Global challenges related to energy security, resource sustainability and the environmental impacts of burning fossil fuels have led to an increasing need for switching to the use of clean and sustainable resources. Bio-oil produced through pyrolysis has been suggested as one of the sustainable alternatives to fossil resources for power generation as well as chemicals and biofuels production. Pyrolysis is a thermochemical process during which the biomass feedstock is heated in an inert atmosphere to produce gas, liquid (bio-oil) and solid (char) products. Microwave heating has been considered a promising technique for providing the energy required for biomass pyrolysis due to its volumetric and selective heating nature which allows for rapid heating in a cold environment. This helps to preserve the product quality by limiting secondary reactions. The aim of this research was to study the interactions between biomass materials and microwave energy during pyrolysis, and to develop a reliable and scalable microwave pyrolysis process. The dielectric properties of selected biomass materials were studied and found to vary significantly with temperature due to the physical and structural changes happening during pyrolysis. The loss factor of the biomass materials was found to reach a minimum value in the range between 300 oC and 400 oC followed by a sharp increase caused by the char formation. A microwave fluidised bed process was introduced as an attempt to overcome the challenges facing the scaling-up of microwave pyrolysis. The concept of microwave pyrolysis in a fluidised bed process was examined for the first time in this thesis. A systematic approach was followed for the process design taking into account the pyrolysis reaction requirements, the microwave-material interactions and the fluidisation behaviour of the biomass particles. The steps of the process design involved studying the fluidisation behaviour of selected biomass materials, theoretical analysis of the heat transfer in the fluidised bed, and electromagnetic simulations to support the cavity design. The developed process was built, and batch pyrolysis experiments were carried out to assess the yield and quality of the product as well as the energy requirement. Around 60 % to 70 % solid pyrolysed was achieved with 3.5 kJ·g-1 to 4.2 kJ·g-1 energy input. The developed microwave fluidised bed process has shown an ability to overcome many of the challenges associated with microwave pyrolysis of biomass including improvement in heating uniformity and ability to control the solid deposition in the process, placing it as a viable candidate for scaling-up. However, it was found to have some weaknesses including its limitations with regards to the size and shape of the biomass feed. Microwave pyrolysis of biomass submerged in a hydrocarbon liquid was introduced for the first time in this thesis as a potential alternative to overcome some of the limitations of the gas-based fluidised bed process. Batch pyrolysis experiments of wood blocks submerged in different hydrocarbon liquids showed that up 50 % solid pyrolysis could be achieved with only 1.9 kJ·g-1 energy input. It was found that the overall degree of pyrolysis obtained in the liquid system is lower than that obtained from the fluidised bed system. This was attributed to the large temperature gradient between the centre of the biomass particle/block and its surface in the liquid system leaving a considerable fraction of the outer layer of the block unpyrolysed. It was shown that the proposed liquid system was able to overcome many of the limitations of the gas-based systems.

The study is concerned with theoretical examination of thermo-acoustic instabilities in combustors and focuses on recently discovered flame intrinsic modes. These modes differ qualitatively from the thoroughly studied acoustic modes in a combustor. Despite being intensely studied, primarily numerically and experimentally, their properties remain poorly understood. Here an analytical investigation produced a comprehensive picture of the properties of linear intrinsic modes within the framework of a one-dimensional model of closed-open and open-open combustors with temperature and cross-section jump across the flame, and a linear n  law of heat release, where n is interaction index and  is the time lag. It has been shown there is always an infinite number of intrinsic modes present. In the limit of small n the frequencies of these modes depend neither on the properties of the combustor nor on the position of the flame. For small n these modes are strongly damped and they become unstable only if n exceeds a certain threshold. Remarkably, on the neutral curve the intrinsic modes become completely decoupled from the environment. The main results of the study follow from the discovered decoupling on the neutral curve and include explicit analytic expressions for the exact neutral curve on the n  plane, and for the growth/decay rate dependence on the parameters of the combustor. A new type of thermo-acoustic instability has been discovered. The instability occurs out due to coupling between intrinsic and conventional acoustic modes. Unstable or weakly decaying coupled acoustic modes behave exactly like an intrinsic mode, which increases the possible number of unstable intrinsic modes. The overall picture of intrinsic mode instabilities found for ideal boundary conditions has been shown to be robust with respect to modifications of the end boundary conditions. The explicit corrections to the position of the neutral curve and growth/decay rate have been found. The analytical results have been verified by numerics.

