The hostile conditions encountered in flames makes it difficult to obtain measurements by conventional techniques. This work describes attempts to obtain time-resolved fluid mechanical information in jet diffusion flames using laser-optical techniques. Several different instruments were used in this study. Laser Doppler anemometry, using a tracking filter processor, was developed for measurement of the time varying velocity structure of these flames. The limitations and applicability of such a technique are investigated. Measurements of various statistical quantities, such as rms intensities and power spectra, are presented. A quantitative Schlieren technique was used to obtain information of flow structure over a wider range of experimental conditions than was possible using the LDA. This provided an insight into the larger scale processes within the flame. A hybrid correlation technique using both Schlieren and LDA was developed. This was used to investigate the near field of both jets and flames. The experiments show that significant differences in structure exist between isothermal and combusting flows. This manifested itself in several ways, including changes in vortex structure and a well defined low frequency instability of the flame.

A novel technique is presented in which two colour, two dimensional, time-resolved Laser induced incandescence (2C-2D-TiRe-LII) is used to produce planar spatially resolved, quantified, soot particle sizing. The technique is applied to a well characterized laboratory flame (Santoro burner) for validation. Accordance with other research efforts of spatial distribution of soot particle size was demonstrated.

Dans le contexte actuel de durcissement de la réglementation thermique visant à améliorer l’efficacité énergétique des bâtiments, il est nécessaire de repenser les installations de chauffage, de rafraîchissement et d’eau chaude sanitaire. Un système thermodynamique multifonction, appelé thermofrigopompe (TFP), produisant simultanément de l’énergie frigorifique et calorifique, semble alors une solution intéressante. L’emploi d’un fluide frigorigène à faible potentiel de réchauffement global (le GWP exprimé en émissions équivalentes de CO2), dans les machines frigorifiques, permet de répondre de manière efficace à la problématique de réduction des émissions de gaz à effet de serre. Cette thèse présente un prototype préindustriel de thermofrigopompe utilisant le propane comme fluide frigorigène. Le propane (R290) est intéressant d’abord pour son faible impact environnemental (ODP nul et GWP100ans =3) et pour ces performances énergétiques. Le prototype est le fruit d’une collaboration entre le Pôle cristal, centre technique froid et génie climatique de Dinan, et le laboratoire LGCGM de Rennes. Une nouvelle architecture du circuit frigorifique de la TFP est proposée avec une réduction significative du nombre d’électrovannes. Cette architecture permet une récupération efficace de la charge en fluide frigorigène lors des basculements entre les différents modes. Les besoins en chauffage, rafraîchissement et eau chaude sanitaire de trois types de bâtiments situés dans différents climats sont obtenus par simulation sous TRNSYS. La nature du bâtiment ainsi que le climat influencent fortement le caractère simultané des besoins. Un indicateur de besoins simultanés (TBS) est proposé afin d’identifier le bâtiment le plus adapté à une solution de production simultanée. Des essais en chambre climatique ont permis de valider le fonctionnement du prototype et de caractériser ses performances. Ces résultats expérimentaux ont servis à calibrer les modèles de composants et de machines frigorifiques pour chaque mode de fonctionnement développés avec le logiciel EES. Un bâtiment résidentiel collectif et un immeuble de bureaux ont été choisis dans l’étude comparative, afin d’évaluer l’influence de la nature des besoins sur les performances de la TFP. Les performances annuelles simulées par la méthode de corésolution (EES-TRNSYS) de la TFP sont comparées à une solution référence qui combine une PAC air/eau réversible pour le chauffage, et le rafraîchissement et un ballon thermodynamique pour l’ECS. Les résultats des simulations des performances saisonnières ont démontré que la piste des bureaux est intéressante dans le cas de zones nécessitant un fort besoin en rafraîchissement tout au long de l’année. / In the current context of hardening of thermal regulations to improve the energy efficiency of buildings, it is necessary to reconsider the heating, cooling and domestic hot water installations. A multifunctional heat pump system for simultaneous heating and cooling (HPS), which simultaneously produces cooling and heating energy, seems to be an interesting solution. The use of a refrigerant with low global warming potential (GWP expressed in equivalent emissions of CO2) in the refrigeration machinery can respond effectively to the problem of reducing greenhouse gas emissions. This work presents a pre-industrial prototype of HPS using propane as refrigerant. Propane (R290) is interesting firstly for its low environmental impact (zero ODP and GWP100 = 3) and for the energy performance. The prototype is the result of the collaboration between the Technical Centre for refrigeration and HVAC, Pôle Cristal and LGCGM laboratory. A new architecture of the refrigerant circuit of the HPS is proposed with a significant reduction in the number of valves. This architecture allows for efficient recovery of the refrigerant charge when switching between modes. The needs for heating, cooling and domestic hot water for three types of buildings in different climates are obtained by simulation using TRNSYS. The nature of the building and climate strongly influence the simultaneous nature of the needs. A ratio of simultaneous needs (RSN) is proposed to identify the most suitable building for the simultaneous production of heating and cooling energy. Climate chamber tests were used to validate the operation of the prototype and characterize its performance. These experimental results were used to calibrate the models of components and refrigerating machines for each operation developed with EES software. A collective residential building and an office building were selected in the comparative study, in order to evaluate the influence of the nature of the requirements on the performance of the HPS. Annual performance simulated by the co-solving method (EES-TRNSYS) of HPS is compared to a reference solution that combines a reversible air / water heat pump for heating, cooling and thermodynamic water heater for domestic hot water. The simulation results of the seasonal performance showed that the office building is interesting in the case of areas requiring a strong need for refreshment throughout the year.

