Single and double cycle simulation techniques have been employed to investigate the structure and properties of the heat affected zone in HY-80 steel due to a single submerged arc weld run and to the deposition of a tempering bead on a pre-existing edge bead heat affected zone. The thermal cycles used for simulation had peak temperatures of 1275°C, 930°C, 7650C and 6500C and corresponded to those experienced in the parent material during the submerged arc welding of 1.5 in. thick plate using a nominal heat input of 54kJ. in and a preheat temperature of 120°C. Double cycling utilized specific combinations of these thermal cycles. Half the total number of simulated specimens received post cycle heat treatment at 650°C. Charpy V-notch impact transition curves and tensile and hardness data have been obtained for each condition studied and structural examination has employed the carbon extraction replica technique in the electron microscope. The results indicate that the properties of the weld heat affected zone in HY-80 steel are superior to those obtained for QT-35 steel but inferior to those of the HY-80 parent material. The tempering bead technique has been shown to have, at best, only a limited tempering effect on the grain coarsened region of an edge bead heat affected zone and can also cause an increase in the susceptibility of this region to brittle fracture. Post weld heat treatment at 6500C is recommended, wherever possible, for HY-80 weldments. The calculation of weld heat affected zone thermal cycles, which correlate well with the practical situation, has been made possible by the development of a series of computer programs. These programs include functions to account for the release of latent heat from the solidifying weld pool and the variation of thermal conductivity with temperature.

Local heat transfer coefficients were measured experimentally when steam condensed on a cooled disc rotating on a horizontal axis when the surface of the disc was non—isothermal. A theoretical analysis was made for laminar film condensation on a cooled rotating disc with an isothermal surface in an infinite medium, using the integral form of boundary layer equations, and the theory was extended to include a non—isothermal surface also. From this theoretical analysis an expression was devised which can be used for calculating a theoretical value of the heat transfer coefficient. ,:ratios of the experimental to the theoretical values of heat transfer coefficient were studied and were found to be a function of the temperature drop across the film and the rotational speed but not of the radius. The theoretical solution for an isothermal surface was also compared with the more exact analysis by Sparrow and Gregg using the partial differential form of the boundary layer equations. The two sets of results show good agreement for the conditions under which steam usually condenses. To gain some knowledge of conditions in the condensate layer, an investigation was made of the flow of a thin film of water on a disc rotating on a vertical axis. This investigation showed that the film was not in plane laminar motion but had waves and vortices visible on its surface. Measurements of the maximum thickness were made for a range of disc speeds and Reynold's Numbers. Due to the waves the maximum film thicknesses were as much as 2.5 times the thickness predicted by a theoretical analysis based on plane laminar flow. These maximum film thicknesses show good agreement with Kirkbrides' measurements on wave flow on a thin film flowing down a vertical tube, and also with results derived from a theoretical formula given by Kapitsa. Due to the presence of waves, the measured heat transfer coefficients

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Biofuels have been identified as a potential short-term solution for reducing greenhouse gas (GHG) emissions from road transport. In order to ensure that they successfully deliver emission savings, the overall GHG balance of producing them must be calculated accurately, and compared with conventional fossil fuels. Life cycle assessment has dominated the process of assessing the GHG emissions from biofuels, though the results can vary significantly, due to real variation caused by biomass feedstock types, processing stages, and uncertainty in GHG emission from certain processes, but also due to how the GHG emission balance is calculated. This study has examined the relative importance of ‘scientific’ variation, and that caused by different methodological approaches. Three case studies with different methodological issues associated with accounting for their GHG emissions were developed. The variation in the LCA results due to the variability of inputs and outputs and from uncertainty of emissions from certain processes were assessed. The results showed that there is a high amount of variation in GHG emissions from fertiliser use, nitrous oxide emissions from soil and direct land use change. The GHG emissions from the full bioethanol production system were then calculated according to three specific GHG regulatory methodologies that are either currently used, or have been used in the UK for biofuels, products and services. This study has found that LCA methodology can cause considerable variation in differences in LCA results. The variation is sometimes caused by arbitrary decisions concerning how GHG emissions from a process should be accounted for and attributed to the main product. Variation in the results that is caused by methodology is comparable to that caused by scientific variation, which is ‘real’ that sometimes cannot be avoided without detailed study. The different results are due to the approach the methodology takes to LCA; whether the method tends toward attributional or consequential LCA. For reporting purposes, the European Directive’s Renewable Energy Directive (RED) states that attributional LCA (ALCA) is best as it provides a snapshot of emissions that are released, and attributable to the production and use of the product or service. A consequential LCA (CLCA), on the other hand is better suited for policy analysis as the potential impacts are applicable to a wider, even global scope. None of the methodologies studied completely adheres to ACLA or CLCA. It appears that they have confused the two within their calculation rules; therefore they do not fulfil their goal and scope. A critical assessment of the accounting methodology within RED is also performed here. The results show that the methodology is inconsistent and arbitrary, and currently too vague to be practical for GHG reporting. The results from this study indicate that the RED penalises the use of renewable energy and its calculation methodology does not support 2nd generation bioethanol production.