No description available.

This thesis concerns numerical investigations of the combustion behaviour of various combustion regimes. The simulations are based on modelling the flow of the fuels in the combustion devices. Computational fluid dynamics (CFD) modelling and analysis were used in three different works. FLUENT software, which is based on the finite volume method, is used to carry out all the simulations. Firstly, numerical simulations were carried out to investigate the turbulent non-premixed combustion of a mixture of methane (CH4) 90% and nitrogen (N2) 10%, on volume basis, inside an axis-symmetric cylindrical chamber (base case). The objective is to investigate the turbulent flow, flame propagation, temperature and species concentration and evaluate the effects of different reduced reaction mechanisms of methane and the influence of various turbulence models on them. The turbulent combustion inside the chamber occurs under a condition for which the equivalence ratio (ɸ) of 1.04 is used. Instead of using fully detailed chemical kinetics schemes and to reduce the computational costs, four global reduced chemical kinetics mechanisms are employed in the combustion model and they are named as (M-I, M-II, M-III and M-IV). The simulations, in which M-I is used, are performed by Renolds-Averaged Navier Stokes (RANS) approach with the three two-equation k-ϵ closures (standard, realizable and RNG) employed to model the turbulent flow. Concerning the chemistry-turbulence interaction, the finite-rate/eddy-dissipation model (FR/ED) is used. The first two of the above kinetics schemes are two-step reaction mechanisms and the other two are first-step and five-step reaction mechanisms, respectively. The latter one is used to assess the capability of FR/ED model for modeling such a mechanism. The influence of thermal radiation is also investigated by means of P-1 model. The standard k-ϵ model and realizable k-ϵ model are also modified and used in the course of simulations. Moreover, the reaction mechanism (M-II) is optimized to see its effects on the combustion process. The results are compared with the experimental data and gave good agreement. It is found that the best results are generally obtained using the modified standard k-ϵ model. Moreover, the simulation results using the realizable turbulence model are found to have large discrepancies compared to the experimental data. In comparison with the experimental data, the optimization of M-II (Em = 1.6x108 J/kmol) is found to have good results in terms of temperature. Increasing the dilution of the fuel by N2 is investigated. Four cases, CH4 (85, 80, 75 and 100%) on volume basis, are performed. The latter one concerns the combustion of pure methane. The results are compared with the base case and found that the base case is the best compromise to obtain the highest temperature in inside the chamber. Secondly, an axis-symmetric combustion model based on the Euler-Lagrange approach was formulated to model the combustion of pulverized bituminous coal. Three cases with three different char oxidation models are presented. In case1 and case 2, the diffusion and kinetic/diffusion global char models are used, respectively. Whereas, to model char oxidation in case 3, the multi-surface reactions model is used. The volatiles released during the devolatilization stage, which is modelled using a single kinetic rate model, are treated as one species and its combustion is modelled using the FR/ED model. The predicted results have good agreement with the available experimental data and the best predictions are obtained from case 3. The results showed that the combustion inside the reactor was affected by the particulate size. It is found that the burnout of the particle with the diameter of 16 μm at the exit of the furnace is 100%. Whereas, the burnout of the particles with diameters of 84, 154, 222, 291 μm is approximately 86, 75, 35, 33, 29 %, respectively. A number of simulations were carried out to find the best values of parameters suitable for predicting NOx pollutants. The chemical formation and reduction rates of NO are calculated by post-processing data obtained from the previously reacting flow simulations. This method is computationally efficient. For volatile-N is assumed that the nitrogen is released via the intermediates HCN and NH3. For char-N path way, it is assumed that all the nitrogen is released via the intermediate HCN. It is found that the assumption of the partition of volatile-N by 52% HCN, 10% NH3 and 38% NO has the best agreement with the experiment data. The influence of different operating parameters on the combustion process and NOx formation was investigated as well. For the same operating conditions and the same particles size distribution, the combustion of pulverised biomass alone, represented by straw, was investigated followed by the investigation of its firing with coal. The former one show a promising results under such operating conditions. It is found that the temperature distribution when burning straw particles is nearly the same as that obtained from burning coal because all the saw particles are completely burned out inside the furnace when compared with the coal particles. The NOx model, in which the ratio of HCN to NH3 is suggested to be for the partitioning of volatile-N, shows that NO formation is reduced by approximately 20% for case I and 26% for case II at the exit of the furnace when compared to coal. For the latter one the results of co-firing blends of coal with 10, 20, 30 and 40% share of biomass are presented and show the influence of co-firing on the combustion process. Co-firing of straw with coal enhances the combustion behaviour and increases the burnout of coal particles compared to that of coal firing only. It is seen that the burnout of the particles with sizes 84, 154 and 222 μm is remarkably increased. On the other hand, the burnout of the other two particles (291 μm and 360 μm) does not show a great change. The share of 10% of straw shows the highest temperature. Thirdly, Two-phase computational modelling based on the Euler–Euler was developed to investigate the heterogeneous combustion processes of carbon particles inside a newly designed combustion chamber. A transient simulation was carried out for a small amount of carbon powder situated in a cup which was located at the centre of the combustion chamber. A heat source was provided to initiate the combustion with the air supplied by three injection nozzles. The combustion simulations are performed for particle sizes with different diameters (0.5mm, 1mm, 1.5mm, 2mm, 2.5mm and 3mm). The particle of 1mm diameter is assigned to the baseline case. The results show that the combustion is sustained in the chamber, as evidenced by the flame temperature. It is shown that, up to a time of 0.55 s, the higher temperature was gained from the case of carbon particles with the diameter of 3 mm and burning the carbon particles with a diameter of 0.5 mm produces lower temperature. This may be attributed to the residence time of the carbon particles and the design of the burner. The larger particles stay longer than the smaller ones inside the chamber. This may due to the reason that the smaller particles follow the streamlines of the continuous phase and increasing the particle size leads to that the larger particles may deviate from the streamlines of the continuous phase and their slip velocity may increase resulting in enhancing convective transports of heat and species concentrations. The influence of the chamber design was also investigated. The height of the chamber is doubled. With the same operating conditions, up to a time of approximately 0.55 s, it is found that burning carbon particles in the doubled height chamber produces higher temperature than the baseline case (particle diameter 1 mm) and after this time the opposite takes place. Most of the other cases do so.