Numerical modeling of free convection in porous enclosures is investigated in order to determine the best approaches to solve the problem in two and three dimensions considering their accuracy and computing time. Two case studies are considered: sloping homogeneous porous enclosures and layered porous enclosures due to their relevance in the context of geothermal energy. The governing equations are based on Darcy's law and the Boussinesq approximation. The mathematical problem of free convection in 2D homogeneous porous enclosures is solved following the well known stream function approach and also in terms of primitive variables. The numerical schemes are based on the Finite Volume numerical method and implemented in Fortran 90. Steady-state solutions are obtained solving the transient problem for long simulation times. The case study of a sloping porous enclosure is used for comparison of the results of the two models and for validation against results reported in the literature. The two modeling approaches generate consistent results in terms of the Nusselt number, the stream function approach however, turns out a faster computational algorithm. A parametric study is conducted to evaluate the Nusselt number in a 2D porous enclosure as a function of the slope angle, Rayleigh number and aspect ratio. The convective modes can be divided into two classes: multicellular convection for small slope angles and single cell convection for large angles. The transition angle between these convective modes is dependent on both the Rayleigh number and the aspect ratio. High Rayleigh numbers allow multicellular convection to remain in a larger interval of angles. This study is extended to the three-dimensional case in order to establish the range of validity of the 2D assumptions. As in the 2D modeling, two different approaches to solve the problem are compared: primitive variables and vector potential. Similarly, both approaches lead to equivalent results in terms of the Nusselt number and convective modes, the vector potential model however, proved to be less mesh-dependent and also a faster algorithm. A parametric study of the problem considering Rayleigh number, slope angle and aspect ratio showed that convective modes with irregular 3D geometries can develop in a wide variety of situations, including horizontal porous enclosure at relatively low Rayleigh numbers. The convective modes obtained in the 2D analysis (multicellular and single cell) are also present in the 3D case. Nonetheless the 3D results show that the transition between these convective modes follows a complex 3D convective mode characterized by the interaction of transverse and longitudinal coils. As a consequence of this, the transition angles between multicellular and single cell convection as well as the location of maxima Nusselt numbers do not match between the 2D and 3D models. Finally in this research, three-dimensional numerical simulations are carried out for the study of free convection in a layered porous enclosure heated from below and cooled from the top. The system is defined as a cubic porous enclosure comprising three layers, of which the external ones share constant physical properties and the internal layer is allowed to vary in both permeability and thermal conductivity. A parametric study to evaluate the sensitivity of the Nusselt number to a decrease in the permeability of the internal layer shows that strong permeability contrasts are required to observe an appreciable drop in the Nusselt number. If additionally the thickness of the internal layer is increased, a further decrease in the Nusselt number is observed as long as the convective modes remain the same, if the convective modes change the Nusselt number may increase. Decreasing the thermal conductivity of the middle layer causes first a slight increment in the Nusselt number and then a drop. On the other hand, the Nusselt number decreases in an approximately linear trend when the thermal conductivity of the layer is increased.