Global energy consumption is projected to increase the world over from 546 EJ in 2010 to an estimated 879 EJ in 2050 (Frei et al., 2013). Several factors contribute to this projected increase including growing global population, better quality of life globally, and continued electrification of services and products. Three serious issues arise from the increase in consumption and production, that is, fuel supply, and availability and increased anthropogenic emissions. To meet demand, developing countries, such as Pakistan, are investing in power generation research and technologies. Whilst a number of technologies are available, fluidised bed combustion (FBC) is an attractive technological option because of its ability to handle fuels with variable calorific content, moisture content, mineral content and high alkaline content. FBC offers reliable thermal output because of the large thermal mass (fluidised bed) associated with the method. This thesis set out to explore the possibility of using low grade fuels in FBC and investigate the impact the fuels have on agglomerate formation rates and combustion efficiencies. To explore the potential of FBC in the first experimental investigation presented in this thesis, a 350KW pilot scale FBC rig was used to perform a series of combustion experiments on ten Pakistani coal blends from the Northern Punjab salt rage coal seams. The coals had high sulphur and alkaline content and presented challenges in both combustion and emissions control. Operational variables including bed temperature, bed additives (limestone), sulphur: calcium fuel ratio, additive particle size and co-firing with wood biomass were employed to evaluate the effect of fuel blending, combustion and emissions optimisation. This thesis argues high SO2 emissions resulting from the combustion of high sulphur coals can be reduced in emission concentration when optimising operational variables. The high alkaline content, because of pyrite (FeS) concentrations in the fuel caused bed agglomeration and slagging in the beds. The investigation analyses the agglomerates and defines the mechanisms involved. This research allows for remedies and implementation choices when considering the coals application in full scale systems. It is not only coal which can be utilised. Further work investigated the effects of five different biomass fuels in FBC. Biomass can be classified as a CO2 neutral fuel as the CO2 released during combustion is relatively equal to the CO2 absorbed in the growth of the original plant. However, biomass is known to contain high concentrations of alkaline species such as potassium (K) and sodium (Na) which were shown in the literature to cause agglomeration. The biomasses were combusted in the FBC rig to evaluate the combustion, emissions, agglomerates, temperatures and pressure outputs associated with each fuel. Following tests the air distribution plate was modified to simulate both a uniform air distribution system and a non-uniform air distribution system. This allowed for comparisons of the fuels in a system with uniform air flows and non-uniform airflows/distribution which would be experienced in damaged systems. Thus, this thesis argues biomass is significant and relevant to industrial application and allowed for identification of significant chemical components in the agglomeration mechanisms of each fuel as well as establishing the performance of each fuel in variable systems. In order to understand the fundamental chemical and physiological makeup of the low-grade fuels it was necessary to conduct an extensive series of fuel characterisation. The fuel characterisation research undertaken yielded information as to the fuels energy content, chemical makeup, combustion characteristic and identify key components such as alkaline species associated with the negative mechanisms seen in pilot scale testing. In order to analyse the fuels x-ray fluorescence (XRF) was used. This technique identifies major and minor oxides in coal samples. However, as demonstrated in the fuel characterisation work, there were limitations, inaccuracies and repeatability issues when analysing low grade fuels with XRF. Thus, a significant effort was made to improve the sampling, ashing, XRF medium and normalisation process. The results of this research led to a more reliable XRF method for analysing low grade fuels and their bi-product of combustion which is applicable for any industry utilising these types of fuels and techniques. The final part of the investigations focused on the prediction of agglomeration and slagging tendencies of the fuels. This was done by applying the results seen in the pilot scale tests and the results of the fuel characterisation work with slagging indices and the application of a thermodynamic model (FACTSAGE). FACTSAGE can be used to predict slagging tendencies of the fuels by modelling chemical species released over temperature ranges. The results showed correlation between the theoretical results and the experimental results Together this research demonstrates the implications of using low-grade fuels in small scale FBC. This thesis explores how this research can then be used in full scale FBC operations. This thesis not only highlights the problems with using low grade fuels in FBC but suggests remedies and potential solutions to the problems based on the results from experimental data and FACTSAGE modelling. It also presents suggestions on how to continue development of the technology to reduce or avoid some of the difficulties in combusting low grade fuels.