Analytical, computational and experimental studies were carried out to investigate heat transfer and fluid flow in micro-channel absorber plates for compact (thin and light-weight) solar thermal collectors. The main objective of the work was to study different design and/or operating scenarios as well as study the significance of various micro-scaling effects. Analytical investigation showed that, under similar conditions, the proposed design yields a much higher fin efficiency, F and collector efficiency factor, F’ compared with the conventional solar collector design. An analytical model combining convective heat transfer in the collector fluid with axial conduction in the metal plate was developed. The predicted plate temperature profiles from the analytical model were in close agreement with the measured profiles. The model further showed that axial thermal conduction can significantly alter the plate temperature profile. Experiments were designed to represent real life operation of the proposed system. A CFD study, using the same design and operating parameters, produced results comparable with experiments. This numerical simulation also gave further insight into the heat transfer and fluid flow patterns in the micro-channel plate. The effect of channel cross section geometry was studied. The Nusselt number was observed to increase as the aspect ratio approached unity. Measured friction factors were similar in trend to the predictions for rectangular channels, although the overall rise in fluid temperature resulted in slightly lower friction factors. Thermal performance reduced slightly with increase in hydraulic diameter. The significance of various scaling effects was also investigated experimentally and numerically. Most of the typical scaling effects such as viscous dissipation and entrance effects were found to be insignificant however, conjugate heat transfer, surface boundary condition, surface finish and measurement uncertainties could be significant. The results showed a Reynolds number dependent Nusselt number which has been attributed to axial thermal conduction. It was also observed that only three walls were transferring heat; the walls of heat transfer had a uniform peripheral temperature while the heat flux varied peripherally. The closest simplified thermal boundary condition to represent heat transfer in these channels is the H1 with three (3) walls transferring heat. Increased surface roughness (obtained by using an etching technique to create the channels) was found to have a detrimental effect on heat transfer. The results showed that thermal improvement can be achieved by increasing the fluid velocity; however, pumping the thermal fluid above a pump power per plate area of 0.3 W/m2 resulted in marginal improvement. In practice, optimum microchannel geometry in plates should be sized based on fluid properties and operating conditions. The micro-channels should also have thin walls to minimise the effects of conjugate heat transfer. A Photovoltaic pump should be installed alongside the collector in order to provide pumping power required and minimise the overall fluid temperature rise. The results are beneficial for the design of micro-channel absorber plates for low heat flux operation up to 1000W/m2.

