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.

In the further advancement of the energy transition towards renewable energy sources, a significant role can be played by energy storage systems. A relatively new energy storage technology is the reversible solid oxide cell. A reversible solid oxide cell is an electrochemical device that can operate as an electrolyser to produce a fuel from electricity, and as a fuel cell to produce electricity from fuel. At Delft University of Technology, a hydrogen and oxygen fuelled solid oxide fuel cell and gas turbine integrated system has been designed. In order to redesign this solid oxide fuel cell and gas turbine integrated system into a reversible solid oxide cell system, the design of a solid oxide electrolyser system is required. One of the main challenges when designing a solid oxide electrolyser system is the heat requirement of the system. However, little research on the design of solid oxide electrolyser systems has yet focussed on thermal optimization of the system. In this thesis, three solid oxide electrolyser systems, displaying different heat integration layouts, were designed and examined. The results show that the highest energy and exergy efficiencies, respectively 75.52% and 75.42%, are obtained by the solid oxide electrolyser system that evaporates the water in the system using the heat that is generated by the hydrogen and oxygen storage compression. This solid oxide electrolyser system is combined with the solid oxide fuel cell and gas turbine integrated system to design a reversible solid oxide cell system. A round-trip energy efficiency, based on the lower heating value of hydrogen, of 48.40% is reached by this reversible solid oxide cell system.

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

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

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

This report analyses the improvement potential of zeotropic refrigerant mixtures on the performance of vapour compression heat pumps integrated into industrial dryers. The report reviews the scope of total global energy consumption and the percentage of this attributable to industry to demarcate application area.

It proposes that by targeting industrial process heating, significant reductions to energy consumption and carbon emission by industry can be achieved. Heat pumps are introduced as the preferred system to accomplish this for temperatures up to 200 ◦C. A performance limitation is identified in the form of heat transfer between process streams that reject and receive heat at non-constant temperatures.

Zeotropic refrigerant mixtures are introduced in heat pumps to improve the interaction with non-isothermal heat sources and-sinks. The considered mixture compounds are limited to future-proof refrigerants and the mixtures are limited to binary ones. Humid air encountered in industrial drying is targeted as one such non-isothermal process stream with a large potential for waste heat recovery. The most prominent industries that use dryers are identified and evaluated as to provide representative operating conditions at which a heat pump integrated dryer would have to work. Heat pump cycles are described both on a cycle scale as well as per component and the most important design aspects and considerations are presented. The methods used to allow heat pumps to interact favourably with temperature glides are presented namely zeotropic mixtures and trans-critical heat pump cycles.

A modeling strategy is proposed to simulate heat pump integrated dryers in large numbers in order to investigate the effects of dryer inlet and outlet conditions as well as refrigerant mixture composition both in terms of constituents and mixing ratio. The total process flow diagram is given and divided into definable thermodynamic states for both the (humid) air as well as the refrigerant. Governing equations for each component are presented and the calculation method for the thermodynamic states is discussed in terms of known state variables and used thermodynamic libraries. Finally a novel heat pump cycle is proposed, motivated by a desire to decouple the glide matching in the evaporator from that in the condenser, that attempts to use the fractionation risk present in zeotropic mixtures as an advantage instead. The criteria placed on refrigerant are presented both within the context of the future-proof limitation, legislative limitation as well as those introduced by the process requirements.
The most promising refrigerant candidates from literature are presented for applications in the defined process conditions. Finally a selection is made of 13 refrigerants namely water, ammonia, (iso)butane,
(iso)pentane, methane, ethane, propane, CO2, propylene, ethylene and hexane. The thirteen refrigerants are combined into 78 refrigerant pairs and modelled using the described modeling strategy. The modelling reveals that using zeotropic mixtures does improve the COP for many refrigerants when compared to their pure cycle performance. For drying at 180 ◦C two heat pumps are proposed at different outlet temperatures. The cycles use 87.5%mol Isobutane mixed with Ethane and 87.5%mol NH3 mixed with Propane for a high temperature and lower temperature outlet respectively. COPs of 3.38 and 3.44 were calculated with PRs of 16.53 and 6.84 for the Isobutane-based and NH3-based cycles respectively. A lower drying temperature of 120 ◦C was explored and here CO2-based mixtures were identified as highly desirable refrigerants due to their non-toxic, non-flammable nature as well as low global warming and ozone depletion potential. The mixtures 87.5%mol CO2-Isopentane and 90%mol CO2-Isobutane were proposed, both feasible with single stage compression and possessing a COP of 3.96 and 4.02. Zeotropic mixtures improved the COP in at least 48 out of 78 possible refrigerant combinations when compared to the pure cycle COPs and allowed the dampening of flammability and toxicity where the pure refrigerant possesses those properties.

