produced in an ”eco-friendly” way, reducing the emissions also during the production process. In this study, the working characteristics of a dual fuel engine, considering methanol and PODE are analysed, as well as the production process of PODE from methanol, process that happens on board of the vessel itself. For the dual fuel engine the analysis showed that different ratios of methanol and PODE can be considered for the dual fuel engine and that this system can deliver the required power output. The plant design resulted in a plant consisting of two reactors and a separation system. At last, the necessary fuel can be produced by a plant that is small enough to fit on the ship and not have to reduce the capacity of the vessel significantly.

To answer the need for renewable fuels, Zero Emission Fuels is designing a microplant to create methanol from carbon dioxide and water from the ambient air using only renewable energy from solar panels. For this microplant, gaseous carbon dioxide must be compressed from 1 to 50 [bar] with the low mass flow of 336.7 [g/h]. This thesis examines the compression system which uses rolling piston compressors. This thesis also examines a solution to compress the carbon dioxide optimally.

Because of the low mass flows, the compressors are very small and have relatively high friction and leakage losses. Meanwhile, due to the high pressure ratio of the system, the temperature increase due to compression also poses a problem for the lubrication of the system. Therefore, this thesis looks for solutions for these problems while trying to gain a better understanding of what exactly is happening inside the compressors.

For the temperature regulation, different options for active cooling were assessed with a dimensionless analysis. Eventually, the concept of cooling through the compressor housing was chosen. Afterwards, a computational model was made to calculate the inner workings of the compressors and assess whether the concept of active cooling would still work. Then the computational model was verified by doing experimentation.

After that, the results from the model and the experimentation were compared. The model was able to predict the mass flow and the outside heat transfer coefficient accurately. However, the model was still off from the experimental values by quite a margin. After assessing all the data, the system seemed to perform better as a 2-stage system then as a 3-stage system. However, due to the inaccuracies of the model and experimentation, a definite recommendation on the number of stages of the system cannot be formed.

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.

As the world population is increasing and cities grow faster than before, food scarcity becomes a bigger issue. To depend less on import and decrease CO2 footprint, food should be produced nearby its consumers. A good solution for food production is a greenhouse which provides a steady high yield throughout the year. Some of the world's fastest growing cities are in Subtropical climates. Subtropical climates are characterized by high temperatures, abundance of sun and high humidity. Due to the high temperatures, cooling in greenhouses in subtropical climates is necessary, but evaporative cooling is not sufficient due to the high humidity. Outside air can be dehumidified by a liquid desiccant to cool the air further with an extra evaporative cooling step. After dehumidifying the air, the absorbed moisture from the air in the desiccant must be removed from the desiccant solution for re-use. This requires a lot of energy, which makes this cooling technique for greenhouses in subtropical climates not economically feasible. The proposed solution is to remove moisture from the desiccant by making use of the hot air that can be collected in the top of the greenhouse. The potential of this solution is assessed in this study. To predict the transfer of mass and heat between air and a calcium chloride solution as desiccant, a mathematical model of the process of mass and heat transfer over a structured cardboard pad is made. An experiment is conducted to validate the mass and heat transfer coefficients of the model. The model is used to evaluate the workings of the proposed system of regeneration. Results show that dehumidification can significantly lower the inlet temperature of the air in the greenhouse, provided that the inlet temperature of the desiccant is low. Regeneration of the desiccant solution with hot air is also possible, provided that the air in the greenhouse is significantly heated by the sun and pre-heating of the desiccant before regeneration is beneficial. The direct coupling of these two systems is not sufficient due to the big difference in temperature of the desiccant that is required at the inlet of the dehumidifier and regenerator. In future work attention should be paid to this temperature difference which is currently limiting the performance of the system. Then it's possible for the concentration of the desiccant to be brought back to the starting conditions for the dehumidifier.