The Externally Fired Combined Cycle (EFCC) is a clean coal technology power generation cycle that has the potential to compete with the currently cheaper and cleaner gas fired cycles. The critical component in the EFCC is an Ultra High Temperature Heat Exchanger (UHTHE) at maximum temperatures in the region of 1500C^ . A conceptual design of the UHTHE utilises bayonet element ceramic tubes in order to eliminate thermal stresses in the tubes and to facilitate sealing between the tubes and the tubesheets. A computer code known as COHEX has been developed to model the heat transfer and fluid flow processes in the heat exchanger by calculating the shell-side and tube-side outlet conditions for given hardware geometry and inlet conditions. A 250 kW capacity laboratory scale heat exchanger was designed and built to carry out an array of tests for validation of !t.e COHEX code. Comparison of the results showed that COHEX performed well, to within – I 0%, at predicting trends as heat exchanger inputs were varied, which validates the fundamental theory used in the code. However, the inaccuracy was apparent in the prediction of outlet values. This has been attributed to assumptions and equations used in COHEX that were made on the basis of modelling larger scale heat exchangers; precipitating errors when modelling the laboratory scale heat exchanger used in this study. A study was undertaken to assess the feasible process intensification devices (PIDs) for use in the heat exchanger based upon current ceramic fabrication technology. The dimensions of annular ring type PID were optimised for heat transfer and pressure drop through a computational fluid dynamics (CFD) study. Results of this study were used to perform experimental tests using PIDs on the heat exchanger which showed an average (over the smooth tube case) of a 23% increase in heat transfer enhancement with a 132% increase in pressure drop across the whole bayonet tube. The 180 ^ annular bend within the bayonet tube end-cap was identified as generating complex fluid flows. Further particle image velocimetry (PIV) experimental and theoretical (CFD) studies have identified the existence of recirculation zones and vortex shedding at the inlet to the annulus. The fluid dynamics theory utilised in COHEX has been shown to be oversimplistic at modelling the pressure drop incurred in the end-cap. The study concludes that COHEX, on condition of further validatory tests, is a viable tool to be used in the design of large scale heat exchangers such as the UHTHE. Also, that simple annular ring PIDs significantly enhance the heat transfer between the shell-side and tube-side flows.

Reciprocating energy-conversion devices, such as ORCs using reciprocating expanders or Thermofluidic Oscillators (TFOs), are promising technologies for the conversion of low- or medium-grade heat from waste heat, solar, or geothermal sources at power outputs below 100 kW. In this thesis, three topics associated with reciprocating machines are investigated. Firstly, a novel two-phase TFO engine concept, named ERPE, is modelled using a linear dynamic approach. Secondly, the use of working-fluid mixtures in ORCs is examined. Thirdly, unsteady heat-transfer losses associated with reciprocating motion are investigated. The ERPE is a theoretical TFO concept especially well-suited as a prime-mover in combined heat and power applications with power outputs below 10 kW. Based on thermal/fluid-electrical analogies, the theoretical engine is conceptualized in the electrical analogy domain as a linearized closed-loop active circuit model. By comparing model calculations with measurements of an existing prototype that is similar to the concept, it has been shown that the linear model provides realistic results. The effects of liquid inertia, viscous drag, hydrostatic pressure, vapour compressibility, and two-phase heat transfer in the various engine components/compartments are examined. Measures for improving engine performance are provided which demonstrate how this engine concept can outperform competing TFO solutions. The methodological approach implemented in this study can be used to guide the early-stage design and verification of these engines, while offering important guidelines for optimizing performance. For the second topic of the thesis, the use of working fluid mixtures in ORCs and their influence on thermodynamic performance is investigated. Mixtures can potentially reduce exergy losses due to their non-isothermal phase-change behaviour. The efficiency, power output, and cost are calculated as a function of working-fluid mixture composition in a mathematical model of a sub-critical, non-regenerative ORC. A new version of the statistical associating fluid theory equation of state is used to predict unavailable thermodynamic property data required for the ORC model. When unlimited quantities of cooling water are used, cycles with the lighter pure working fluid exhibit the highest efficiencies and power outputs and lowest costs. Only at low evaporation pressures do the investigated mixtures perform better than pure fluids. When the quantity of cooling water is constrained by the application, overall performance deteriorates and mixtures emerge as the optimal working fluids. A general conclusion from this work is that the choice of the optimum working fluid (pure or mixture) depends in a non-trivial and occasionally counter-intuitive fashion on the particular external heat source and heat sink conditions. The third topic revolves around unsteady heat-transfer losses which occur in reciprocating energy-conversion devices. These exergy losses occur even if the overall time-averaged heat transfer between the gas and the cylinder wall are zero. The specific aim is to compare the behaviour of real gases (such as those used in ORCs) to ideal gases by simulating an oscillating gas-spring in CFD . Results show that unsteady heat-transfer losses are not negligible as they can reach up to 30 % of the work required to compress the gas. Further results indicate that simple mono- and diatomic gases exhibit negligible differences between ideal and real-gas models. However, when considering heavier (organic) molecules, the ideal gas-models overestimate the pressure by as much as 20 % and underestimate the unsteady heat-transfer losses by 25 %. The increase in loss is caused by different compressibility factors of the gas during compression and expansion.

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