The use of hydrogen in gas turbines is promising to help with the energy transition since there are no carbon-based pollutants released during its combustion. However, the transition from fossil fuels to hydrogen in gas turbines involves significant challenges. Hydrogen flames exhibit higher flame speed, making burners more prone to flashback, which in turn can cause damage by propagating upstream into the premixing sections. To avoid this type of failure, it is necessary to understand the different mechanisms that can cause flame flashback in gas turbines and develop strategies to help reduce the flashback tendency of hydrogen flames. The FlameSheet™ burner, originally developed by Power Systems Manufacturing (PSM), is a two-stage burner that can be used with fuels having high content of hydrogen. The geometry of the main stage includes a 180-degree bend (U-bend). This feature, which abruptly changes the flow direction of the air-fuel mixture, creates a trapped vortex which helps in stabilizing the flame in the main stage. Such a design makes the burner more resistant to flame flashback.

In this thesis, a numerical study is done on an academic trapped vortex burner, inspired by the FlameSheet™ design using Ansys Fluent. Initially, a cold flow simulation of the flow inside the main stage of the burner was performed using RANS. This was followed by a parametric study (using RANS) of the cold flow patterns while varying several parameters like the shape of the inner liner tip, channel height ratio and the distance between the tip and outer wall of the U-bend. From these results, useful insights were gained by analyzing and comparing the velocity profiles in different parts of the U-bend. To gain more information from some of the relevant cases, large eddy simulations (LES) were used. With the LES results, two-point correlations and cumulative distribution functions of the velocity field were made to describe the cold flow behaviour in the main stage of the burner. These results confirmed that the tip shape and its distance from the U-bend are significant parameters since they affect the velocity profile in the boundary layers of the flow. This change in the velocity profile of its boundary layer can alter the flashback tendency of hydrogen flames in the burner. Along with these cold flow simulations, a reactive simulation was also performed with the original geometry of the trapped vortex burner to understand the flow features inside, in the presence of a flame. Super adiabatic flame temperature and non-uniform diffusion of hydrogen into the flame were observed at various points in the flame.

Energy efficiency is a key goal for the Quooker system. To further increase the energy efficiency of the Quooker system, this study investigates a possible heat integration system where the waste heat from the refrigeration cycle for chilled drinking water is implemented to preheat water before entering the boiling water reservoir.
Due to the intermittent nature of both the supply of heat, which is coupled to the control scheme of the chilled water reservoir and the demand for heat, which is determined by the user, a thermal storage system is required. Literature showed that the best refrigeration system for this application was a vapour compression system using a natural refrigerant such as isobutane. The most efficient heat storage could be done using organic phase change materials (PCM). Using a PCM allows the system to retain more energy in the same volume due to the latent heat in the system. To enhance the heat transfer between the fluid streams and the PCM, a fin-and-tube heat exchanger concept was designed. This concept, coupled with existing Quooker system demands, leads to a preliminary set of design requirements as well as a set of variables left to be optimised by modelling. The heat transfer from the refrigerant to the tubes was modelled using known correlations for condensation in tubes. The heat transfer between tubes and fins was modelled using a two-dimensional finite difference scheme. The heat transfer between the tubes and the water was modelled using forced convection models. The models gave dimensions for the size of the PCM container, the number of passes for each fluid stream and the thickness and spacing of the heat transfer fins. An experimental setup based on the optimal design was created to validate the models. The results showed that PCM storage is an effective manner to store thermal energy. Heat transfer was significant in the regions surrounding the tubes. Further away from the tubes, the fins did not provide enough heat transfer to utilise the whole storage capacity effectively. In its current design state, the system would have an economic payback time of around 20 years. With small design improvements, such as increasing the fin thickness and decreasing the fin distance, the payback period can be brought down significantly. The added product value from being a more efficient product makes the concept promising for future implementation.

Hydrogen is a promising fuel for both reducing greenhouse gas emissions and being used as an energy carrier for renewable energy sources. The adaption of hydrogen instead of natural gas as a fuel in gas turbine combustors introduces additional challenges regarding flame stability. One instabilities is boundary layer flashback. This phenomenon occurs if the flame speed exceeds the local flow velocity. Then the flame can propagate upstream, which can result in equipment failure. Lean premixed hydrogen mixtures are more prone to boundary layer flashback because hydrogen flames have both a smaller quenching distance and higher flame speed compared to natural gas. Therefore, operating at 100% hydrogen fuel content is an enormous challenge. Active control strategies are a promising strategy to prevent and withstand possible occurrences of boundary layer flashback in gas turbine combustors. In this work, the detection, prevention, and counteraction of flashback are investigated in unconfined Bunsen burners. After an experimental analysis of suitable sensors, a tracking controller is therefore designed and different control design approaches evaluated.