In the report that is before you, a method has been investigated to increase the capacity of domestic hot water tanks. To do so without having to increase the volume of the tank, a phase change material has been included in an annular space around the tank. To test the performance of such a system, three main topics are dealt with in this report. First of all, the most suitable phase change material was selected. There exist many types of phase change materials, all having completely different properties, advantages and disadvantages. Secondly, weak spots were indicated during melting of the phase change material. These spots indicate the locations of the worst heat transfer rate, and to improve the performance of the system, these spots can be treated separately in future experiments or models, for example by placing fins at those locations. Finally, the response time of the phase change material was investigated during a typical discharge cycle to investigate the amount of latent heat that was released to the water. To model the charging- and discharging cycle, the vorticity stream method was used to create a two dimensional axisymmetric model. This model was implemented into MATLAB. The Boussinesque approximation was assumed to take into account buoyancy effects. Furthermore, only diffusive heat transfer was assumed to take place throughout the PCM and the effect of convection was neglected. Results showed that paraffin based PCMs are most suitable for applications in domestic hot water tanks. They are inert and stable, and their thermal properties do not degrade in time. They have suitable melting temperatures and high latent heat. To increase the thermal conductivity, dispersion of high density alumina particles was applied. Although the charging model did not reach the asymptotic range of convergence, it was shown that melting 5.0 cm PCM required long periods of time. After 9 hours, part of the PCM was still in its solid phase. Due to thermal stratification in the tank, the slowest melting rate was seen at the lower outer edges of the tank. During the discharge cycle, it was shown that not all PCM was able to release its latent heat to the water: 16.9% did not solidify. This occurred at an adapted flow rate to match the discharge time for both tanks. For higher flow rates, used in domestic hot water tanks, the amount of liquid PCM would even be greater, resulting in less latent energy release to the water. Based on the fact that 5.0 cm PCM did not increase the heat capacity of the tank, and all the previously mentioned factors, it is concluded that the performance of a hot water tank enhanced with PCM in an annular space does not increase the performance of a domestic hot water tank. Some suggestions were made regarding the possibilities of increasing the performance of the tank in future models and experiments.

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.

The vehicle industry is increasingly exploring emission-free mobility. Transforming the mobility sector to zero-emission vehicles that consume renewable and low carbon fuels is necessary to reduce the impacts on climate change. Hydrogen powered electric vehicles, or Fuel Cell Electric Vehicles (FCEVs), could make a significant contribution to reduce GHG-emissions and energy use in the transport sector. The current refuelling network for hydrogen vehicles is in its very early stages. The Cogeneration of Hydrogen,
Heat and Power project (CH2P) aims to impact the sector with a more efficient solution for the growing hydrogen infrastructure. It does so by offering very efficient decentralized H2 refuelling stations. The goal of this project is to design a refuelling station that has the ability to refuel hydrogen vehicles and to charge electric vehicles. The onsite CH2P production system is based on SMR and SOFC technology. A thermodynamic model has been designed to size and optimize all the station components. This includes the compression, intercooling, storage, throttling, precooling and dispensing of hydrogen, and the storage and charging of power. Main messages to take away from this analysis are the following: CH2P as a decentralized production method is protable. For higher station utilization the NPV has the potential to increase further by 55 %. Cascade filling of vehicles reduces the total storage and energy demand, and greatly reduces the CAPEX of the station. The optimal configuration consisted only of 950 bar High Pressure Storages. By adding a battery to the station, the system can power five fast-chargers to charge BEVs. However, power production by CH2P is more expensive than grid power and the current electric system is not sufficient for coping with sudden power fluctuations.

Heatpipes are promising devices for geothermal energy extraction owing to their
effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data.

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.

