This research investigates the potential for integration of heat storage into district heating networks, focusing on three varied case studies in the city of Sheffield. Each case study has implications for the future development of district heating and heat storage. The first case study concerned the potential for heat storage operating alongside proposed CHP units at the University of Sheffield. Heat demand data from the university was analysed to understand variations due to occupancy, weather conditions and other factors. Scenario modelling using Visual Basic algorithms simulated the operation of a new gas-fired CHP installation. Using heat storage was demonstrated to enhance the commercial and carbon benefits for the university from the CHP. The second case study involved working with Sheffield Forgemasters to assess potential for waste heat recovery and supply to an emerging district heating scheme. Site visits and dialogue allowed for estimation of the quantity, intermittency and temperature of waste heat resources. A novel computer programme was developed to simulate the effects of various parameters on the viability of heat storage with results highlighting a role to manage the production of waste heat as well as wider benefits for the CHP plant and the heat network. The third case study considered the operation of a city-wide heat network where established and emerging heat networks were interconnected. The running priority for the two CHP plants was optimised by an algorithm selecting heat production from the source of minimum cost. The environmental and economic impacts of interconnection and the addition of heat storage were evaluated. Overall, this thesis improves understanding of the role of heat storage as the importance of both energy storage and heat networks are growing. This work has benefited from collaboration with private and public sectors, providing valuable information for policy makers and for identifying future research directions.

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This thesis describes a study to evaluate the energy recovery potential and challenges associated with the application of Organic Rankine Cycle (ORC)s. Application of ORCs for both waste heat streams and deep geothermal sources are considered. A model which calculates the thermodynamic performance of ORCs for any source heat or sink stream and cycle configuration was developed. Simulations for three waste heat case studies showed a potential thermodynamic benefit from using zeotropic working fluids. An experimental rig was built to explore the reported discrepancy in performance between theory and experimental observations for zeotropic mixtures. Experiments were carried out with a near azeotropic working fluid, R410a, and a zeotropic mixture R407c. Results show that the global heat transfer coefficient of the zeotropic mixture was lower than for the azeotrope. The availability of theoretical models to accurately calculate heat transfer for zeotropic mixtures was explored. Appropriate models are available in the literature. However, to incorporate these into the ORC model is a significant bit of work beyond this project. The thermodynamic performance, footprint and cost of an ORC plant are key parameters that will determine the feasibility or otherwise of an ORC plant. These factors are considered together and the interdependence of them is discussed. Three deep geothermal heat sources are considered, within the context of these three factors. The reasons for the feasibility or otherwise of fuelling an ORC with each of these heat sources is discussed. Ultimately, while simulations show there is potential improvement in thermodynamic performance, by using zeotropic working fluid, experimental work shows there may be a penalty to pay in terms of the size of the system. The analysis of the deep geothermal case studies shows that finance and social factors also have a huge influence on whether a project to recover low enthalpy heat will evaluate or not.