Both the increase in flame fluctuations and in flame angle are reasonable as indicators for a flame moving towards flashback as the flow velocity is reduced. However, the complex nature of a flame makes it challenging to measure these effects in a flame. Four types of sensors, i.e. an ion sensor, a thermocouple, a photo-detector and a microphone, were investigated for their ability to detect flashback and to find precursors that indicate the onset of flashback. A Bunsen burner equipped with the different types of sensors was used in an experiment to determine the most suitable sensor type. Of the investigated sensors, the thermocouple is the most promising sensor for both flashback detection and use the temperature signal as a control variable. The temperature is a precursor for the onset of flashback since the temperature increases with increasing flame angle. Also, by placing multiple thermocouples in $360^oC$ configuration on the burner rim, it is possible to estimate the position, where the flame entered the burner.

A complete fault-tolerant framework to control a flashback event is proposed. This framework increases the flashback resistance of the system and it counteracts possible flashback events, which is considered as a fault in the system. A supervisor block has two functions. Firstly, the supervisor uses a fault detection method, known as trend checking, to detect flashback. Secondly, it decides which strategy should be applied, based on the temperature measurements. The fault-tolerant control strategy is threefold. First, the prevention controller closes the loop, so it can steer away from flashback conditions by rejecting disturbances and tracking a desired reference path using the temperature data. Different tuning methodologies based on the traditional proportional and integral (PI), linear-quadratic-integral (LQI) and pole placement regulator (PPR) are explored for this purpose. The robustness of the proposed controller was verified through loop shaping interpretation. However, flashback could be consider as an stochastic process that still could be triggered. Therefore, the second task is to detect present flashback events and apply counteraction to oppose the fault. A design is proposed by the injection of pressurized air upstream of the burner rim to push back the flame by diluting the boundary layer and increasing the flow speed. A proof-of-concept is validated in the experimental set-up as a possible and feasible counteraction. The third task is a final safety measure. A safety switch would be applied to shut off the fuel supply to fail safe after a potential unsuccessful counteraction attempt. The fault-tolerant control framework was simulated in Simulink for a demonstration purpose, where the models were identified from experimental data.

The proposed fault tolerant control framework is intended to increases the flashback resistance of the system by being more robust towards disturbances and flashback events that would normally require a restart of the system. Its potential to do so has been confirmed by simulations. However, further research on the prediction of flashback and controller implementation are still required to run burners on 100% hydrogen without flashback.

Currently, the paper producing industry is a significant energy consumer and contributes to the excessive greenhouse emissions. In order to become carbon neutral by 2050, it is evident that the paper industry must become more sustainable as well. Most of the energy is consumed in the dewatering process. Therefore, optimizing this process is key for efficient operation and reducing the carbon footprint. Since the late 1950's, there have been several models proposed to describe the dryer section of a paper machine. However, the majority did not capture internal transport phenomena resulting in inadequate models for heavier paper grades. In this work, a comprehensive physical-numerical model is developed that describes the internal transport phenomena in multicylinder paper drying. The model solves a set of two differential equations describing the moisture content and temperature in the thickness direction of the sheet. The model can be solved by imposing time-varying boundary- and initial conditions. The model includes unfelted and double-felted cylinders. Correlations for multiple thermodynamic properties are proposed and evaluated. Heat and mass transfer coefficients at the open surface are determined using the Chilton-Colburn analogy. Furthermore, the sorptive characteristics are fully accounted for. In order to find the most applicable thermodynamic properties, the model is validated against data gathered in a field survey, carried out as part of this project. In this field survey, a paper grade of 203 [g/m^2] running at 403 [m/s] was investigated. Temperature measurements were performed in order to determine the conditions per cylinder. A final moisture content and two temperature profiles were used as test criteria to validate the model. Furthermore, the survey gave direct insight on the state of the dryer section: Cylinder 23 was found to be in the flooded state and the warm-up time was too long. The model was found to slightly overestimate the final moisture content: the calculated final wet-basis moisture content at the end of the pre-dryer section was 0.235 [kg/kg], which is 0.022 [kg/kg] above the measured final moisture content. The calculated temperature profiles closely followed measured values. The effect of changing three operating conditions was evaluated. This involved fixing the relative humidities in the pockets, felting the first 10 cylinders and varying the machine speeds. By fixing the relative humidities in the pockets, the final moisture content increased by almost 20%. However, a 60% reduction in volumetric flow rate over the fans could be achieved. Felting the first 10 cylinders results in a 50% reduction in the warm-up length and a 8.9% decrease in the final moisture content. Lastly, it was found that for an increased machine speed, the final moisture content increases as well, due to reduced heat transfer and drying time. This Master Thesis Project was carried out by the author at the company Kadant Johnson in the Netherlands.