In the Netherlands most houses and buildings are connected to a gas network since the nineteen seventies to heat our houses and cook. In march of this year the new Dutch government announced that to meet the climate accord of Paris all dutch household should be of the gas net in 2030. To do this multiple alternatives to heat homes are available and one of those which has proven to be feasible over the last years is district heat. In a district heating network hot water is pumped round a city through a network of pipe lines to houses and other buildings. The network is split into a primary and a secondary network. In the primary network the heat is transported from the source, and in the secondary network the heat is pumped from a substation to the customers. The heat usually is rest heat of a STEG- or waste incineration power plant. With the increasing demand for heat due to the disconnection of the gas network, and at the same time the search for a more sustainable heat source for the district heating network like geothermal heat the prices and costs rise rapidly. This makes the heat loss in the system a more important issue also from a financial perspective. The current installed networks are all series 1 single pipe lines of ST/PUR/PE, this means the inner pipe through which the water flows is steal, followed by an insulating PUR layer which is covered by a protective PE layer. To decrease the heat loss from the pipe lines Nuon is looking at two possible options. Series 2 single piping which has a thicker insulating PUR layer and second the twin pipeline where both the steal supply and return lines are
embedded into one insulating PUR layer and a protective PE shell. The heat loss calculations for the buried pipelines in the ground are done using empirical formulas found in literature. These are checked numerical using a pde-solver. Comparing both results it was the concluded the empirical formulas give a good approximation of the heat loss. It also showed the heat loss is for a great deal depended on the temperature in the pipe and the ground temperature. In current heat loss calculations done by Nuon the temperature gradient of the water entering the network and leaving the network in not taken into consideration. By using the ambient temperature and a formula formulated in literature an estimation of the ground temperature at the buried depth of the pipelines can be made. By dividing the network into a number of pieces with length 푑푥 and an individual temperature 푇 the temperature gradient over
the entire network is taken into account. Collected data by Nuon concerning the user demand and the ambient temperature makes it possible to simulate what is the mass flow through the system at any given time and to analyze how the system responds to a demand fluctuation. By analyzing what happens in the networks in term of heat loss during a diurnal heat curve the results show that the heat loss stays nearly constant during a day. There are however big differences between different days in different times of the year. Analyzing what happens over the course of a year especially in the summer during times of very low demand the system under performs. A main reason for this is caused by the
minimum required temperature of 70 degrees Celsius at the customers due to salmonella regulations. This causes a lot of extra mass flow of hot water pumped around the system which heats up the return flow. This is also clearly notable from the efficiency of the network which drops tremendously in the summer. Comparing the difference in heat loss for the series 1, series 2 and twin system the results where as expected an decrease in the amount of heat lost, by 14.6% for series 2 and 39.70% for twin compared to series 1. The overall image of what happens actually stays the same with high return temperature and relative high heat loss and low efficiency in summer. There are a few options to further improve the performance of the system with a few percent by changing the inlet temperature or the location of the bypass valve. Financially speaking the twin system is also the better choice of the three. Compared to series 1 for series 2 the investment costs will increase because the materials
and instalment costs will increase. For the twin system the prices of the materials will increase and the placements of the welds will become more expansive however only half the number of pipe lines and joints is needed, so in total the prices will stay nearly the same compared to series 1. There is a lot of discussion on whether or not the maintenance costs will increase for the twin system, arguments for both cases are given. However the maintenance costs are so small compared to the costs of the heat loss that in every case the twin system is clearly the better choice.

The production process of milk powder consists of multiple stages. This report focuses on the falling film evaporator, which role in the production process is to evaporate the water content from the milk. A falling film evaporator is a large vertically-placed vessel whereby the inside is filled with smaller tubes. Steam enters the vessel and heats up the outside of the smaller tubes. The milk flows in a thin layer only along the inside perimeter of the smaller tubes, so these tubes are not completely filled. The advantage of this flow is that a thin layer of liquid is continuously in contact with the wall so the heating process is equal along the tube. The thin layer is called a falling film because the thickness compared to the length of the flow is very small.

The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.

Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.

The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.

Environmental regulations are continuously raising the bar towards more advanced diesel engine designs ca- pable of minimizing the emissions of polluting substances. The recent IMO Tier III legislation, entered into force in 2016, is forcing the engine manufacturers to meet a NOx reduction of more than 70% from Tier II for all the ships sailing in designated NOx Emission Control Areas (NECA). Although these zones are, up to now, limited to the North American and U.S. Caribbean Sea NECAs, future regions, such as the North Sea and Baltic NECA in 2021 as well as stricter national and port regulations, are increasing the uncertainty regarding the possible compliance methods.
The Selective Catalytic Reduction (SCR) represents a flexible, proven and commercially available technology capable of reducing more than 80% of the NOx in the exhaust gas. Its adoption started in the ’70s and was aimed at the reduction of stationary source emissions. From early 2000, it has been extended to the automotive industry in order to meet the Euro and EPA legislation for heavy duty and light diesel engines. Despite the evident disadvantages of the high installation cost and the considerable space requirements, the application of the SCR is subjected to a number of operating limits such as the narrow optimal temperature window and the slip of byproducts originated from the chemical reactions. These limitations can be even more severe when the diesel engine undergoes transient and dynamic operations.
The main objective of this research is to gain insights into the behaviour of the SCR technology under steady-state and dynamic operations of the diesel engine. After an accurate literature review of the working principles, reaction kinetics and modeling approach, a first principle model of the SCR has been developed to fulfill the scope. A 1D single channel approach, discretized along the reactor length, has been adopted to sim- ulate the heat exchange and the chemical reactions occurring in the catalyst. To facilitate the integration of the designed model with an existing diesel engine model available in the Maritime and Transport Technology Department of the 3me faculty, the resistance and volume approach has been selected. After the verification of the SCR model, the integrated system "SCR+Engine" has been tested under steady-state conditions, to an- alyze the effect that the added back pressure has on the engine performance, and under dynamic conditions, to simulate "real" load case scenarios that the SCR might experience.
The designed model, although limited from the chemical reaction point of view, is able to predict NOx reduction and ammonia slip under steady-state and transient operations. It can be further used in an early design phase to investigate the feasibility of the SCR inclusion in the drive trains of ships. Future studies are recommended to optimize the model, by taking into account the sulphur influence on the reduction efficiency and the catalyst aging, and to validate it with real experiments on marine SCR systems.