In order to decrease reliance on fossil fuels, renewable energy has become an important topic of research in recent years. The development in the renewable energy source will help in meeting the requirements of limiting greenhouse-gas effects, and conserve the environment from pollution, global warming, ozone layer depletion, etc. There are various naturally available renewable energy sources. One of these sources is solar energy. Solar energy is available in abundance throughout the world and is the cleanest of all known energy sources. There are various devices that can be used to harness solar energy. One of such devices is a thermo-syphon. Thermo-syphon converts the solar energy obtained from the Sun into thermal energy of a working fluid. This thermal energy in the working fluid can be used for various industrial and household activities. In a closed loop thermo-syphon system, the working fluid circulates within the thermo-syphon loop via natural convection phenomenon and does not need any external devices, such as a pump. Therefore, it is considered to be one of the most efficient devices for the heat transfer. Moreover, the absence of a pumping device reduces the manufacturing and maintenance costs of a thermo-syphon system. The heat exchange process in the thermo-syphon is a complicated process, which considers the heat convection phenomenon. Therefore, to understand the natural convection process in the thermo-syphon and their effect on the thermal performance of the system a Computational Fluid Dynamics (CFD) based techniques have been used. Numerical results obtained have been verified against the experimental results, and they match closely with each other. The comparison between the CFD and experimental result, suggest that CFD can be used as an effective tool to analyse the performance of a thermo-syphon with reasonable accuracy. In order to investigate the flow structure within the thermo-syphon system, detailed qualitative and quantitative analyses have been carried out in the present study. The qualitative analysis of the flow field includes descriptions of the velocity magnitude and the static temperature distributions contours within the closed loop thermo-syphon system. Furthermore, the variation in the temperature of water within the storage tank, temperature of the working fluid, heat transfer coefficient, wall shear stress, and local velocity and temperature distribution of the working fluid within thermo-syphon loop have been quantified as a function of time. In addition, numerical studies have been conducted to identify the effects of various geometrical parameters, which include the number of the riser pipes, length-to-diameter ratio of the riser pipe on the thermal performance of a closed loop thermo-syphon system. Moreover, a further investigation has been carried out to analyse the effect of various heat flux conditions and different transient thermal loadings on the thermal performance of a closed loop thermo-syphon system. Based on these analyses some novel semi-empirical relations have been developed to predict the thermal performance of the thermo-syphon, which is one of the focal points of this research. Another goal of the current study is to improve the thermal performance characteristic of thermo-syphon solar water heating system using an enhancement device to improve the heat transfer. This aspect of the work focuses on the increasing energy conversion from the riser pipes to the working fluid within the thermo-syphon loop. This is accomplished by increasing the surface area of riser pipes by employing several design modifications, such as straight, wavy and helical pipes, within the riser pipes, while maintaining the amount of the working fluid constant within the closed loop thermo-syphon system. In this study, a comparative analysis has been carried out for these new design modifications to identify the best in terms of heat transfer coefficient, heat gain in collector etc., as an indication of thermal performance. According to the findings of this analysis, the model comprising of pipe inside the riser pipe depict better thermal performance as compared to other models. After defining the best design modification, a further detailed investigation has been carried out between the traditional and modified design (straight pipe inside the riser pipe) using experimental and numerical method. Established methods regarding the design process of thermo-syphons are very limited, and they are severely limited in estimating important design parameters, such as useful heat gain and heat transfer coefficient, which have a significant impact on the thermal performance of thermo-syphon system/loop. A design methodology has been developed to enrich the design process of a closed loop thermo-syphon solar water heating system. The developed methodology is more efficient and reliable since it is capable of estimating various geometrical and thermal parameters, such as collector area, diameter and length of the riser pipes, distance between the centers of the riser pipes, heat transfer coefficient, temperature of the working fluid and the mass flow rate. This design methodology is user friendly and robust.