This thesis investigates enhanced methods for analysing non-linear effects in propagating laminar flames, enabling more accurate evaluation of laminar flame characteristics such as Markstein length and unstretched flame speed whilst proposing a new method for evaluating extinction stretch rate. Furthermore, a new cloud-combustor is developed and commissioned enabling laminar flame characteristics through droplet fuel mists to be explored again utilising advanced non-linear analysis. Re-analysis of previous low-ignition energy methane-water flames reveals the analytical non-linear characteristic. The analysis also demonstrates the need for a larger chamber to avoid pressurised effects during the latter stages of propagation, potentially reducing the accuracy of the adopted methodology. Non-linear analysis shows interesting trends concerning Markstein length at higher water loading in particular when it increase to 15% (by volume), and laminar burning rate decreased. The non-linear analysis technique is deployed to analyse four hydrocarbon fuels, two traditional paraffinic fuels in methane and propane, and two alternative alcohol fuels namely ethanol and methanol. It is shown that overdriven flame data can be used to predict flame extinction stretch rate, as long as a sufficient time period is disregarded to allow the effects of the early ignition-affected period to subside. The new technique proposed for evaluated critical extinction stretch rate shows good agreement with the traditional counter-flowing flame technique. Results for the four fuels reveal a common profile for extinction stretch-rate as a function of equivalence ratio, which was anticipated due to the similar fundamental combustion characteristics of the chosen fuels. Based on the non-linear analysis, it is shown analytically that this common profile may be represented by a combination of the iv unstretched laminar burning velocity, the Markstein length and the expansion ratio of the fuel. Ethanol in air is used to benchmark Cardiff University’s new, large 35Litre ‘Cloud Combustor’ for an investigation of flame propagation through fuel mists across a wide range of equivalence ratios. Non-intrusive, in-situ droplet sizing with concurrent flame propagation is achieved for the first time. The fuel mist flame data was subsequently compared to that for pure vapour mixtures at nominally identical ambient conditions in order to study the reported enhancement in flame speed exhibited in previous studies, and to compare qualitatively against conflicting published views reported in literature. It was found that with the onset of instabilities at certain droplet size an enhancement in flame speed could be shown for rich mist flames compared to those of analogous vapour flames. Based on mechanisms detailed elsewhere that provide a possible explanation for this enhancement full discussion and correlations that help to understand the nature of flame speed through droplet mists are presented.

The problems of excessive consumption of fossil resources, oil shortages and greenhouse gas emissions are becoming increasingly severe. Research and development work on new methods of thermal energy storage are imminently required. To effectively store seasonal renewable energy, a novel high capacity heat storage system has been designed and evaluated/validated through laboratory experiments and numerical simulations in this research. The system is driven by direct flow evacuated tube solar collector with enhanced PCM tank and intends to be applied in residential and commercial buildings. Theoretical and experimental approaches and numerical analysis have been employed in this study. Firstly, phase change materials (PCM) with specific heat density, melting point, melting and solidifying time have been investigated. This type of PCMs can maintain a considerable high internal temperature of environment chamber in a frozen ambient temperature. Numerical modelling has been conducted on a passive house (Nottingham H.O.U.S.E) to study whether proposed thermochemical materials can cover relative heating load and be power by solar panel in terms of roof size. Meanwhile, PCMs have been used to give a long duration for temperature-controlled chamber in laboratory, and thermochemical materials have been utilized in closed pumping pipe system for cooling and heating purpose. Secondly, characteristic experiments have been conducted on a modified solar collector working with an enhanced PCM tank that is integrated with a fan coil heat exchanger. The results show that light radiation of tungsten lamps (as a solar simulator) has approximately 70% efficiency to equate to solar radiation under the same Pyranometer reading value. At the same time, the solar system can supply over 50°C heating energy and the PCM tank within it can supply higher output temperature with longer duration than water tank. The efficiency of the whole solar collector heating system is over 50% as a heat absorption chamber in sunny days, while only approximately 10% under mostly cloudy weather. Lastly, proposed thermochemical materials (silica gel, calcium chloride, zeolite 13x, vermiculite and activated carbon) have been evaluated on designed thermochemical absorption chamber to supply fresh high temperature air for space heating. The results show that zeolite holds the highest reacted temperature (over 58°C) and vermiculite has really fast absorbing hydration duration, less than half hour. Silica gel possesses the biggest water absorbing capacity and vermiculite has a worse result. A comparison between experimental and numerical modelling results has been revealed. Considering the complexity of processes in cooling and heating system, the agreement of simulation and experimentation is satisfactory, thus the lumped numerical model is acceptable and significant for investigation of this scaled seasonal high capacity heat storage system. A full size seasonal heat storage system with a nominal heating capacity of 3kW has been proposed and illustrated in economic and environmental issues section. The results from net present value (NPV) and internal rate of return (IRR) sensitivity analysis both shows it is greatly attractive to develop this novel system for application in both household and commercial buildings in consideration of its about 9 years payback period, 20 years life span and zero gas (C02) emissions. An intelligent transpired solar collector system is also introduced and illustrated as future work.