The Beam Dump Facility (BDF) is a new fixed target facility proposed to be installed in the North Area of CERN. Currently in its design phase, the BDF is aimed at the Search for Hidden Particles (SHiP) experiment, which purpose is to investigate the origin of dark matter and other lightly interacting particles. The BDF target/dump sits at the core of the installation, and its aim is to fully absorb the high intensity Super Proton Synchrotron (SPS) beam and produce charmed mesons. The average beam power on target is 305 kw due to which very high thermo-mechanical loads will be generated on the target/dump configuration which can lead to mechanical failure of the target material. This calls for continuous cooling and heat removal from the target material, which requires the design and optimization of a very complex cooling system to ensure the target reliability during operation.
Keeping in view the high velocities that are obtained in the channels of the cooling system of the target assembly, an extensive 3D turbulence modeling of the full scale cooling system is required in order to predict the target water cooling system behavior. A comparative conjugate heat transfer (CHT) study using a simplified 2D geometry of the BDF target for different mesh size was performed for identical y+ values to validate the pressure drop in the cooling channels and the temperature profile in the target blocks with the analytical calculations. Subsequently, a 3D model of the cooling channel was simulated for different Reynolds number and an extensive study was performed to check the behaviour of the flow in the log-law layer. In addition to that the friction factor was validated with the analytical results and with the available literature for various Reynolds number. Thereafter a full scale 3D steady and transient model was simulated using the information obtained from the previous studies. All these simulations were carried out in Ansys Fluent. The energy deposition in space on the target blocks was obtained via FLUKA MonteCarlo simulations. The variation of HTC in different channels and the fluid-solid interface temperature is found out to be in accordance with the analytical calculations. The pressure drop and the temperature rise from inlet to outlet of the cooling system is also in confirmation with the design parameters. Transient simulations were performed subsequently in order to study the impact of time and space varying energy deposition in the target blocks on the overall flow. With the given inlet velocity, the boiling at the surface of the target blocks is not expected to happen. As a final part of this study, in order to make the numerical model more robust, a mesh sensitivity analysis was done in order to optimize the mesh size, especially in the boundary layer region.

This report presents the first simple and comprehensive method to effectively compare different infrastructure configurations for reservoirs that have been found suitable for long-term storage of carbon dioxide. Modelling the different transport phenomena along the chain, this study identified a fixed set of input parameters and used them to evaluate the differences between existing and new infrastructure. The report then explores how these differences affect the conditioning steps the CO2 must undergo before injection.The model was then applied to a a test case, the P18-2 natural gas reservoir 30km north-west of the Hague. The results for this test case show that an auxiliary pumping step is required for supercritical injection at the desired pressure. Subsequently, the results are used to develop a conceptual design for a module that can either be installed on a new platform or retrofitted to existing offshore production facilities, allowing them to be used for conditioning and injection of CO2. Last, the economic feasibility analysis demonstrates that reusing existing infrastructure can reduce the costs of transport and storage of CO2 by over 50%.