Different types of base fluids, such as water, engine oil, kerosene, ethanol, methanol, ethylene glycol etc. are usually used to increase the heat transfer performance in many engineering applications. But these conventional heat transfer fluids have often several limitations. One of those major limitations is that the thermal conductivity of each of these base fluids is very low and this results a lower heat transfer rate in thermal engineering systems. Such limitation also affects the performance of different equipments used in different heat transfer process industries. To overcome such an important drawback, researchers over the years have considered a new generation heat transfer fluid, simply known as nanofluid with higher thermal conductivity. This new generation heat transfer fluid is a mixture of nanometre-size particles and different base fluids. Different researchers suggest that adding spherical or cylindrical shape of uniform/non-uniform nanoparticles into a base fluid can remarkably increase the thermal conductivity of nanofluid. Such augmentation of thermal conductivity could play a more significant role in enhancing the heat transfer rate than that of the base fluid. Nanoparticles diameters used in nanofluid are usually considered to be less than or equal to 100 nm and the nanoparticles concentration usually varies from 5% to 10%. Different researchers mentioned that the smaller nanoparticles concentration with size diameter of 100 nm could enhance the heat transfer rate more significantly compared to that of base fluids. But it is not obvious what effect it will have on the heat transfer performance when nanofluids contain small size nanoparticles of less than 100 nm with different concentrations. Besides, the effect of static and moving nanoparticles on the heat transfer of nanofluid is not known too. The idea of moving nanoparticles brings the effect of Brownian motion of nanoparticles on the heat transfer. The aim of this work is, therefore, to investigate the heat transfer performance of nanofluid using a combination of smaller size of nanoparticles with different concentrations considering the Brownian motion of nanoparticles. A horizontal pipe has been considered as a physical system within which the above mentioned nanofluid performances are investigated under transition to turbulent flow conditions. Three different types of numerical models, such as single phase model, Eulerian-Eulerian multi-phase mixture model and Eulerian-Lagrangian discrete phase model have been used while investigating the performance of nanofluids. The most commonly used model is single phase model which is based on the assumption that nanofluids behave like a conventional fluid. The other two models are used when the interaction between solid and fluid particles is considered. However, two different phases, such as fluid and solid phases is also considered in the Eulerian-Eulerian multi-phase mixture model. Thus, these phases create a fluid-solid mixture. But, two phases in the Eulerian-Lagrangian discrete phase model are independent. One of them is a solid phase and the other one is a fluid phase. In addition, RANS (Reynolds Average Navier Stokes) based Standard κ-ω and SST κ-ω transitional models have been used for the simulation of transitional flow. While the RANS based Standard κ-ϵ, Realizable κ-ϵ and RNG κ-ϵ turbulent models are used for the simulation of turbulent flow. Hydrodynamic as well as temperature behaviour of transition to turbulent flows of nanofluids through the horizontal pipe is studied under a uniform heat flux boundary condition applied to the wall with temperature dependent thermo-physical properties for both water and nanofluids. Numerical results characterising the performances of velocity and temperature fields are presented in terms of velocity and temperature contours, turbulent kinetic energy contours, surface temperature, local and average Nusselt numbers, Darcy friction factor, thermal performance factor and total entropy generation. New correlations are also proposed for the calculation of average Nusselt number for both the single and multi-phase models. Result reveals that the combination of small size of nanoparticles and higher nanoparticles concentrations with the Brownian motion of nanoparticles shows higher heat transfer enhancement and thermal performance factor than those of water. Literature suggests that the use of nanofluids flow in an inclined pipe at transition to turbulent regimes has been ignored despite its significance in real-life applications. Therefore, a particular investigation has been carried out in this thesis with a view to understand the heat transfer behaviour and performance of an inclined pipe under transition flow condition. It is found that the heat transfer rate decreases with the increase of a pipe inclination angle. Also, a higher heat transfer rate is found for a horizontal pipe under forced convection than that of an inclined pipe under mixed convection.

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This thesis provides an analysis of the effects of introducing a novel flow regime into fluidised beds, where swirling structures interact with particles of a similar scale. The motivation behind the work is to investigate the potential of increasing the efficiency of a widespread industrial process by a system that is readily scaled up and possible to retrofit. It begins by providing a review of the scientific literature around swirl flows and fluidised beds, explaining the reasoning behind the research hypothesis and the flow mechanisms in the inventive principle. This covers the background and utilisation of swirling and helical flow structures, followed by the principles of fluidised beds. The research hypothesis was tested empirically on a specially constructed pilot scale rig designed to provide a range of different flow regimes. The apparatus required advanced manufacturing methods and design was guided by Finite Element Analysis to ensure the desired flow regimes were achieved. The empirical studies with the apparatus were backed up through numerical modelling of the pseudo one dimensional fluidised bed, that is, the interaction between one jet and one particle. The empirical studies indicate a significant change in the behaviour of the fluidised bed between the novel flow regime and the standard design control. It was found that the novel flow resulted in improved gas distribution and the creation of smaller bubbles, improving the gas-solid interaction demonstrated by 2% lower bed expansion, 18% shorter drying times, 29% increased bubble frequency and minimal difference in total bubble volume. The numerical model shows change in the direction of momentum transfer and reduced heat transfer rate as the intensity of novel flow conditioning increases. This results in the particle being drawn towards an individual jet by a recirculation zone and a more even spread of heat throughout the gaseous phase. The evidence presented shows that by the introduction of a novel flow conditioning to fluidised beds results in significantly improved gas distribution across the system, leading to improved mixing between the gas and solid phases and resultantly a more efficient process.