It is clearly demonstrated that future-proof binary zeotropic mixtures increase the performance of VCHP-integrated dryers across all relevant temperature ranges, by as much as 21.47%. The highest COPs are found when zeotropic refrigerant mixtures are used together with trans-critical operation. It is concluded that zeotropic mixtures improve the COP of vapour compression heat pump integrated dryers and should be utilised for non-isothermal processes like drying.

Today's electricity supply falls short of current demands, leading to the utilization of gas turbines in both ground based and avionic infrastructures. Nevertheless, these often rely on carbon-based fuels, resulting in escalating CO2 emissions. However, adopting hydrogen as a fuel eliminates carbon emissions. Aside from zero carbon emissions, hydrogen has a higher energy density by weight compared to conventional fuels. This makes it an distinct option for applications requiring efficient energy storage and delivery. Due to its wide flammability range and low ignition energy, hydrogen can combust in scenarios where traditional fuels might not. This unique characteristic, while advantageous in certain contexts, requires detailed study to ensure safe and efficient combustion in gas turbines. However, the combustion of hydrogen inherently results in elevated flame temperatures, thereby generating increased NOx levels. Furthermore, hydrogen's high mass diffusivity translates to a reduced Lewis number. Consequently, it becomes vital to grasp the local dynamic characteristics of the flames, particularly at stable and flashback points. Understanding the thermo-physical behavior of hydrogen flames, especially at stable and flashback points, therefore, requires experimental studies to reflect real life complexities. These tools can offer insights into turbulence-flame interactions, flame stabilization, and emission formation mechanisms.


In this research, an exploration was undertaken to understand the local kinematics and dynamics of Hydrogen and DNG flames, with emphasis on stable states and those approaching flashback conditions. Particle Image Velocimetry (PIV) experiments were employed on a Bunsen burner setup, facilitating the capture of the jet flames' velocity fields. Both low-speed and high-speed recordings were captured by high-speed camera, providing distinct insights into flame dynamics. Flame front detection was achieved using Mie-scattering, capitalizing on the differential seeding particle densities between the unburnt and burnt regions. Intensity differences between these regions were meticulously captured with a bilateral filter, leading to the successful extraction of the flame front. This extracted front was subsequently distinguished via segmentation and superimposed onto the velocity field. Low-speed recordings offered a generalized perspective on flame turbulence characteristics through cold flow validation, while high-speed recordings unveiled specific dynamics, inclusive of flame curvature, local flame and displacement speeds, and both normal and tangential velocities and stretches. It consistently holds the 1-D unstretched flame speed, even as the Reynolds number increases, aligning with the respective flashback points and stable conditions of the flames. As a result, in-depth comparison of DNG and $H_2$ fuels in terms of flame dynamics and kinematics were discerned.

Variable green energy sources were sought to supply the growing demand for energy without impacting the environment. Wind and solar energy are at the forefront of the energy transition. However, their intermittent nature poses a challenge that necessitates the development of energy storage technologies.
Several technologies were studied over the years including pumped hydro energy storage, compressed air energy storage, electrochemical energy storage, and pumped thermal energy storage.
Pumped thermal energy storage provides a mean to store excess electrical energy in the form of heat by employing a heat pump cycle during the charging process and a heat engine cycle during the discharging process. There are multiple variations of pumped thermal energy storage cycles of which Rankine and transcritical cycles. Unlike Brayton cycles, Rankine and transcritical cycles typically operate at low-temperature levels < 200o𝐶 which facilitate their
integration with low-temperature waste heat in order to boost their round trip efficiencies even beyond 100%. Therefore, the focus of this thesis was to model and optimize Rankine and transcritical thermally integrated small scale pumped thermal energy storage.
Critical to the storage system’s efficacy are the performances of its key components: the compressor and expander. Geometric models were developed and validated for these components, facilitating the determination of their isentropic efficiencies and their variation with pressure ratio and rotation speed.
Additionally, a small-scale pumped thermal energy storage system model was developed to study the system performance. The choice of working fluid and operating parameters were guided by both a simplified optimization scheme and a multi-objective approach utilizing the Non-Dominated Sorting Genetic Algorithm-II (NSGA-II). This yielded 𝑅13𝐼1 as the optimum working fluid with source temperature 𝑇𝑠𝑜𝑢𝑟𝑐𝑒 = 80o𝐶 and storage temperature 𝑇𝑠𝑡𝑜𝑟𝑎𝑔𝑒 =140o𝐶. This configuration led to a transcritical heat pump charging cycle and a subcritical heat engine discharging cycle employing pressurized water as the storage medium.
Consequently, the compressor and expander geometric models were integrated into the pumped thermal energy storage system model working with 𝑅13𝐼1 at the source and storage
temperature mentioned earlier which yielded a round trip efficiency of 𝜂𝑟𝑡 = 93%, energy density 𝜌𝑒𝑛 = 12𝑘𝑊ℎ𝑟/𝑚3, and exergy efficiency 𝜓𝑒𝑥 = 29%.
Finally, a case study was demonstrated with the integrated cycle employed in a Dutch household solar system where dual electric and thermal panels are employed for the electrical and heat energy input in the pumped thermal energy storage. The system was developed for a 1𝑘𝑊 electrical input from the solar system to be stored for further use during off peak hours.