This dissertation deals with the development of a co-firing advisory tool capable of predicting the effects of biomass co-firing with coal on the ash deposition and thermal performance of pulverised fired (pf) boilers. The developed predictive methodology integrates a one-dimensional zone model of a pf boiler to determine the heat transfer conditions and midsection temperature profile throughout the boiler, with the phase equilibrium–based ash deposition mechanistic model that utilises FactSageTM thermo-chemical data. The designed model enables advanced thermal analysis of a boiler for investigating the impact of fuel switching on boiler performance including the ash deposition effects. With respect to the ash deposition predictive model, the improved phase equilibrium approach, adjusted to the pf boiler conditions was proposed that allows the assessment of the slagging and high temperature fouling severity caused by the deposition of the sticky ash as well as low-temperature fouling due to salts condensation. An additional ash interaction phase equilibrium module was designed in order to estimate the interactions occurring in the furnace between alumino-silicate fly ash and alkali metals originating from biomass. Based on the developed model, the new slagging/fouling indices were defined which take into account the ash burden, slag ratio in the fly ash approaching the tube banks as well as the slag viscosity corresponded to the conditions within the pf boiler. The developed model was validated against field observations data derived from semiindustrial pf coal-fired furnace as well as a large scale 518 MWe pf boiler fired with a blend of imported bituminous coals and biomass mix composed of the various quality biomass/residues, such as meat and bone meal, wood pellets and biomass mix pellets produced on-site: the power plant typically fired up to 20wt% coal substitution. Good agreement has been found for the comparison between predictions and slagging/fouling observations. Based on the validated model the fuel blend optimisation was performed up to 30wt% co-firing shares revealing highly non-additive ash behaviour of the investigated fuel blends.

No description available.

Large eddy simulation of spray combustion in an HSDI engine is carried out in this thesis. The implementation of the code was performed in logical steps that allowed both assessment of the performance of the existing KIVA-LES and later development. The analysis of the liquid annular jet confirmed existence of typical, annular jet exclusive structures like head vortices, stagnation point and recirculation in the inner zone. The influence of the swirl in the ambient domain was found to have profound impact on the development, penetration and radial spreading of the jet. Detailed results were reported in Jagus et al. (2008). The code was further validated by performing an extensive study of large eddy simulation of diesel fuel mixing in an engine environment. The reaction models were switched off in order to isolate the effects of both swirl and the different numerical treatment of LES. Reference RANS simulation was performed and significant differences were found. LES was found to capture much better the influence of the swirl on the liquid and vapour jets, a feature essentially absent in RANS results. Moreover, the predicted penetration of the liquid was much higher for the LES case and more in accordance with experimental measurements. Liquid penetration and subsequent evaporation are very important for prediction of heat release rates and encouraging results formed a good basis to performing a full simulation with models for ignition and combustion employed. The findings were analyzed in the paper by Jagus et al. (2009). Further modifications were introduced into the LES code, among them changes to the combustion model that was originally developed for RANS and calculation of the filter width. A new way of estimating the turbulent timescale (eddy turnover time) assured that the incompatibilities in the numerical treatment were eliminated and benefits of LES maximized. The new combustion model proved to give a very good agreement with experimental data, especially with regard to pressure and accumulated heat release rates. Both qualitative and quantitative results presented a significant improvement with respect to RANS results and old LES formulation. The new LES model was proved to give a very good performance on a spectrum of mesh resolutions. Encouraging results were obtained on a coarse mesh sets therefore proving that the new LES code is able to give good prediction even on mesh sizes more suitable for RANS. Overall, LES was found to be a worthy alternative to the well established RANS methods, surpassing it in many areas, such as liquid penetration prediction, temperature-turbulence coupling and prediction of volume-averaged data. It was also discovered that the improved LES code is capable of producing very good results on under-resolved mesh resolutions, a feature that is especially important in industrial applications and on serial code structure.