In this thesis work, we strive to describe and apply the modelling methodology for simulating a turbulent jet diffusion flame stabilized behind a bluff body by applying flamelet generated manifold (FGM) model and steady diffusion flamelet model for predicting the pollutant emissions from the flame. The flame under consideration is a CH4/H2 (1:1) bluff-body stabilized flame known as HM1. The numerical calculations of the flow physics has been completed by using a commercial CFD code namely Ansys Fluent 19-R3, where the computational domain is assumed to be two dimensional and axis-symmetric in nature due to the cylindrical symmetry of the burner with an structured grid for meshing. The turbulence is modelled by using a two equation Reynolds averaged Navier Stokes model, namely the standard k – ϵ model with a modification in the Cϵ1 from 1.44 to 1.60. The chemical mechanism used in the project was the GRI 2.11 mechanism developed by the Gas Research Institute, USA. Next, postprocessing tools like the Reactor Network Model (RNM) and Ansys NOx post-processor were used as an alternative, possibly a better way to obtain the NO species
concentrations. The acquired results from the simulation are thoroughly analysed and were compared to earlier results on the HM1 flame in the literature and validated by using the experimental data documented by the University of Sydney in collaboration with the TNF Workshop. The results showed that the steady diffusion flamelet performed better than the FGM model and was in acceptable agreement with the experimental data, although some under-prediction and overpredictions were reported for NO and OH species. In regard with the post-processing tools the RNM model performed better than the Ansys NOx post processor.

In reacting flows, detailed chemistry computations are usually avoided precomputing the thermochemical quantities as functions of a reduced set of variables such as the Flamelet Generated Manifold (FGM) approach[34]. Although it mitigates the calculations of detailed chemical mechanism, the memory requirement associated to store the lookup table and retrieve the information during numerical simulations is usually large (order of Gigabytes). Thereby, extending the FGM approach in order to include other conditions requires to add
other independent variables which will inevitable lead to increase the size of the lookup table. This will generate that Large Eddy Simulations (LES) cannot be performed such as is the case of the Diluted Air FGM (DA-FGM) approach developed by Xu Huang[9], limiting the simulations to Reynolds Average Navier-Stokes (RANS) approach. In this master thesis, the goal is to use Artificial Intelligence (AI)-Machine Learning (ML) techniques in order to reduce substantially the computational cost of storing lookup tables. In order to achieve this, the Artificial Neural Network (ANN) technique is used. First, a 4D FGM lookup table for hydrogen flames is simplified using the aforementioned AI technique. Then, this technique is used to replace a 6D lookup table generated using the DA-FGM approach. The accuracy and stability of the models provided by ANNs is measured by statistical indicators, providing high accurate and stable AI models. Finally, in the middle of this project, unexpected issues regarding the 4D lookup table were encountered, which lead to recreate the 4D lookup table. After studying carefully how the 4D lookup table was created, a new 4D lookup table is generated, providing excellent results and improving the AI models obtained.

Work is presented on a novel hybrid fs/ps two beam CARS application for spatiotemporal gas phase hydrogen thermometry, capable of sampling at a rate of 1 kHz. The aim of the research is to assess the feasibility of directly probing hydrogen for accurate temperature measurements in a combustion environment. Typically, nitrogen is used in applied CARS thermometry because of the inert nature of the gas and the availability on both the products- and reactants side of the flame. Nitrogen is well understood in CARS applications and is therefore used as a benchmark for the results obtained through hydrogen thermometry. No hydrogen was available in the lab, so a rich premixed methane-air flame was used in a regular Bunsen burner. At high equivalence ratio’s sufficient amounts of hydrogen are formed as an intermediate species to be probed in the flame. Quantum-mechanicalmodels were used to predict the molecular responses of the corresponding molecules, after which they are compared to the experimental data. The feasibility of hydrogen thermometry was assessed through a series of three different experiments: using the standard output pulse of the regenerative amplifier at 35 fs for spatiotemporal measurements, using the 35 fs pulse for point measurements, and using a frequency broadened 10 fs pulse produced through filamentation for point measurements. Nitrogen and hydrogen spectra were recorded simultaneously to ensure proper data comparison. The 1 kHz acquisition rate enabled the evaluation of temporal correlation in the temperature data. The spatiotemporal CARS measurements clearly depicted flame dynamics at 1 Hz and 12Hz, but the signal to noise ratio in the hydrogen signal was too weak to properly assess the data. The point measurements using the 35 fs pulse yielded similar average temperatures, but the precision in the hydrogen temperatures was poor (25%) due to the fact that only the first two S-branch transition in hydrogen were excited. In the experiments using the 10 fs pulse, the first four S-branch transitions in hydrogen were excited. This greatly improved the precision for the temperature measurements (2%), surpassing that of the nitrogen thermometry (4%). The nitrogen and hydrogen temperatures also showed great temporal correlation (0.85-0.9). However, there was a very consistent temperature deficit in the hydrogen temperatures of 550K, most like caused by a wavelength dependence in the optics (which are optimized for narrowband applications) which severely affects the higher Raman shifted hydrogen signal. Lastly, an experiment was done using the 10 fs pulse to probe O-branch transitions in hydrogen, and a serious discrepancy between the CARS and CSRS spectra were found, again likely caused by the optics in the setup. These results approximated nitrogen temperatures much better, within 7% at 2000 K. Overall, the results were promising, but not yet conclusive and more research is needed.