The rise in temperature attributed to human CO2 emissions and the escalating energy needs of society necessitates the development of clean energy production. Solar and wind energy, both renewable sources, have emerged as cost-effective alternatives to conventional fossil fuel systems. They now account for a substantial portion of the world's electricity generation (IEA, 2022). However, their intermittent nature poses a challenge to their reliability. To overcome this, the implementation of grid-scale energy storage systems is crucial. Such systems can store excess energy produced during peak periods and release it during low-generation or high-demand periods, ensuring a stable and dependable power supply to the grid.

Pumped Thermal Energy Storage (PTES) is one such type of promising grid-scale storage solution based on the concept of storing electricity in the form of heat. These systems are not reliant on rare earth metals, are not restricted by geographical location, and are relatively economical over their lifetime. They employ a heat pump cycle for charging and a heat engine cycle during times of discharge. Often, in the thermodynamic modelling of PTES systems, a fixed value of turbomachinery efficiency is assumed. This approach holds well for the first estimate of performance, but for better accuracy and further analysis, meanline models could be used to arrive at the efficiency value and preliminary geometric design. Hence, this work presents a method for developing meanline models for centrifugal compressors and radial inflow turbines. Modelling techniques and guidelines from the literature are noted and presented here. The accuracy of these models is dependent mainly on the loss models used. Using suitable models selected from the literature, a fair agreement was found between the meanline model's prediction and experimental data from open literature, validating the methodology.

An essential function of energy storage is to provide load flexibility, meaning its charging and discharging cycles must adjust to match the net load curve. As a result, the turbomachinery would need to operate under off-design conditions to meet these demands. Therefore, this report introduces an approach to extend the PTES model by Radi (2023) for off-design operation based on turbomachine performance.

With variable renewable energy on the rise, large-scale energy storage is needed to assure grid stability. Thermal energy storage (TES) is a new player in the large-scale energy storage landscape, but might prove a cheap, safe and reliable option, with low permitting barriers. TES is not geographically limited, generally compact and has a low levelised cost of storage (LCOS) which comes at a cost of a lower round-trip efficiency compared to other grid-scale energy storage methods. Thermal energy storage is most suited for intraday and multiday storage.
Within thermal energy storage, several types can be distinguished based on the design of the thermodynamic cycle. In this thesis, these types are analysed and compared on basis of technology, cost, and miscellaneous characteristics. An overview of cost and characteristics of current relevant thermal energy storage (TES) storage materials and TES companies is given. A Rankine-type TES, as is used in the E2S project, is generally cheaper and easier to implement, at a cost of a reduced round-trip efficiency. Rankine-type TES have a round-trip efficiency of about 40% as it is limited by the Rankine-cycle efficiency.
The TES studied in the scope of this thesis relies on evaporation of water and a steam turbine to recover power. The work of the thesis comprises of a thermodynamic and hydraulic optimization study regarding the heat transfer during conduction, convection and boiling inside the TES. To this end, a steady boiling model is constructed based on empirical and phenomenological correlations. It is found that during steady state, boiling phenomena incur higher heat transfer coefficients for low to medium wall superheat in comparison to a model that does not regard boiling phenomena. Conversely, lower heat transfer coefficients are attained for high to very high wall superheat in comparison to a model that does not regard boiling phenomena. It is hypothesised that this phenomenon will incur higher overall boiling heat transfer during transient operation.
An optimization study is conducted with the aforementioned boiling model to find the optimum combination of TES parameters for a given power and capacity, with the optimization parameter being the combined material cost of steam generator tubing and storage material. Several design guidelines are concluded. Most notably is found that pressure drop plays a very minor role in TES optimization and therefore a lower pressure drop may be chosen. Furthermore, critical parameters and bottlenecks of the TES are identified in a sensitivity analysis.
Lastly, integration of the TES in a steam cycle is evaluated. The effect of variations in pressure and temperature output of the TES are analysed, and the total round-trip efficiency and its performance during part-load operation are determined. Various qualitative changes are discussed, and a quantitative estimate is made of the sizing of a start-up TES, which may be used for cold, warm and hot start of the steam turbine. As the TES is to be implemented in a thermal power plant, an estimate is made of the sizing and cost of such an implementation using the data from the optimisation study. Various aspects of this plant are evaluated such as system response, start-up time and a techno-economical analysis of multi-day operation.

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.

