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Environmental awareness and the interest in distributed generation caused by electricity market de-regulation are factors that promote research on renewable energies. Fuel cells transform the chemical energy stored in fuel into electricity by means of electrochemical reactions. Among the different fuel cell types, high temperature fuel cells (HTFCS) have many advantages: high efficiency, low emissions, fuel flexibility, modularity and high quality waste heat. The main disadvantage is their high cost - however, this will be reduced when HTFCS are commercialised. The synergy between HTFCS and gas turbines (GTS) makes HTPC/GTS very efficient power systems for the generation of electricity, from kilowatts to just a few megawatts. The present work focuses on HTFC/GT power systems, analysing their performance, studying some particular applications, and making an economic assessment. The final objective of this Thesis is to define a procedure to assist in the preliminary design of HTFC/GT systems. The authors main contribution is the definition of the Green-Cell Code capable of simulating HTFC/GT systems, the study of their interest for several applications, and the generation of a decision-making method for the preliminary design of HTFC/GT systems. The design and off-design simulation of HTFC/GT cycles have been carried out with the integration of a code developed by the author to simulate HTFC performance, and a commercial code to simulate GT performance. This work is even more valuable given the lack of commercial tools to analyse the system. All of the technical and economic work is collected in a set of charts that assist the procedure of HTFC/GT cycle selection. These charts show that HTFC/GT systems currently achieve thermal efficiencies of about 60%, and will be capable of achieving up to 73% in the future. This is of great interest for power generation applications. The use of a recuperate is required to optimise the performance of the gas turbine and the fuel cell; it is also interesting to use it to generate the maximum amount of power from the HTFC, in order to reduce emissions and increase overall efficiency. Results show that Pressurised MCFC/GT Cycles achieve better performance and economic results that Atmospheric MCFC/GT Cycles. For Pressurised MCFC/GT Cycles, the optimum stack operating pressure is between 5 and 10 bars. The installation of a combustor in Pressurised MCFC/GT cycles leads to higher specific power, higher unit costs of electricity, higher CHP efficiency, and lower thermal efficiency. The use of HTFC/GT cycles to generate heat and power must be seen as a way to improve HTFC/GT efficiency by using the waste heat of exhaust gases, rather than as an optimum application. Results also show that SOFC/GT systems achieve slightly higher results than MCFC/GT systems. Thus, the choice between MCFCs and SOFCs will be based on durability and cost issues rather than on performance issues.

An investigation of the flame structure and radiation properties of turbulent hydrogen/air diffusion flames is reported. The laminar flamelet-conserved scalar probability density function approach is used to predict the scalar distributions throughout a laboratory-scale axisymmetric buoyant hydrogen diffusion flame. Predictions are compared with published measurements of mean and root mean square (RMS) temperatures and species concentrations based on the laminar flamelet concept. Predictions of spectral intensity and received heat flux are made with a narrow-band radiation model using mean properties, stochastic and mean emission methods to evaluate the effects of turbulence radiation interactions (TRI) and modelling TRI to predict the received radiant heat flux was very important. The predictions were, on the whole in good agreement with published measured data available in the open literature. Present study centres on the development of novel numerical models to predict TRI in turbulent hydrogen flames, implemented in a sophisticated way using enthalpy perturbation equation to account for radiative heat loss. This thesis highlights novel accomplishments in areas such as modelling lifted hydrogen jet flames, flame structures and external radiation fields where significant findings are reported. Firstly, successful extension of the lift-off model to hydrogen jet flames using strain rate as stretch parameter to accurately predict the lift-off height and affirming the smallscale strain rate model is better than the large-scale strain rate model which is different to methane lifted jet flames. Secondly, different jet flames were investigated using two different probability density functions (PDFs) and two transport equations taking into account fluctuations of temperature T ¢2 and water vapours 2 2 H O X¢ . The new Truncated Gaussian PDF was confirmed to give better predictions than other methods. Lastly, of the three approaches considered in modelling TRI the stochastic method proved the most accurate to predict the spectral intensity distribution and radiative heat flux distribution.