This study presents the numerical modeling of boundary layer flame flashback in premixed tube burners with turbulent flow and high hydrogen content. The modeling approach is based on a RANS turbulence model and a flamelet combustion model using a flame surface density approach in combination with multiple turbulent flame speed correlations to close the chemical source term. The Zimont turbulent flame speed correlation is used to model flame flashback and gain insight in the relevant mechanisms driving the occurrence of the instability. However, this approach has not let to a method to predict in ad- vance that a flame is near flashback conditions. This implicates that the flashback process (when mod- eled with this specific modeling approach) seems to occur after a certain threshold has been passed. When this threshold has been passed the flashback process is self-amplifying. The ability of the mod- eling approach based on the Zimont correlation to predict flame flashback is compared to experimental data. Based on this comparison it is concluded that the numerical model performs fairly well. The most notable result is the flame regime map which shows that the numerical model is able to capture that an increase in the hydrogen concentration in the fuel leads to an increased flashback propensity, which was also seen in experimental results. Furthermore, high hydrogen concentration fuels are character- ized by high flame speeds and, if sufficiently lean, by a Lewis number lower than 1. In combination with differential diffusion this leads to the thermal diffusive instability. At an attempt to improve the nu- merical model, two turbulent flame speed correlations that account for these effects were selected and implemented in the numerical model. However, these correlations did not lead to an improved ability of the model to predict flame flashback and flame shape. This reinforces the initial findings that the numerical model with the Zimont turbulent flame speed correlation already performed fairly well.

As a contribution to the decarbonisation of domestic heating, the graduation project investigates the feasibility of the application of UV sensor technology for flame detection and flame monitoring in hydrogen-powered domestic gas boilers. The research includes empirical studies and an analytical approach to describe influences on the sensor signal strength.

The idea of using hydrogen as a medium for storing excess renewable energy is gathering momentum but to deliver, several technological advances are required. One of the main challenges is boundary layer flame flashback (BLF) as a key operability issue for low NO_x premixed combustion of hydrogen-enriched fuels; thus design of tools to predict flashback propensity is of interest. Extensive research has already been done, and the result is three main BLF models, promise to predict the initiation of boundary layer flame flashback accurately. The first model was developed in the Paul Scherrer Institute (PSI) and was applied to an axial-dump burner operating at elevated pressure and temperature conditions with fuel mixtures of H₂-N₂ and H₂. The second model was developed at the University of California, Irvine (UCI) and was based on the flashback experimental data of 100% H₂ fuel (similar geometry with UCI) at elevated pressure and temperature conditions. Finally, the most recent BLF model was developed at TU Delft based upon the 'generalized' Stratford's turbulent boundary layer separation criterion, which is a further development of the model initially developed in TU Munich (TUM) for atmospheric confined H₂ flames in a horizontal channel burner. In this thesis, the performance of the state-of-the-art BLF models is evaluated in a number of academic burners operating with H₂-rich and pure H₂ fuels at atmospheric and elevated operating conditions. While the TU Delft model showed good agreement with 100% H₂ flashback experiments under atmospheric conditions and simple geometries, further modifications are proposed to perform under conditions relevant for gas turbines (increased pressure and temperature). A new turbulent flame speed correlation to capture the effect of pressure on flashback propensity is applied to the model and validated in high-pressure academic burners. Furthermore, the TU Delft model is validated for a lab-scale size of a gas turbine burner geometry, operating at atmospheric conditions and tested in the TU Delft laboratory. This validation shows that it is essential to review the outcome of the boundary layer flashback model at different locations in the boundary layer, and with minor modifications, the model can capture the test results adequately. The sensitivity of the TU Delft model on different fuel compositions is also investigated. Proper selection of the turbulent flame speed correlation is found to improve the model's performance for fuel mixtures less rich in H₂.