The energy transition from fossil fuels to renewable energy sources is crucial for climate resilience. In particular, hydrogen combustion in gas turbine combustors is expected to play an essential role in the energy transition. Hence, a fuel-flexible gas turbine combustor operating with a wide range of natural gas and hydrogen fuels is necessary. Low Swirl Burner(LSB), developed at Lawrence Berkeley National Laboratory (LBNL), is one of the promising solutions for fuel-flexible gas turbine combustors. However, hydrogen combustion in LSB at lean conditions is accompanied by several challenges, especially flame flashback. Flashback results in damage and shutdown of gas turbines. Recent experimental studies performed on LSB revealed the occurrence of flame flashback at a high volume percentage of hydrogen, but the exact cause of the flashback in LSB is yet to be discovered.

In this study, numerical modelling of low swirl premixed burner with turbulent flow and high hydrogen content is performed. RANS simulations are performed initially for the non-reacting flow and are validated with the experimental measurements. Flamelet combustion model with Zimont Turbulent Flame speed correlation is employed to model the reacting flow. The flame front position obtained from the Zimont correlation closely agrees with the experiments.
Further, Algebraic Flame Surface Wrinkling (AFSW) correlation is compared with the Zimont correlation for hydrogen-enriched flames. Based on the comparison, it is concluded that the AFSW correlation did not lead to a significant difference. However, both the correlations failed to model the flashback process. Therefore, the potential mechanism responsible for the flame flashback in LSB is examined. It has been found that none of the standard flashback mechanisms is accountable for the flame flashback in LSB. Furthermore, the effect of different flame shapes reported in the experiments is evaluated. It has been observed that ’M’ shape flame has more propensity to flashback than ’V’ shape flame. Then, a shear layer and turbulent wakes in the premix nozzle of the LSB were noticed. Hence, an increase in flame speed due to the local enrichment of thermo-diffusive unstable hydrogen flame and all the aforementioned effects might lead to a sudden flashback in LSB.

The growing energy demand and climate change poses a need for alternative energy generation in terms of renewable resources. Renewable energy resources are characterised by their intermittent behaviour. A backup power supply is required that can deliver electricity when the supply from the renewables is not sufficient. Gas turbines operating with hydrogen is an attractive option, since hydrogen combustion has zero carbon emissions and can be used as an energy storage when the supply of renewable energy sources are abundant. However, hydrogen combustion poses several challenges. A hydrogen flame has a higher flame temperature than natural gas, leading to more NOe production. To
reduce these NOe emissions, gas turbines operate in lean premixed conditions. This creates a risk of flame flashback, in particular boundary layer flashback, which can lead to severe damage to the gas turbine.
Recent research performed on boundary layer flashback revealed that two flame configurations, i.e. unconfined and confined, showed fundamentally different flashback phenomena. Unconfined flame flashback refers to the situation where an initially stable flame anchored at the burner rim eventually moves into burner tube. When a flame is partially of completely surrounded by walls and propagates and then starts propagation along the wall, it is called confined flame flashback. Previous studies have focused on one of the two flashback processes at a time. However, the transient flashback process between unconfined and confined flame flashback is not well understood. Research has shown that hydrogen is much more prone to flashback compared to natural gas. This has been attributed to the difference in flame speed between natural gas and hydrogen, but the exact reason for the difference in flashback behaviour between natural gas flames and hydrogen flames has yet to be found.
In this study, a quartz Bunsen burner is used to investigate the flashback phenomena of turbulent premixed hydrogen-natural gas-air flames. To gain more insight in the flashback phenomena, three experiments have been performed. First, flashback maps are obtained to determine the flashback limits of the quartz Bunsen burner. Secondly, the influence of a flame on the flow was investigated using turbulent statistics. Finally, both unconfined and confined flashback are visualised, thereby capturing the transient flashback process between these two configurations. This has been done for a stoichiometric natural gas flame and a lean hydrogen flame. Laser diagnostics like Particle Image Velocimetry (PIV) and Miescattering are used to obtain the turbulent flow statistics and to visualize the instantaneous flashback process. The results show that regions with negative velocity fluctuations in the unburned mixture are the predominant physical mechanism for the start of unconfined flashback and for the transient flashback process of the flame propagating into the burner. However, the start of a flashback event depends on the combination of several parameters: the bulk velocity, the position of the flame front before it interacts with a region with negative velocity fluctuations, the magnitude of the negative velocity fluctuations and whether a region with positive velocity fluctuations is absent after interaction of the flame with the region consisting of negative fluctuations. So, unconfined flashback is rather a statistical phenomena, where the chance of the occurrence of a flashback event is increasing for a decreasing bulk flow velocity. Experiments showed that the transient process between unconfined and confined flashback is very short and fast. After a distance of approximately 5 mm upstream of the burner rim, a backflow region starts to develop in front of the flame, which denotes the start of confined flashback. The time needed for a natural gas flame propagating upstream from the burner rim to reach a confined configuration is approximately 17 ms and only 5.6 ms for the hydrogen flame. The suggested physical mechanisms leading from unconfined flashback to confined flashback are the convex shape of the flame towards the reactants during upstream flame propagation and the reduced cross-sectional flow area of the burned gases at the flame tip. Due to the created backflow in front of the flame and the abovementioned mechanisms, the upstream flame propagation is strongly enhanced, which explains why the flashback propensity for confined flames is much higher than for unconfined flames. The experiments showed that the hydrogen flame propagates closer to the wall than the natural gas flame, indicating higher backpressure effects. The hydrogen flame is thermaldiffusive unstable and the convex shape of the flame tip during flashback strongly enhances the local flame speed and thus the upstream propagation velocity. In contrast, the hydrodynamic instability encountered in the natural gas flame only retards the flow in front of the flame tip, but does not affect the flame speed. This might explain the difference in flashback behaviour between natural gas and hydrogen flames.