UK energy prices have doubled over the last decade, which has driven the UK Iron and Steel Industry to invest in energy efficient technologies. However, even with these relatively high prices the industry still finds it difficult to build a business case to justify waste heat recovery projects. The Steel Industry has large quantities of waste heat and there are technologies readily available for its capture, but often the issue has been finding a cost effective ‘end use’. Individual schemes incorporating both capturing and an ‘end use’ for the waste heat often incur high capital costs with resulting long payback times. This thesis defines the development and modelling of a strategy and methodology for the utilisation of waste heat recovery in a UK based Steelworks. The methodology involves the utilisation of the existing steam distribution circuit to link the possible waste heat schemes together with a single ‘end user’ thus limiting the capital requirement for each subsequent project. The thesis defines the development of a numerical model that is initially verified through extensive comparison to actual plant data from a series of pre-defined operational scenarios. The model is used to predict the pressure and temperature effects on the steam distribution system as the waste heat recovery boilers from various areas of the case study steelworks are connected up to it. The developed strategy stimulated significant capital investment for the CSSW and has generated over 100,000 MWh and is therefore saving over £7m and 50,000 tonnes of indirect CO2 emissions per annum. The thesis discusses and recommends further research and modelling for low, medium and high grade waste heat as well as the potential of a partial de-centralisation of the steam system. The output of the thesis is referenced by the DECC as an example of waste heat recovery in UK industry.

Conjugate heat transfer is a crucial issue in a number of turbulent engineering fluidflow applications, particularly in nuclear engineering and heat exchanger equipment. Temperature fluctuations in the near-wall turbulent fluid lead to similar fluctuationsin the temperature of the solid wall, and these fluctuations in the solid cause thermalstress in the material which may lead to fatigue and finally damage. In the present study, the Reynolds Average Navier-Stokes (RANS) modelling approachhas been adopted, with four equation k−ε−θ2−εθ eddy viscosity based modelsemployed to account for the turbulence in the fluid region. Transport equations forthe mean temperature, temperature variance, θ2, and its dissipation rate, εθ, have beensimultaneously solved across the solid region, with suitable matching conditions forthe thermal fields at the fluid/solid interface. The study has started by examining the case of fully developed channel flow withheat transfer through a thick wall, for which Tiselj et al. [2001b] provide DNS dataat a range of thermal activity ratios (essentially a ratio of the fluid and solid thermalmaterial properties). Initial simulations were performed with the existing Hanjali´cet al. [1996] four-equation model, extended across the solid region as described above. However, this model was found not to produce the correct sensitivity to thermal activityratio of the near wall θ2 values in the fluid, or the decay rate of θ2 across the solid wall. Therefore, a number of model refinements are proposed in order to improve predictionsin both fluid and solid regions over a range of thermal activity ratios. These refinementsare based on elements from a three-equation non-linear EVM designed to bring aboutbetter profiles of the variables k, ε, θ2 and εθ near the wall , and their inclusion is shownto produce a good matching with the DNS data of Tiselj et al. [2001b].Thereafter, a further, more complex test case has been investigated, namely an opposedwall jet flow, in which a hot wall jet flows vertically downward into an ascendingcold flow. As in the channel flow case, the thermal field is also solved across the solidwalls. The modified model results are compared with results from the Hanjali´c modeland LES and experimental data of Addad et al. [2004] and He et al. [2002] respectively. In this test case, the modified model presents generally good agreement with the LESand experimental data in the dynamic flow field, particularly the penetration point ofthe jet flow. In the thermal field, the modified model also shows improvements in the θ2predictions, particularly in the decay of the θ2 across the wall, which is consistent withthe behaviour found in the simple channel flow case. Although the modified model hasshown significant improvements in the conjugate heat transfer predictions, in some instancesit was difficult to obtain fully-converged steady state numerical results. Thusthe particular investigation with the inlet jet location shows non-convergence numericalresults in this steady state assumption. Thus, unsteady flow calculations have beenperformed for this case. These show large scale unsteadiness in the jet penetration area. In the dynamic field, the total rms values of the modelled and mean fluctuations showgood agreement with the LES data. In the thermal field calculation, a range of the flowconditions and solid material properties have been considered, and the predicted conjugateheat transfer predicted performance is broadly in line with the behaviour shownin the channel flow.