Flameless combustion, or MILD combustion is a technology that enables stable combustion with high efficiency while maintaining very low NOx and soot emissions. It is characterized by uniform heat flux and can be achieved when fuel and preheated oxidant jets are mixed into a strong recirculating flow. It is a complex process which combines multiple physical phenomena such as, heat transfer and reactions in a turbulent flow and is difficult to capture numerically via simple Computational Fluid Dynamics (CFD) simulations. Therefore, appropriate turbulence, chemistry, radiation and turbulence-chemistry interaction models are required to address this combustion regime via computational modeling. The present work tries to tackle this issue by conducting a comparative study of different radiation, turbulence and chemistry models for the simulation of a labscale MILD combustion furnace using RANS CFD simulations. This assessment was conducted in two steps. Firstly, the importance of radiative heat transfer in furnaces was investigated, by simulating an axisymmetric oxycombustion furnace using advanced radiation models. The SLW, Domain-based WSGG and Cell-based WSGG models were evaluated using simple chemistry and turbulence models. The results were validated based on the work of Webb et al and experimental results. It was concluded that detailed simulation of the radiative gas properties is important and directly influences the resulting temperature field, especially in the bulk flow region. Secondly, a detailed evaluation of a Dutch natural gas fired Delft lab-scale furnace that is operating in MILD combustion was conducted. For the turbulence modeling, the Standard, the Modified Standard, the Realizable k − ε and the k − ω SST Models were compared firstly qualitatively, based on velocity and reaction field contours, and secondly, quantitatively, based on available LDA velocity measurements. The influence of the chemistry modeling was evaluated in two parts. Firstly, the qualitative effect of changing the Cξ constant of the EDC model on temperature and reaction zones of the GRI-Mech 3.0, the DRM-19, the KEE58 and Chen mechanisms, was investigated. And secondly, the quantitative effect on the use of the GRI-Mech 3.0, the KEE58, Chen and Lu-30, based on CARS temperature measurements was researched. Lastly, the influence of the radiation model was investigated qualitatively and quantitatively using the standard 1-RTE domain based WSGG, a 4-RTE WSGG, a cell-based WSGG and the SLW model.
Out of the mechanisms coupled with the EDC, the GRI-Mech 3.0 performs well. Of the radiative properties models the domain based WSGG performs worse compared to the other models. The cell based WSGG performs similarly to the more advanced spectral and non-grey (4-RTE) models and has lower computational cost. For these reasons, the best overall combination for the studied flameless furnace is found to be the combination of Standard k−ε model, standard EDC with GRI-Mech 3.0 and cell based WSGG.

Steel plants are one of the largest sources of waste heat. Waste heat recovery throughout the steelmaking process is not a new phenomenon. One of the sources of waste heat is found in the coke plant. Cokes are an essential part of steel production. During the coking process hot cokes are cooled down. Most coke plants in the world traditionally use water, so called wet quenching. In this process, all the heat put into the cokes is dispersed to the environment as steam. This waste heat source has huge potential. The goal of this study is to find a method to utilise this waste heat and to evaluate this method’s technical and economic feasibilities.
The chief issue that must be tackled in a design is the fact that the steam generated during a quench is close to atmospheric pressure. Another issue to be solved is that of the solid particles suspended in the steam. Several potential designs were produced to use the steam from a quench to recover the waste heat. Based on several criteria, the design using the Synext engine was found to be the superior one and was developed further.
This design is divided into three sections, capture, cleaning and storage. A water wall and capture valve are used to capture the steam. An impaction and a cyclone separator are used to rid the steam of the solid particles to the extent that their detrimental effect to the Synext engine is minimised. The sepa-rators’ dimensions are derived based on the steam input and Synext engine requirements. The steam is then stored in a storage vessel.
A Simulink model of the design is composed to simulate the process and evaluate its efficiency and technical feasibility. The model’s findings show that the outputs for certain cases require unreasonable dimensions for the design. The economic analysis showed the designs costs make it an unlucrative investment.