At present, there is a worldwide awareness and concern about how global warming could impact daily lives and the need to switch to renewable sources of energy. Commercially, various green energy technologies such as hydro, wind, solar, nuclear fission, etc. are being used to harness their stored energy and convert it into electricity. However, amongst these, the natural sources of energy are limited in the operation by their intermittent nature. This potentially leads to the instability of the power grid system. Hence, it is difficult to replace the existing conventional fossil-fired energy generation systems with their renewable counterparts, unless and until, flexibility is introduced to damp the intermittency
of renewable sources of energy.

A Pumped Thermal Energy Storage (PTES) system, which uses a combination of a heat pump and heat engine to store electricity in form of heat and convert the heat back to electricity, could efficiently provide intraday to multiday flexibility. However, the literature reveals that this technology is not mature enough and requires enhanced research and experimental work.

The scope of this thesis includes a conceptual design, steady-state modelling, and optimization of a waste heat integrated PTES, alternatively called Compressed Heat Energy Storage (CHEST) system. Two configurations of the CHEST system were analysed: (i) CHEST with latent heat storage, and (ii) CHEST with sensible heat storage. A combination of R1233zD as a working fluid and sunflower oil
as a sensible thermal energy storage medium was selected and a Phase Change Material (PCM) as a latent heat storage medium. Towards this end, a sensitivity analysis is performed to examine the effect of various design parameters on the performance of the system. It is found that CHEST systems are not suitable for high-temperature storage applications (> 200°𝐶). Further, the effect of superheating in a heat pump, and the effect of waste heat source temperature on system performance is investigated.

An optimization study is conducted on CHEST with a sensible storage configuration to use a single heat exchanger for the charging and discharging cycle during sensible heat transfer with a storage medium. A
detailed comparison between the two configurations have been performed. Most notably a trade-off has to be set between the performance and economic viability of the system.

Finally, the integration of the optimised sensible heat storage configuration with a solar-powered alkaline electrolyser is evaluated. A 30 MW of heat from the electrolyser unit is considered to be recovered with the help of the CHEST system. Towards the end, a preliminary design of the CHEST components (heat exchangers and turbomachinery) is provided to estimate the overall sizing and performance characteristics of the components.

The issue of climate change has fostered the innovation of new technology in an attempt to curb the rising emissions, with the energy sector being one of the biggest contributors of carbon emissions. Renewable Energy has shown promise in reducing the emissions by up to 32 %. However the problem of intermittency of
renewable energy sources is still a big reason why they aren’t more mainstream. Therefore, hydrogen has been seen as an energy carrier to provide integration amongst the various sectors of energy generation, transportation, household heating and industrial use. The use of hydrogen as a fuel for re-electrification and power production can be done via IC engines, gas turbines and fuel cells.
This thesis is interested in the possible applications of re-electrification of hydrogen using a system consisting of an SOFC and a gas turbine. It aims to design a small lab scale, test-setup of such a power cycle. The setup
would consist of a small gas turbine and a solid oxide fuel cell.
To that end, this thesis introduces the importance of hydrogen as an energy storage medium followed by the advantages of the SOFC-GT systems in providing higher thermal efficiencies. This is followed by the introduction of the H2-O2 power cycles, namely the Graz and the Toshiba cycles. From previous research done at TU Delft, a basic schematic of the system has been shown, consisting of an SOFC, a combustor, and gas turbine, heat exchangers and an ejector for re-circulative cooling of the SOFC. This is the setup that has to be
designed and so the theory behind the components is presented.
Firstly, the theory behind solid oxide fuel cells, and their modelling is presented. The cooling of the SOFC has been looked into in greater detail. This is followed by the theory of combustion and turbulence modelling.
Two combustor types, a swirl based and a micromix have been considered. Out of these the micromix has been chosen because of its superior mixing capabilities. Finally, the theory behind ejectors, heat exchangers and
sizing of turbomachinery is presented. This concludes the theoretical background of the study. A 0-D model of the SOFC is made based on the Sunfire/Staxera Mk-200 ISM with the 3YSZ KeraCell 11 by Kerafol. This model is then used as a basis of design for the entire thermodynamic H2-O2 cycle. The designed thermodynamic cycle has an overall efficiency of 67.28 %. LHV of hydrogen.
The micromix combustor design is introduced next. A conceptual micromix geometry is presented following which, 2 different configurations are scaled and designed using the theory of jets in cross flow. The two configurations are compared after performing a CFD analysis using the k−ε model with equilibrium chemistry for combustion. The comparison is based on mixing performance, pressure drop and temperature contours. Following the design of the combustor, the supplementary components such as the heat exchangers and the
ejector are designed. The turbine is selected as a small radial turbocharger with a pressure ratio of 8 and is sized using the Balje diagrams. material recommendations are given for the combustor and high- temperature
heat exchangers. The final H2-O2 thermodynamic cycle is presented with the updated component designs. The final thermal efficiency is found to be 64.09 % LHV.
Finally, a basic control scheme is developed for the system and the start-up procedure is briefly discussed.