Combined cycles are subject to degradations and hence performance deterioration. According to the author's survey nothing was found in the open literature on this subject. Therefore, it was anticipated that it would be of great achievement if a tool for analysing and diagnosing the deterioration of combined cycle could be produced. So this thesis presents a procedure for combined cycle performance analysis and fault diagnostic by way of simulation. l order to accomplish this task successfully it was necessary to developed two pieces of software. These are STEAMOMATCH for steam cycle performance deterioration analysis, and GOTRESS for GPA of any system. STEAMOMATCH, which is built on the basics of combined cycle thermodynamics, can simulate up to three levels of pressure with reheat. On the other hand GOTRESS uses a Gas Path Analysis technique that enables the user the choice of conducting either linear or non-linear GPA at the same time. I both cases single or multiple fault can be diagnosed. GOTRESS was built in such a way that it makes it a generalised code that can be used not only for combined cycle but to diagnose a wide range of power cycle plants. The deterioration simulation results of the gas turbine power plant showed that the isentropic efficiency deterioration of the turbine unit has the uppermost sever effect on overall gas turbine power output and thermal efficiency. This is also the case with steam turbine (bottoming) cycle power and Rankine efficiency. Also, the simulation results obtained showed that the relationship between the gas turbine size and its performance deterioration is almost constant, i.e. performance deterioration follows the plant's size. Among the two major gas turbine parameters that affects the steam bottoming cycle performance of a CCGT power plant, the gas turbine exhaust temperature has a predominant effect on steam cycle efficiency over the exhaust mass flow.' As a general result, the obtained simulation results showed that the behaviour of CCGT power plant performance is more affected by gas turbine cycle conditions than by steam turbine cycle conditions. The obtained results showed that GPA can be successfully applied to either gas turbine cycle, steam turbine cycle, or the combination of the two in a form of combined cycle. The GPA diagnostic results obtained showed that it would be possible to detect some faults that might occur within the gas turbine that is a part of a combined cycle power plant by monitoring the dependent parameters of the steam turbine (bottoming) cycle such as live steam pressure and temperature and steam turbine power. In contrast, it would not been possible to detect the problems (implanted faults) that might occur within the steam turbine by monitoring the dependent parameters of the gas turbine unit.

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This study is aimed to investigate and demonstrate the feasibility and validity of various flame flashback resistance techniques for premixed fuel conditions. It presents a series of experiments to determine the impact of different configurations on flame flashback mechanisms. Experiments were performed using a 150 kW tangential swirl burner working on premixed mode with various swirl numbers; the flow field characteristics were measured by 1D LDA. The first part of the project targeted the effect of central fuel injector geometries on flame flashback mechanisms, especially combustion induced vortex breakdown (CIVB). It was found that changing the central fuel injector outside diameter can significantly alter the flame flashback mechanism. Large injector diameters result in boundary layer flashback (BLF), contrary, the use of small injectors diameter led to CIVB. Thus a dimensionless number (χ) which represent the ratio between the injector outside diameter and the nozzle inside diameter was introduced. Using this dimensionless number the critical value of transition from CIVB to BLF has been defined, the value being χ= 0.280 for Sg=1.12 and χ= 0.320 for Sg= 0.9. The second part was about the effect of using axial air injection instead of central fuel injectors. It was found that axial air jets have a considerable potential for flame stability requirements, they producing wider stability operation than that of central injectors. Moreover, the stability limits increase regarding both equivalence ratio and inlet tangential velocity. It appeared that using such air jets could reduce the combustor maintenance cost that arises due to a continuous harsh environment. However, it was found that axial air jets could enforce flame propagation during flashback via wall boundary layer. Thus, the third part of the study was about the validity of using micromeshes to improve BLF resistance in addition to axial air injection. It was found that using both techniques produced high flashback resistance for both mechanisms, i.e. CIVB and BLF.