Optical measurement techniques are widely used in fluid mechanics to measure quantities like velocity, vorticity, and temperature. They are usually non-intrusive visualization techniques that provide a major advantage by not influencing the flow properties. Most of the developed measurement techniques estimate vorticity by computing the gradients of the measured velocity fields. In this project, the idea is to directly measure the mean vorticity using polarized emission of nanoparticles. The nanoparticles are excited using vertically and horizontally polarized laser. Accordingly, the emission from these nanoparticles is separated into vertical and horizontal polarized emissions respectively. Depending on the percentage of fluorophores in the nanoparticles whose absorption dipole moment is parallel to the excitation electric vector of the laser, the polarized emission is varied. The nanoparticles in the quiescent fluid state have a characteristic polarization (or anisotropy) depending on the chemical composition of the nanoparticle. However, in turbulent flows, depolarization of the particles can occur because of the rotation of the nanoparticles due to vorticity. Based on this idea, the vorticity can be estimated by quantifying the depolarization in turbulent flows. To verify the above phenomena experimentally, steady-state polarization measurements were conducted. An experimental setup was built specifically to measure depolarization from these nanoparticles. Important preliminary tests like lifetime measurements, spectroscopic studies, and photo-bleaching experiments were conducted. Based on these results, hybrid europium based chelated spherical particles were chosen. To further understand the properties of these particles in a quiescent flow, a consolidated set of anisotropy measurements were conducted to study their oxygen sensitivity in the surrounding medium, viscosity dependence and the effect of its concentration in the fluids. Analytical correlations which was developed in-house to translate the depolarization of the nanoparticles to vorticity was used in this thesis. Flow in a square duct was studied to estimate the vorticity along the edges and further compare it with the DNS data.
Two additional non-dimensional numbers ND1 and ND2 were introduced to interpret the underlying physics behind the problem. The variation of synthetic signals was studied for different Re, fundamental anisotropy r0 and the non-dimensional number ND2. These synthetic signals showed a drop in anisotropy near the edges of the square duct that has occurred probably due to vorticity. Emission polarized intensity signals and the anisotropy contours for Re = 0 and Re = 4434 were analyzed. Due to the measurement error, the contours were column averaged and respectively compared their variation along the width of the square duct. A drop in anisotropy was observed for Re = 4434. Using the ratio of emission intensities for each excitation polarization, the vorticity was estimated using the optimization algorithm. The estimated vorticity Ѡy shows similar trends compared to DNS data which is a promising evidence for the proof of concept. However, the results are not overlapping from the repeatability studies due to the high sensitivity of the optimization algorithm and the random error from the measurement equipment. To improve this technique in the future, transient experiments have to be conducted to estimate vorticity accurately and to capture the underlying physics.

Shifting from coal and oil towards sustainable energy sources is currently an urgent and global challenge. Compared to coal and oil, natural gas is a cleaner fossil fuel and may serve as a transition fuel. To meet future demands for gas there is a desire to optimize the production from existing reservoirs with their production facilities. In case of wet natural gas wells this gas production can be hampered by the presence of liquids, namely water and condensate, thus creating a multiphase flow in the production tubing. The maximum gas well capacity is limited by the ever reducing upward gas flow velocity due to the subsurface reservoir pressure that is reducing over time. The reduced gas velocity results in the inability to transport the liquid upward from the downhole location to the surface. The accumulated liquids will then block the well bore (so-called liquification), making further gas production impossible. One solution it to decrease the cross sectional flow area by inserting an inner pipe in the existing tubing, thus creating an annulus. This results in a higher gas velocity. The multiphase flow characteristics in the annulus tubing are not yet fully understood, such as the pressure drop, the liquid hold-up and the different flow regimes. Therefore in the present study lab experiments were carried out to further investigate those characteristics to enable optimisation of predictive flow modeling. The main focus is on the behaviour of the liquid film along both pipe walls. In this study a 12 m mid-scale vertical annulus with a 124 mm diameter outer pipe and a 100 mm diameter inner pipe was designed and constructed. The experiments were conducted at atmospheric conditions using an air-water mixture as the working fluids. Multiple combinations of gas and liquid throughputs were studied. The liquid at the inlet could either be injected on the inner pipe, the outer pipe or equally divided on both pipes. The position of the inner pipe with respect to the outer pipe was adjustable to create different eccentricities. To measure the local film heights flush mounted conductance sensors were designed, built and installed in the half circumference of both pipes. The efficacy of the sensors was thoroughly studied and the sensors were subjected to multiple test cases. The concentric annulus experiments showed that the pressure drop does not depend on the method of liquid injection. However, the liquid hold-up fraction at lower liquid throughputs was observed to be lower for the single wall liquid injection as compared to injection on both pipes. The redistribution of the liquid film for the single liquid injection depends on the liquid throughput. Larger liquid throughputs gave thicker liquid films containing a wavier gas-liquid interface from which droplets are atomized and subsequently migrated to the other pipe wall. Furthermore, it was found that the critical superficial gas velocity at which liquid loading occurs is 14 m/s, which is neither dependent on the method of liquid injection nor on the liquid throughput. The designed sensors were able to provide a good indication of the film heights. Single phase experiments, using air as the working fluid, showed that the pressure drop is reduced by 30$\%$ in the fully eccentric case in comparison to the concentric case. The eccentric annulus configuration with air-water flow showed that the liquid films along both pipe walls may merge. This so-called liquid bridging mainly occurs at higher eccentricities and causes flow reversal in the narrow gap. This results in a much higher liquid hold-up and pressure drop. Eccentricity induces redistribution of the liquid film from the inner pipe to the outer pipe at higher superficial gas velocities. It was found that eccentricity promotes an unequal film height along the circumference of each pipe: the film height decreases when going from the narrow gap towards the wide gap.