Zero Emission Fuels B.V. develops a standalone System that produces renewable methanol from air and sunlight. In this thesis the steady-state model of this System is replaced with a Simulation Tool that can predict its behaviour for every minute of a year. The goal is to verify how technical design parameters influence methanol production and to determine if the business case target of 6.7 mole methanol per day is achievable. This thesis will contribute to the understanding of the System, speeds-up the design process and makes it possible to see the impact of fluctuating weather conditions throughout the year at different locations on earth. The System as it was at January 1, 2018 is implemented in the Simulation Tool and its production determined for Tucson, Arizona in the year 2005. The result is an average daily production of 3.0 mole methanol with fluctuations from 1.0 mole during a cloudy winter day to 4.2 mole during a sunny summer day. The minute by minute production is found to be influenced by all System design parameters. They must thus be designed together to make sure the System performs optimally. The Alkaline Electrolysis uses the most energy of all subsystems. Four options to improve the System are identified. These are 1) improving the power the Solar Panel produces, 2) improving the efficiency of each subsystem, 3) improving how the System is controlled and 4) integrating heat. A selection of these improvements shows that it is possible to produce 4.8 mole methanol per day. The observed seasonal and daily fluctuating methanol production demonstrates the importance of the dynamic model developed in this thesis. Compared to the steady-state model it gives a more realistic view of methanol production and can be used to improve System design. However, to reach the business case target more technical improvements are needed. It should be realised that the design decisions will be based on technical, economical and political arguments.

In this thesis diffuser performance will be simulated with computational fluid dynamics, which will be done with the program Ansys Fluent. The expanding geometry creates an adverse pressure gradient and under certain conditions there will also be separation.

At first a relative simple simulation will be done of a one-directional incompressible turbulent flow. It will show how different turbulencemodels performat a Reynolds number of 15,000 inside a conical diffuser with an angle of 2θ=8°, no separation is expected in this geometry. The turbulence models investigated are the k-ε, the k- ω and the RSM model. The mesh refinement is tested on produced accuracy of the results. The final results of the simulations are compared with both DNS (direct numerical simulation) and experimental results. All models produce different behavior, due to transport equations, algebraic models and empirical constants used. The deviations are dependent on geometry and flow conditions, where certain turbulence models are better for specific cases. The simulations showed different representations. The flow behavior of the k- ω model was the most realistic, due to the correct core velocity although it showed flow reversal at the wall. Various parameters are reviewed, such as velocity profile, flow reversal, pressure coefficient, friction coefficient and turbulent statistics.

The oscillating flow will represent a case which is more closely to the flow seen inside a thermoacoustic engine. The geometry is a rectangular diffuser and the flow is compressible. It is a much more complex flow with frequencies ranging from 6-21 Hz during the simulations. Laminar, transitional and turbulent cases are simulated by Reδ numbers of 380, 580 and 740 with varying displacement amplitudes. The transitional k-kl-ω model is used, because of its ability to also simulate laminar and transitional cases besides only turbulent cases. Additional the k- ω SST model is tested. The velocity profiles are not simulated well with the k-kl-ω model, which was caused by the under estimation of simulated turbulence near the wall. It was found that for both models separation will induce early and low in the diffuser and expand downstream as the cycle passes. As a result the Reynolds shear stresses show higher values earlier in the cycle. It was also seen that the reattachment would differ in pattern. The trend is found that separation begins earlier with increasing Reynolds number and increasing displacement amplitude. The minor losses, or irreversibilities, vary in accuracy, the effect of the displacement amplitude is not always seen for variables which are dependent on the magnitude of the pressure. In addition turbulence would not show an increase at the point of transition compared to turbulent cases. Both models seem to deliver deviating results, but the k- ω SST models the cases better.