The combustion of fossil fuels, in particular coal, meets the majority of energy demand worldwide, but produces carbon dioxide, which is believed to be the main cause of climate change. Since the majority of energy comes from coal-fired power stations, the deployment of carbon capture and storage (CCS) technologies, which remove the CO2 by either utilisation or storage, are necessary to mitigate climate change. Oxy-fuel combustion is one of the leading options for CCS. The fuel combusts in a mixture of oxygen and recycled ue gas, rather than in air and the change in the oxidiser environment poses questions relating to combustion characteristics such as heat transfer, emissions and burnout. To gain a further understanding of the process, the use of modelling and simulation techniques can be employed and in this thesis, Computational Fluid Dynamics (CFD) is used to model air and oxy-fuel environments using advanced combustion sub-models. An in-house Large Eddy Simulation (LES) CFD code has been updated to include models suitable for the prediction of NO. The model is verified and compared against available experimental data for three cases involving methane, coal and oxycoal combustion. Advanced simulations of a 250 kWth combustion test facility (CTF) are validated against experimental measurements of air-coal combustion. The geometry set-up and simplifications are discussed followed by a sensitivity study of grid refinement, turbulence models and approaches in modelling gaseous radiative properties. The validated CFD simulation of the facility were then numerically examined under a number of oxy-fuel environments. Finally, CFD simulations were performed on a full-scale utility boiler at 500MWe to examine the effects of firing coal and biomass under air and oxy-fuel environments. This included an assessment of the heat transfer as a method of addressing the performance of the boiler under these conditions.

Natural convection is a common phenomenon occurring both in nature and in industrial applications. Among the various confined enclosures, the rectangular cavity (empty or partitioned) is the most extensively studied enclosure because many engineering geometries can be simplified to this configuration. Natural convection in a rectangular cavity is also a very good vehicle for both experimental and theoretical studies. In experimental terms, the geometry of the rectangular cavity is simple and its boundary conditions are relatively easy to realise so that researchers can focus on the measurements of important quantities such as velocity and temperature profiles. In numerical terms, the flow phenomena in the cavity are so complicated and so plentiful that they intrigue both physicists and engineers. The first part of the thesis introduces the background of this project and an extensive review of the available literature. The literature review is divided into two parts. The first part presents an overview of natural convection in standard cavities with particular emphasis on previous studies pertaining to the simultaneous measurement of local velocity and temperature. The second part reviews and discusses previous studies, especially in the turbulent flow region, in natural convection in partitioned rectangular cavities. The experimental and numerical results are compared. Problems encountered in earlier studies are discussed. A proposal for the present study is given at the end of this part, which is then followed by the mathematical description and definition of the problem. The second part of the thesis introduces the existing experimental system at South Bank University together with the modifications made to suit the present study. The third part of the thesis presents the experimental results, which are then compared with earlier work. The thermal and fluid flow fields were systematically surveyed. The experimental contour plots of T, T', u~, v~ and u'v' and the velocity vector plots are reported for the first time for turbulent natural convection in both standard and partitioned square cavities. In addition to these quantities, the experimental turbulent quantities, u'T' and v'T', are also reported for the first time for natural convection in a standard square cavity. Also, in this part, the results of numerical simulations performed with the commercial software CFX-5.4.1 will be presented and compared with the experimental results. Despite the numerical simulations, this thesis is mainly concerned with the experimental investigation of low level turbulence natural convection in an air filled standard or partitioned square cavity. The experiments were performed with high precision and avoided some of the problems and shortcomings of earlier work. The results form benchmark data for low level turbulence natural convection and are useful for comparison with numerical predictions and CFD code validation and development.

The current work provides fundamental understanding of combustion regime transitions from distributed reactions towards the corrugated flamelet regime through a novel application of the multi-fluid approach of Spalding. Aerodynamically stabilised premixed flames were studied in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re ≃ 18,400 and Ret > 350). The chemical timescale was varied via the mixture stoichiometry, fuel reactivity and excess enthalpy with rates of strain exceeding the laminar flame extinction point. Rayleigh thermometry was performed to quantify the reaction zone broadening with large low temperature regions observed. Simultaneous Mie scattering, OH-PLIF and PIV were used to quantify the encounter of intermediate fluid states (i.e. mixing, mildly and strongly reacting) in addition to reactants and combustion products. A physical interpretation was provided for the individual fluid states. The analysis showed self-sustained flames in low strain regions with a collocated and pronounced dilatation for higher Damköhler numbers. By contrast, highly strained regions resulted in an auto-ignition related burning with attenuated dilatation and increased vorticity levels. The variation of the excess enthalpy - in particular for low Damköhler number combustion - illustrates the dominant influence of the burnt gas state on the dilatation and burning mode, with a distinct impact on the scalar flux also evident. The fuel reactivity showed a clear effect on the burning mode transitions, with explicit differences in the resulting flow field. The flow conditions were analysed in terms of Damköhler and Karlovitz numbers based on chemical timescales corresponding to laminar flames and auto-ignition events. The thesis provides novel insights into the underlying conditions leading to combustion regime transitions by means of (i) the evolution of multi-fluid probability, (ii) interface, (iii) mean flow field, (iv) conditional velocity and (v) conditional strain statistics evaluated as a function of the Damköhler number. (vi) The combustion mode influence on the scalar transport is discussed and (iv) a tentative 3D regime diagram is provided. The data illustrate the potential of a multi-fluid delineation to quantify a wide range of burning modes of relevance to low polluting combustion technologies.