Trying to improve gas turbines to be flexible to use different fuels requires a big challenge for gas turbine designers when working with current operation stability issues. Flashback is the major challenge for low NOx premixed combustion of high hydrogen content fuels. Flashback in gas turbine combustors is usually assigned to four mechanisms; core flow flashback, combustion instability flashback, boundary layer flashback (BLF) and combustion induced vortex breakdown (CIVB). The last two mechanisms are most common in swirl combustors, which are used to have better operation stability and low emissions. Improvement of swirl combustors against CIVB has been studied extensively to improve the combustion stability. The most promising solutions of CIVB are limited because the improvement against CIVB worsens the system against boundary layer flashback. Boundary layer flashback is theoretically based on the Lewis von Elbe’s formula for laminar flame, with formulas also used in turbulent flames (with some reservation) by most recent studies. However, the majority of studies take the flame side of the formula, parameters such as fuel type and blends, pressure and preheat temperature, and try to improve the understanding of the boundary layer flashback hoping to find ways to reduce its onset. However, the effect of the burner nozzle has not been studied in many types of research, especially the internal nozzle surfaces. Therefore, this work aims to study the effect of regular surface roughness on the boundary layer flashback in a 150 kW tangential swirl burner. The first part of this study is a numerical simulation using the in-house code Hydr3D to simulate the flow over riblets with different geometries (blade, triangular, scallop, diamond, lotus and sharkskin). The numerical results demonstrate that the blade riblets were the best at reducing the boundary layer thickness and consequently showed the best drag reduction around 11% compared to the smooth surface while the sharkskin geometry was the worse in drag reduction with only 0.5%. vi Although the blade showed the best drag reduction, its weak structure and complex machine specifications make the scallop, lotus, and diamond and sharkskin riblet to be chosen for manufacturing on small discs. The scallop riblet on the nozzle was manufactured using wire electrical discharge machining (WEDM). The second part of the study was isothermal experimental tests for manufactured surfaces. The flow structure was measured using a 1D LDA. The results show that the riblets alter the flow structure near the wall and increase the velocity gradient which helps the flow to reach a velocity 0.99 from mainstream velocity at y+ < 10 compared to y+ > 30 for a smooth surface. The third part of the study was obtained with combustion and isothermal experiments using two different stainless steel woven meshes that served as a liner for the nozzle burner. A 50μm and a 150μm wire diameter meshes were used. The isothermal test showed that the 150μm mesh denoted the best shift of the velocity gradient close to the wall. The combustion experiments showed that the two meshes help to improve system against the boundary layer with the 150μm being the best. Thus, it was demonstrated that BLF could be reduced using microsurfaces, which in conjunction with other techniques, have the potential of increasing optionality, an essential feature for fuel flexibility in Gas Turbines.

Micro-geometries or 'microfluidics' are commonly utilised in a widespread variety of applications such as, bioengineering devices, microelectronic devices, electronics cooling, chemical micro-reactors and mini or micro-heat exchangers. In the microscale systems (with 'small' dimensions typically less than 1 millimeter), however, fluid mixing has been understood as one of the most fundamental and difficult-to-achieve issues because the flow of Newtonian fluids becomes increasingly controlled by viscous forces rather than inertia (as molecular diffusion is dominant at these small scales). As a consequence, the enhancement of convective heat transfer is problematic under these conditions (steady and laminar flow regime). In this thesis, two different regimes of instabilities, namely 'purely-inertial' and 'purely-elastic', have been adopted to enhance the convective heat transfer in the micro-scale geometries. Purely-inertial instability refers here to the secondary flow that arise in curved channels, also known as Dean flows, due to the centrifugal forces and also in crossed channels (cross-slot), symmetry-breaking bifurcations, which results in an axially-oriented spiral vortex along the outlet channels. While, purely-elastic instability is created in the flow of non-Newtonian viscoelastic fluids through curved channels due to the non-linear interaction between elastic stresses generated within the flowing viscoelastic solutions and the streamline curvature or through cross-slot device as a consequence of the planar extensional flow field (strong elongational flow) at the stagnation point. Fluid flow and convective heat transfer characteristics have been investigated experimentally and supporting numerical calculations for Newtonian flow within two different micro-geometries: a square cross-section serpentine microchannel and a square cross-section crossslot micro-device. A group of Newtonian fluids, aqueous glycerine solutions and aqueous sucrose solutions, was utilised to carry out the experiments for purely-inertial flows whilst high-viscosity polymeric viscoelastic fluids, shear-thinning and approximately constant-viscosity Boger solutions, were used for the experiments to investigate purely-elastic instabilities.

The dynamic characteristics of diesel engines have been examined using mathematical models and simulations. The mathematical approach uses sampled data analysis and small perturbation techniques in conjunction with frequency response methods. Extensive test work on a three cylinder compression ignition engine has shown that the sampled data model is valid for stability prediction. A method of plotting the experimental results is suggested which has the advantage of generality and as it takes into account both engine and load torque speed characteristics gives a unique plot for all engines. It shows clearly how the stability of the control loop can depend on the number of cylinders and the engine speed. Two simulations of engines are presented - an analogue computer simulation which is restricted in the form presented to a three cylinder engine, and a digital computer simulation which can be used for any naturally aspirated engine with a variety of governor characteristics. Both simulations accurately predict engine performance. The experimental methods used involve the use of electro- hydraulic systems and test techniques. These are shown to have great advantages in providing the appropriate inputs and loading conditions.

The program for solving one-dimensional unsteady flow by the field method of characteristics was developed further in a form suitable for pulse combustors designed for boiler application to be analysed. A pulse combustor was simulated on the pulse generating rig to optimise a smoothing circuit design and the experiments indicated that static pressure gains of up to 16% can be obtained. Results obtained from running an actual combustor are used to refine the assumptions made in the program and it is shown that a satisfactory agreement can be achieved between predicted and measured values.