One of the challenges in gas turbines is in the reduction of NOx and CO emissions due to simultaneous
dependence on stringent emission norms and subsequent demand for change in operating conditions. To cater the volatile demand of power supply, most of the gas turbines have to be flexible to changing loads. Closer the load is to full load, more is the impact on gas turbine emissions. In order to mitigate the effects of emission rise, a shift towards emission reduction techniques are being adopted. Of the available options, Exhaust Gas Recirculation (EGR) is a promising option for good part load performance and emission reduction. EGR is a technique to reduce the flame temperature by recirculating the products of combustion into air intake and replacing part of the oxygen content with inert constituents such as CO2. This leads to reduced NOx emissions and increased CO2 content at the exhaust for effective carbon capture. Despite these advantages, the introduction of an oxygen-depleted oxidizer leads to changes in combustion behaviour giving rise to flame stability issues. In this thesis, a chemical kinetic model to predict emissions in a lean premixed combustor coupled with EGR, is built using chemical reactor networks (CRN). A CRN model was developed by splitting the whole combustor volume approximately in half before and after the recirculation zone. The first half was further partitioned into the flame reactor and the recirculation reactor. The flame reactor volume
is set based on the chemical time scale approach. This approach quantifies the flame zone volume based on the difference in reaction kinetics for different oxidizers. The model was also validated with results from the literature for various flame cases with similar flow fields and was in good agreement with the data. Various chemical mechanisms and their emission prediction ability are investigated. GRI-Mech 3.0 is found to accurately predict the emissions for a given set of operating conditions. The effect of change in oxidizer composition with different amounts of CO2 is investigated. As a consequence of O2 starvation in the oxidizer, NOx formation is inhibited and CO levels are escalated.
At the same flame temperature, NOx levels reduced by a maximum of 40%, whereas the CO levels rose simultaneously as hgh as 50%. A parametric study on varying the combustor pressure indicated a rise in NOx levels and drop in CO levels with increase in pressure. The pressure rise augmented the NOx formed through thermal pathway and lead to the rise in NOx beyond 1800 K. On the other hand, CO2 dissociation to CO is suppressed by the rise of pressure, hence decreasing its magnitude. The impact of using wet EGR is investigated and compared with results of dry EGR. It is observed that NOx reduction is enhanced by 5-10%. Whereas the CO levels is escalated by 10-20%. The combined kinetic effect of CO2 and thermal effect of H2O is found to be reason for the change in emissions. This indicates an enhancement in combustion reactivity. Hydrogen injection raised the NOx levels by 5-10% and the CO levels dropped by a maximum of 12.5%. CO level suppression was a clear indication of the augmented flame stability and was found to be achieved with a slight compromise on NOx levels. Additionally, the effect of premixer efficiency is tested by discretizing the volume as parallel reactors with varying equivalence ratio inputs. Due to decreased premixing, unmixedness leads to various rich and lean pockets inside the combustor. This is a reason evident rise in both NOx and CO levels with decreasing premixer efficiency.

Direct Numerical simulations (DNS) belong to the class of simulations that strive to emulate the real physical flow by trying to simulate the complete range of scales involved in the flow. Though the computational power has steadily increased in the past few decades, resolving the complete range of scales from the largest scales up to the Kolmogorov scales can be very complicated and time consuming for moderately high Reynolds numbers as well. Hence, DNS is still expected to be a research tool to study relatively simple flows and geometries for the forseeable future unlike its counterparts such as Large Eddy Simulations (LES) and Reynolds Averaged Navier--Stokes (RANS) methods. The main objective of DNS studies is to study the flows so that turbulence models which are parametrized can be improved upon with the physical insight from the results of DNS. It also helps in studying the effect of numerical methods and techniques applicable to real flows and their stand-alone effects without any type of modeling as done for LES or RANS.

This thesis is concerned with the DNS study of a compressible turbulent jet. A turbulent jet belongs to the class of free turbulent flows, in the sense that, it is not bounded physically. In this thesis, the transport of a passive scalar through the jet is studied. At higher Reynolds numbers, for convection dominated equations such as the scalar transport equation, discontinuities can occur due to the hyperbolic nature of the equations. When central methods are used, oscillations are observed in the regions of the discontinuities. These oscillations can make the solution unphysical. Therefore, in this thesis the scalar transport equation has been modeled with an additional Weighted Essentially Non-Oscillatory (WENO) interpolation to accurately capture the discontinuities without oscillations. The WENO interpolation is combined with central compact finite difference methods to reduce the numerical dissipation while maintaining the order of accuracy in the smooth regions which is essential in high Reynolds number flows.

It was observed that a high dissipation setting for the WENO interpolation removed the oscillations but introduced artificial numerical viscosity. Therefore a relatively large domain was used with a low dissipation which while removing the oscillations and reducing the numerical dissipation. The WENO method for the compressible turbulent jet was first validated with experimental results to ascertain the accuracy of the grid resolution. The same grid resolution was used to study some properties of the jet at two different Schmidt numbers at a slightly lower Reynolds number. It was observed that the results obtained with the WENO interpolation matched well with the experimental results while being physical valid solutions because they had no oscillations. The results also showed different modes of instabilities for different Schmidt numbers and that the decay rate is a good characterization of the properties of the turbulent jet.

In conclusion, the WENO methods can be a very helpful method to accurately capture the discontinuities in an efficient manner and is also suitable for methods such as LES and RANS to model flows that have some hyperbolic nature of terms in the governing equations.