The detrimental influence of nitric oxides (NOx) on humans and the environment has been widely discussed by researchers.
The dominant part of nitic oxides emissions comes from combustion - majority of NOx is being produced by reaction of nitrogen and oxygen at high temperatures.

This thesis project is carried out in cooperation with Siemens Energy, one of the gas turbines manufacturers.
The gas turbine combustor design requires extensive work in many fields: in addition to temperature distribution and flow field prediction, modelling of acoustics and emissions is necessary. As NOx are influenced by many variables, detailed sensitivity analyses of design features is required. Since that gas turbine producers are developing their own tools for emissions predictions.

The aim of this study is improvement and assessment of the existing tool used by Siemens Energy to predict NOx emissions by simulating the combustor system (both the flow and chemical kinetics).
As NOx emissions depend on the air-fuel mixing quality, the tool is equipped with a Monte Carlo-based turbulent mixing model. The combustor flow is divided into small parts - particles, that react and exchange properties between each other according to the Curl's turbulent mixing model.

The tool was improved by adding useful features (such as prescribed unmixedness in the flame front). After that, the Siemens Energy axially staged combustor was modelled (with different equivalence ratios, pressure levels and pilot fuel flows). In the end the tool was validated - the simulation results were compared to the experimental ones.

Industry is a major contributor to the rise in global CO2 emissions, constituting one fifth of the global energy consumption, of which significant amount is provided by fossil fuel combustion. Following the Paris agreement, emphasis has been made on decarbonization of the industrial sector. This thesis focuses on industrial decarbonization by employing Carbon Capture and Storage for Combined Heat and Power (CHP) gas turbine plants. The scope of this thesis includes conceptual modelling and thermodynamic analysis of potential decarbonization options for zero carbon CHP plants. The studied options include post-combustion capture, exhaust gas recirculation, pre-combustion capture and oxyfuel combustion. As conventional air Brayton cycles are not applicable for oxy-fuel combustion in gas turbines, different working fluids and cycle configurations are proposed and thermodynamic performance is evaluated. Selected cycles were then compared based on thermodynamics, economics and off-design performance at a typical constant power to heat ratio of 0.78. It was observed that oxyfuel CHP cycle with CO2 working fluid is a promising solution for zero carbon CHP with 100% CO2 reduction. However, this solution requires new turbomachinery design. In view of this, a retrofit analysis is also performed in this thesis which evaluates if an existing air designed gas turbine can be used for CO2 operation. It was concluded from this analysis that it is possible to operate an air gas turbine on CO2 by incorporating proposed modifications of higher compressor inlet temperature (473K) and a turbine inlet nozzle area 20% larger than design. These modifications, however also lead to serious performance deterioration.