Computational Fluid Dynamics (CFD) has become one of the main instruments for the prediction of many commercial and research oriented fluid flow and heat transfer problems. While single phase flow analysis through CFD has gained grounds within the commercial industry, multiphase flow analysis is still the subject of further research and development. Heat Pipes and thermosyphones are no exception to this. However, the involvement of more than one fluid phase within these devices has made their analysis through CFD more challenging and computationally more demanding to perform. In this thesis, computational fluid dynamics is used as a modelling tool in order to predict the thermal hydraulic behaviour of multiphase environment within thermosyphones and heat pipes. Eulerian two-fluid method is used to solve the conservation equations for mass, momentum and energy, for each phase along with the inclusion of interfacial heat and mass transfer terms. Numerical predictions are obtained for the steady-state and transient operation of stationary thermosyphon, while rotating heat pipes operation is also simulated using axially and radially rotating heat pipe models. Apart from using the commercially available CFD code for the analysis of thermosyphones related simulation, numerical work is performed regarding the coupling of momentum equations based on Eulerian two-fluid modelling scheme. OPENFOAM open source code is used and modified to include the Partial Elimination Algorithm (PEA) for the coupling of interfacial exchange terms, including interfacial mass transfer term, in the momentum equations of both phases. Results obtained from above discussed studies provide good agreement with corresponding experimental and analytical observations.

This study investigates the effects of a split injection strategy on combustion performance and exhaust emissions in a high speed direct injection optical diesel engine. The investigation is focused on the effects of injection timing, quantity, and the dwell angle between the injections using commercially available diesel fuel. Three different split injection strategies including 50:50, 30:70, and 70:30 have been investigated. Additionally, the effect of total injected fuel quantity using total fuel quantities of 10 mm3 and 20 mm3 has been investigated. Moreover, the effect of variable and fixed dwell angle in split injections has been examined for five different values between 5o CA and 25o CA in the case of variable and 10o CA for the fixed dwell timing. The last parameter investigated was the injection timing, nine injection timings have been tested for each of the strategies. A Ricardo Hydra single cylinder optical engine running at 1500 rpm was used in this investigation. Conventional methods such as direct in-cylinder pressure measurements and heat release rate analysis have been employed. In addition, optical techniques such as high speed video imaging and two-colour have been applied, aimed at in depth analysis of the effects of the aforementioned parameters on engine performance and emissions. Furthermore, a significant amount of effort was devoted to the development and application of the Laser Induced Excipex Fluorescence (LIEF) technique so that simultaneous fuel liquid and fuel vapour distribution could be visualised. This investigation concludes that split injection strategies have the potential to reduce diesel exhaust emissions while maintaining a good level of fuel economy, provided that injection timings and the dwell angle between injections are appropriately selected. Further investigations are required in order to examine the effect of split injection under different engine operating conditions and speeds. In addition, the effect of alternative fuels must be considered. Moreover, the application of LIEF technique for quantitative fuel vapour concentration measurement should be considered through further optimisation of the LIEF system and careful calibration experiments.

Coal firing power stations represent the second largest source of global NOx emissions. The current practice of predicting likely exit NOx levels from multi-burner furnaces on the basis of single burner test rig data has been proven inadequate. Therefore, to further improve current NOx reduction technologies and assist in the assessment of NOx levels in new and retrofit plant cases, an improved understanding of the impact of burner interactions is required. The aim of this research is two-fold: firstly, to experimentally investigate isothermal flow interactions in multi-burner arrays for different swirl directions and burner pitches in order to gain a better understanding of burner interaction effects within multi-burner furnaces. Secondly, to carry out numerical modelling in order to determine turbulence models which give the best agreement to experimental data. Experimental investigations were carried out using flow visualisation for qualitative and 3D laser Doppler anemometry for quantitative measurements. Numerical modelling was performed using the computational fluid dynamics software, Fluent, to compare performance between k-ε, k- ω and RSM turbulence models. Experimental investigation showed that the recirculation zone of the chequerboard configuration is more sensitive to the change in pitch than that of the columnar configuration. Further, it was found that the smaller pitch is more sensitive to change in configuration than the wider pitch. The analysis of fluctuating components, u’, v’ and w’ showed that the burner flow is highly anisotropic at burner exit. Numerical investigation showed that the k-ω turbulence model consistently performed below the other two models. The statistical comparison between k-ε and RSM turbulence models revealed that, for prediction of the swirl velocity profiles, the RSM model overall performed better than the k-ε turbulence model. The visual and statistical analyses of turbulent kinetic energy profiles also showed that the RSM turbulence model provides a closer match to the experimental data than the k-ε turbulence model.