On all levels of rowing, a general rule is that you have to row together. Rowing together has no clear-cut definition. However, it is known that each rower has his or her own style, which can be registered in movement patterns and force curves. The big question is how to combine these individual styles such that the crew works together in the best way. The Dutch Rowing Federation showed interest in this topic, wanting to know how to adjust the rigging dimensions of the boat to allow the best racing performance. The goal of this thesis is to provide advice on what features of the rowing stroke should be synchronized and whether and how this could be promoted by individualized rigging.

The theoretical foundation for this study was a literature study about the current knowledge on the rowing stroke and differences within and between individuals and crews. Current used measures on performance and synchronization of rowers were described, and finally a proposal was done for which methods to use in the ongoing of the study.

Data was obtained from five female athletes of elite level, doing trials in a quadruple sculls of approximately 30 seconds at 30 SPM and 32 SPM in four different combinations. The strokes were identified and analyzed, based on performance and synchronization measures. Performance measured as Average Speed, Work per Stroke, Blade Losses, Velocity Fluctuation Losses and their respective and combined efficiencies. Synchronization measures were defined as Mean Standard Deviation of the Phase, Standard Deviation of the Time to Half Impulse and Standard Deviation of the Time to Half Work.

The chosen synchronization measures were not completely independent. Standard deviations of time to half impulse and half work were found to be highly similar (r = 0.970). An opposite effect was found between kinematic synchronization and the other two, Mean Standard deviation of the Phase was not in line with the empirical rule that better synchronization leads to better performance. The kinetic and energetic measures did show this effect: Lower standard deviations of time to half impulse and time to half work meant higher average speed (r = −0.193) and higher Work per Stroke (r = −0.574).

The best performing synchronization measure was time to half impulse synchrony. A drawback on this measure was that the sampling period was long, compared to the interpolated time differences. Athletes were found to achieve their half impulse moments in a consistent order.

To find out whether it is possible to promote synchronization and thus performance by individualizing rigging, the oar angles at the time to half impulse were analyzed. This new measure correlated moderately (r = 0.624), meaning it quantifies more or less the same effect. The kinetic similarity actually worked better (r = −0.292 with Average Speed and r = −0.748 with Work per Stroke) than the synchronization measure.

Similarity of half impulse angles enables the coach to adjust the rigging such that the timing should improve too. However, this should be tested in a follow-up study.

A liquid film falling vertically along a wall results in the formation of waves at the liquid-gas interface. The principal forces in these flows are gravity, viscosity, and surface tension, which are characterized by the Reynolds number, Re and the Kapitza number, Ka. Experimental characterization of the film flows for high viscosity liquids (Ka in the order of 1) at low Re » in the order of 10 was performed for flow along the inner circumference of a vertical pipe. The three-dimensional liquid-gas interfacial profile is quantitatively reconstructed using the laser induced fluorescence technique. The waves observed had higher surface steepness at the front when compared with the back of the wave crest. The film thickness at the back of the wave crest is higher than at the front, resembling the streak-like waves observed in the literature for low-viscosity liquids (Ka in the order of 1000) at comparable Re. When the lateral surfaces of the waves are in contact, merging in the transverse direction is observed. The experimentally determined mean film thickness was approximately 15 % lower than the Nusselt’s flat film thickness, while the experimental values of average wave velocity was approximately 4 times higher than the Nusselt’s velocity for the corresponding film thickness. On the time-averaged film thickness field, a transverse variation in the film thickness, called as ridges and valleys, were observed. These transverse variations are found to be unsteady in their behaviour. Probability density distributions of the film thickness measurements showed two distinctive time-dependant patterns for the distribution of film thickness values in the ridges.