In the battle against climate change, humankind needs to develop alternative energy systems if it wants to survive on this planet. Fossil fuels, polluting the air with CO2, must make way for renewable sources so that climate change is put to a stop. Several new energy initiatives based on renewable sources are deployed worldwide, these however are mostly for electricity generation. There are elatively few zero emission alternatives developed for the energy demand from the transport sector, therefore, new small scale energy systems need to be developed in order to achieve the goals stated in the Paris agreement. The Hydrogen Roadmap” is a vital part of the European Union in achieving the Paris climate agreement, which uses hydrogen for future energy generation. Gas turbines, steam turbines and fuel cell applications are investigated thoroughly for small scale energy systems throughout the years. Fewer research is conducted about the integration of these three energy technologies and specially sparse research has been conducted in the area of pure hydrogen oxygen combustion for these energy systems, resulting in opportunities for development. This report introduces an alternative small scale power system suited for the newest inland shipping vessels. It proposes a basic design for a 3 MW SOFC integrated hydrogen oxygen fired combined power cycle, feasible within 10 to 20 years of research and development. First, the theoretical background for the research is elaborated covering the thermodynamics of traditional power cycles, exergy analysis theory and zero emission cycles such as hydrogen oxygen fuelled power cycles. The electrochemical research containing the fuel cell operating specifics and the advantages of integrating fuel cell stacks in power cycles is introduced next. The theoretical background ends with an elaboration on the preliminary design steps for power cycle components regarding turbomachinery and heat exchange equipment. After the relevant subjects regarding power cycles and fuel cells are elaborated, a basic cycle was designed. From literature, several starting points such as the maximum turbine inlet temperature and inlet temperatures of the SOFC were defined. With a few determining thermodynamic states known on beforehand, a pressure and temperature analysis was carried out to determine the limiting operating conditions and the remaining thermodynamic states in the cycle. The cycle was eventually designed as a SOFC integrated Brayton Rankine cycle coupled via a single pressure heat recovery steam generator, where the SOFC is situated upstream of the combustor in the Brayton part of the cycle. All components in the cycle are evaluated using an exergy analysis, from which the final exergetic efficiency was determined as 73.09%. From the exergy analysis of the proposed basic cycle became clear that there was potential for improvement regarding the HRSG design. The improved cycle is therefore designed with a dual pressure HRSG, while the rest of the combined cycle remains equal compared to the basic cycle. With the extra high pressure regime, a high pressure turbine is added, thus creating an extra point in the system from which useful power is extracted from the medium. The exergetic efficiency increase of the HRSG alone amounts to 0.74%, but due to extra compressor and pump work and losses, the improved cycle results in a total efficiency of 73.47%, implying an increase of 0.38% compared to the basic cycle. To give a first practical indication about the size and specifics of the proposed cycles, a preliminary design of the components is made. The turbomachinery is sized according to the appropriate non dimensional numbers, the specific speed and diameter. An indication about the SOFC stack size and its operating specifics are given, together with the preliminary designs of the sensible and latent heat exchangers. A total preliminary size estimation of the cycle predicted a system with the size of approximately 4.5 shipping containers.

Gas turbines for power generation use lean premixed flames to adhere to the strict pollutant emissions regulations. Unfortunately, this often leads to thermoacoustic instabilities. These instabilities can lead to failure of system components, flame blow-off or flashback and a reduction in efficiency. Especially the high frequency transverse thermoacoustic instabilities can lead to big problems in the successful operation of gas turbines. These instabilities can be reduced by adding damping devices or mitigating the excitation mechanisms. In this work, Large Eddy Simulation (LES) is used to identify the thermoacoustic instability which tends to occur in a specific heavy duty gas turbine combustor design. In order to successfully counteract this instability, identification of the acoustic mode affecting the burner is required. Results from the LES are compared with experiments to test the predictability of LES and research mitigation techniques. The oscillation amplitudes do not match due to numerical damping, but the frequency of the oscillation in LES is within 1% of the experimental value. Furthermore, a coherence between the heat release and pressure oscillations of 80% is achieved and the phase of the oscillations are within 90° of each other. This confirms that the Rayleigh criteria is satisfied and the instability in the simulation is indeed a self-excited thermoacoustic instability. After identifying the mode, Fast Fourier Transform (FFT) of the pressure data at the combustor walls is used to visualize and locate the mode. Its location suggests that fuel supply oscillations could be a possible driving mechanism. However, the frequency of the oscillations in the fuel supply did not match that of the thermoacoustic instability so this hypothesis was rejected. After identifying the mode, the effect of different simulation and operation parameters is researched. First a model coefficient which influences the flame length is changed. Shortening the flame results in a higher amplitude instability, due to the location of maximum heat release moving closer to the location of the source of the instability. Next, the fuel distribution is changed in order to break flame symmetry and reduce the instability amplitude. This is done successfully, as the amplitude of the instability at the peak frequency is reduced to the same level as the surrounding noise frequencies. Finally, the effect of adding Helmholtz resonators specifically designed for the instability of interest is researched. A transfer function is defined to measure the response of the resonators to a pressure fluctuation in front of them. The result is compared to an analytical solution to determine the effectiveness of the resonators. Like the fuel biasing, also the resonators performed well and resulted in a decrease of the instability amplitude.