An effective design for a kettle reboiler is dependent on fitness for purpose while reducing costs. Thus, accurate information concerning two-phase flow behaviour within it is important. Experimental and numerical studies have been carried out in this research to gain a more detailed understanding of the phenomena associated with two-phase flow in a thin-slice kettle reboiler. The kettle reboiler contained 241 electrically heated tubes arranged as 17 rows of 17 columns in an in-line layout with an outside diameter of 19 mm and a pitch-to-diameter ratio of 1.34. The working fluids used in this investigation were pentane and the refrigerant R113. They were boiled at atmospheric pressure at uniform heat fluxes in the range of 10 to 40 kW/m2. The patterns of flow inside the kettle reboiler were investigated experimentally using ordinary and high speed cameras. Visual observation of the flow patterns showed that the flow in the tube bundle was two-dimensional at heat fluxes of 20 kW/m2 and above. The quantity of foam and recirculation above the tube bundle were found to depend on both the heat flux and the working fluid used. Observations of the two-phase flow pattern in the shell indicated that the movement of fluid from the centre column of the bundle was affected by the down flow into the top of the tube bundle. Two flow patterns in the tube bundle were identified: bubbly and intermittent. At low heat fluxes, bubbly flow dominated, then, with increasing heat flux, bubble coalescence led to the development of vapour slugs and intermittent flow was observed. Pressure drop measurements were made in three columns within the tube bundle. The results showed that at heat fluxes below 20 kW/m2, the pressure drop remained nearly constant and equal to the all-liquid value. At a heat flux of 20 kW/m2 and above, the pressure drop was found to increasingly fall below the all-liquid value as the bundle row number increased. This effect was especially evident in the centre of the bundle. A change in the flow pattern caused the pressure distribution up the tube bundle to change from roughly constant to decaying with height. Based on a number of assumptions, the two-fluid model has been applied. The two-fluid model’s drag coefficient and tube resistance were deduced from a one-dimensional model. The two-fluid model predictions show good agreement with the experimental results for the pressure distribution and flow distribution. Grid sizes of 10, 8 and 4 mm for the bundle and the pool were considered. It was found that the predicted bundle results were not affected by changing the grid size. However, in the pool region, a small grid size was needed. A grid size of 10 mm was used in the bundle while 4 mm was used in the pool. The pool velocity predictions compared well with measured values available in the open literature. The results indicated that the bundle flow is not significantly affected by the pool flow. This allows the two-fluid model to be further refined: simplifying it and reducing the computational time. A bundle-only two fluid model has been developed to accurately predict two-phase flow behaviour in the kettle reboiler tube bundle. Information available from earlier studies has been used to develop this model because of the difficulties associated with measuring the void fraction and velocities within the tube bundle. The model uses two different boundary conditions: (1) static liquid pressure in the pool and (2) variation of pressure in the pool based on the flow pattern transition. The results predicted by the model have been compared with experimental data and with one and two-fluid models at different heat fluxes. Boundary condition (1) was found to be in good agreement with experimental data and the one and two-fluid models at a heat flux of 10 kW/m2. This was because the transition flow pattern was not achieved and the bundle was surrounded by a static pool. Boundary condition (2) is based on the Kutateladze number (Ku), which sets the transition point from bubbly to intermittent flow at a certain height in the bundle. For Ku ≤ 1.09, the bundle flow would be surrounded by liquid, and if Ku > 1.09, the bundle flow would be surrounded by two-phase flow. At heat fluxes of 20 kW/m2 and above, boundary condition (2) has been found to be in good agreement with experimental data and the values predicted from the one and two-fluid models for liquid velocities, vertical mass flux and void fraction. The bundle-only model accurately predicts the trend line of constant and decaying pressure drop measured at low and high heat fluxes, respectively, and the observed flow phenomena in the kettle reboiler. The key feature of the model presented is that it allows two-phase flow in the kettle reboiler to be simulated by only modelling the tube bundle. Thus the model is simplified and less computational time is required. A central column model was developed using the minimum pressure gradient approach. The predicted results from this model were compared with experimental data and the values predicted by the two-fluid model and the bundle-only model. Reasonable agreement was obtained indicating that the flow distribution may be linked to the minimum pressure gradient.