The climate change agreements require large investments in renewable energies. However, with the increasing demand for energy, immediately stopping the production and use of fossil fuels is not realistic. Natural gas is twice as clean as coal and hence could be considered as a transition fuel, which can help to find a balance between the requirements of fighting climate change and producing more and cleaner energy.The high-pressure, underground reservoirs supply natural gas which is usually wetted with liquid hydrocarbons and water. Natural gas is lifted through a production tubing to the surface, with a relatively high velocity and the drag between the gas and liquid will also bring the oil (or condensate) and water to the surface. However, over time the gas supply from the reservoirs will deplete, which gives a decreasing gas velocity, until liquid starts to fall. This is known as liquid loading, which can finally lead to the killing of the gas production. One of the solutions to operate such aging reservoirs is to reduce the cross-sectional area of the production tubing through making use of an annulus (which actually is the space in between an outer pipe and an inner pipe).Liquid loading is noticed to occur when the flow regime changes from annular flow to churn flow. A good understanding of this multiphase flow behaviour in an annulus is desired. Therefore, this Master Thesis project was devoted to obtaining new experimental data and to develop a model for upward gas-liquid flow in a vertical annulus. Experiments with air and water were carried out in the small-scale flow loop of TNO in Delft. The air and water throughputs were varied, and both concentric and eccentric pipe-in-pipe configurations were measured. Both the pressure drop and liquid holdup were measured. The model is able to predict the pressure gradient and holdup for a range of superficial gas and liquid velocities, using different eccentricities in the annulus configuration. The model predicted the pressure gradient within 10% of the experimental values at high superficial liquid velocities (>0.5 cm/s). However, for a superficial liquid velocity of 0.5 cm/s, the model was overpredicting the pressure drop by 100%, which could be attributed to the partial dry-out on the tube walls. To determine the superficial gas velocity, for a given pressure drop and holdup, a modified version of the Wallis correlation was used for the interfacial friction factor. The gas velocity was predicted within 5 to 10% at low holdups (< 0.075). For higher holdups the gas velocity was overpredicted by about 40%. The primary modifications made to the model for the eccentric cases are based on applying a number of grid cells in circular direction allowing for the inclusion of a closure for the variation of the circular film thickness both at the inner and at the outer pipe wall. A verification of the closure of the amplitude or of the closure of the film thickness variation is required to confirm the dependency between this parameter, the eccentricity and the holdup. However, with the applied closure of the amplitude the pressure gradient with respect to the holdup was predicted within 6% accuracy for the eccentric cases.

The efficient and clean combustion of liquid fuels is a fundamental requirement in the design of future energy systems. Simulation plays a more and more important role in the design of such burners. In this work the spray combustion simulation approach introduced by Ma (2016) is improved, and validated against the CORIA Jet Spray Flame database (Verdier et al., 2017). The database presents droplet temperatures measured by global rainbow refractometry technique, which gives a unique insight in the flame structure. The two phase flow is treated with an Eulerian-Lagrangian approach. Flamelet Generated Manifold (FGM) is used to model the gas phase combustion. The RANS equations are solved using final volume method, with standard k − ε turbulence modelling. The turbulence-chemistry interaction is addressed with assumed probability density function method. The spray cloud is modelled with the Lagrangian transport of droplets including heat and mass transfer. Ma (2016) developed a solver based on the OpenFOAM 2.3.x libraries. His development is complemented in this work with a novel spray model. The improved spray modelling allows the treatment of droplet evaporation in the context of FGM without limiting the complexity of the chemical mechanism. This improvement is crucial for the modelling of complex fuels and the correct prediction of emissions. The modelling concept is rather light-weight considering the RANS approach. Despite the low computational expenses, most of the results agree fairly well with the measurement data. However the correct prediction of droplet temperature remains an an unresolved problem.

Ma, L. (2016). Computational modelling of turbulent spray combustion. PhD Thesis, Delft University of Technology.

Verdier, A., Santiago, J. M., Vandel, A., Saengkaew, S., Cabot, G., Grehan, G., and Renou, B. (2017). Experimental study of local flame structures and fuel droplet properties of a spray jet flame. Proceedings of the Combustion Institute , 36(2):2595-2602.

Prediction of emissions from combustion systems is a complex problem involving the coupling between the flow field and chemistry. CFD analysis is the most commonly employed approach. However, it has the drawback that a very high computational cost prevents the use of detailed chemistry models. This master thesis, which focuses on NOx emissions, uses a more unconventional method of emissions prediction: Chemical Reactor Network (CRN). The advantage of this method is that, as it does not use fine discretisation, closure models nor fluid dynamics equations, it allows the implementation of detailed chemistry mechanisms.

A CRN is developed first for a single-element Lean Direct Injection (LDI) combustor and then the CRN is adapted for a Multi-Point LDI (MPLDI) combustor. CFD and experimental results are used to set up the CRN. In the base case scenario, in which kerosene is the fuel choice, the NOx emissions predicted are very close to experimental measurements. This is particularly meaningful given the high uncertainties of modelling this highly complex turbulent combustion process.

The developed CRN is also used to predict NOx emissions for the same combustor when the fuel choice is varied. Namely, the alternative fuels considered are kerosene enriched with hydrogen, methane and methane enriched with hydrogen. In comparison to kerosene, higher Lower Heating Values (LHVs) of these fuels lead to lower combustor temperatures for the same power input. Consequently, the thermal NOx pathway is weakened and the NOx mass flow generated is reduced. Nevertheless, these fuels with a higher LHV come with an increased operational risk that must be overcome before their implementation in aviation becomes possible.