Gas turbines (GT) play a crucial role in the transition to fully sustainable energy. Conversely, the expected growth of the GT market is met with more stringent regulatory requirements. Clearly, an increasing turbine inlet temperature (TIT) is beneficial for the efficiency of GT cycles. With increasing TIT, however, the magnitude of the thermal loads becomes unbearable, causing thermal fatigue of the component in a transient setting such as GT start-up or shut-down. Therefore, it is crucial that the thermal loads are well managed, which is typically achievable only by adequate external and internal cooling. Generally, such cooling systems are modeled in the industry using an uncoupled heat transfer (HT) approach, assuming that the heat coefficients (HTCs) are only influenced by the aerodynamics of the flow field: HTCs are obtained in a separate computational fluid dynamics (CFD) simulation and are extracted and mapped on the solid domain of the component. Firstly, the report presents a comparison between the uncoupled approach and the increasingly popular conjugate heat transfer (CHT) approach, in which the fluid and solid domains are solved in a single simulation. To do so, the commercial code (STAR-CCM+) is first validated for both laminar flat plate and turbulent offset jet in a steady-state setting. The latter is done by comparing three Reynolds-Averaged Navier-Stokes (RANS) models. The popular wall treatment methods: blended and high-y+ wall functions (WFs) are also compared. The comparison between the two HT methodologies is done by a parameter sensitivity study, involving the thermal conductivity of the solid (for both laminar and turbulent cases), Turbulent Prandtl number, and turbulence intensity. It was concluded that the realizable k-epsilon model is the most accurate for this flow. Conjugation played a marginal role for all parameters. with only a considerable difference in the stagnation regions, when the solid conductivity is low. The results using a high-y+ WF displayed high numerical dissipation due to the coarse mesh size and hence the difference between the HT methodologies was artificially low. It is therefore advised that the study of the high-y+ WF is repeated on a different flow arrangement/setting (e.g. increased inlet velocity). Secondly, the thesis details a study around the internal cooling of Siemens GT transition piece (TP), using CHT. An introduction of perpendicular ribs into a fully developed turbulent channel flow can theoretically significantly increase the heat transfer coefficient through the introduction of large-scale vortices downstream. An optimization routine was run with a hybrid algorithm to study the effect of different rib configurations in a steady RANS simulation, aiming to minimize both mass flow and transition bond coat temperature. The performance is compared to the baseline smooth channels used in the Siemens TP. No better results were achieved for both objective functions in comparison to the baseline case mostly due to the length of the channels. However, a considerable decrease in the coolant consumption was present with a limited increase in the BC temperature. The study showed that the optimization of such ribs can minimize the effect of skin friction (which influences the amount of coolant), and presents a possible further improvement of the TP cooling, without relying solely on the experience of the designer.

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.

Due to the growing energy demand and global warming renewable energy is currently of interest. To deal with the production intermittency of renewable energy, energy might be stored in hydrogen. Hydrogen can be converted into electricity with a fuel cell or by combusting hydrogen in a gas turbine. Gas turbines are more suitable for large power outputs. Large power outputs require large volumetric flow rate through the gas turbine. The large volumetric flow rate causes high gas velocities and results in a turbulent flow. Using hydrogen instead of natural gas as energy resource changes the stable operating regime and increases flame flashback propensity. Current studies done at the Technical University of Munich and the Technical University of Delft suggest flame confinement, burner wall temperature and velocity fluctuations are vital to the flashback process. Des-pite the recent studies flashback is yet not completely understood. \\ This research focuses on premixed turbulent hydrogen and hydrocarbon combustion to further investigate the possibilities of using hydrogen in a gas turbine. The aim of this research is to gain more insight in the flashback process by varying velocity fluctuations while keeping the bulk velocity constant. The first objective is to gain insight in the relation between bulk velocity at flashback and velocity fluctuations. The second objective is to gain insight in the relation between bulk velocity at flashback and low velocity streaks upstream the flame front. The third research objective is to gain insight in the relation between flame front, boundary layer obstruction and bulk velocity at flashback. \\ All experiments were conducted with a Bunsen burner. A screw, acting as boundary layer obstruction, was placed below the burner rim to locally disturb the flow. The influence of the local disturbance on bulk velocity at flashback was investigated by mapping flashback propensity and by examining non-reacting and reacting pipe flow with planar Particle Image Velocimetry (PIV). The fuel compositions studied in this research are DNG, hydrogen and mixtures containing 20, 40, 60 or 80 volumetric percentage $H_2$. \\ The flashback maps show that bulk velocity at flashback increases with obstruction height for all gas mixtures. The obstruction height is the height from burner wall to screw tip. The results obtained through the non-reacting field experiment confirmed that the flow was fully-developed and turbulent. Furthermore, the results showed that obstruction height locally increased velocity fluctuations. The average velocity downstream the obstruction slightly decreased with increasing obstruction height. The reacting field was examined during flashback and during stable operation to relate flashback propensity and time-averaged flow characteristics with statistical analysis. The results showed flame front axial distance from the burner rim decreases with increasing obstruction height, indicating that a flame produced by a burner with boundary layer obstruction is more prone to flashback. The flow characteristics just upstream the flame front were analyzed because once the conditions upstream the flame front are suitable, flashback occurs. \\ A flashback prediction model developed at the TU Munich was modified to account for the obstruction height. The modelled results indicate that, in an unconfined set-up, flashback might be described with time-averaged flow characteristics and that the turbulent burning velocity is strongly correlated to the bulk velocity at flashback. Histograms of the instantaneous velocity showed that obstruction height has no influence on low velocity streaks and thus it seems low velocity streaks are not dominant in the flashback process of premixed unconfined flames. The absence of low velocity streaks for different obstruction heights indicates that flashback of unconfined flames is a different process than flashback of confined flames.