The presented work attempts to extend the conditional moment closure method for noon-premixed. turbulent combustion to predict extinction and reignition phenomena in turbulent flames. The conditional moment closure method is one of a class of conserved scalar modelling approaches in turbulent non-premixed combustion. where chemistry is treated as mainly dependend on the mixing of oxidizer and fuel. However. as designers of combustion devices aim for higher turbulence rates to enhance mixing and promote combustion, chemical conversion is not solely determined by the rate at which fuel and oxidizer are mixed, but kinetic effects become important. Therefore it is necessary in these cases. to consider a second variable to govern the evolution of the chemical system. This variable will parameterize the chemical conversion process from cold. mixed reactants at fixed eguivalence ratio to an eguilibrium state. Equations describing the chemical system as a function of these two variables, the conserved scalar, commonly referred to as mixture fraction and the progress variable. can be derived and constitute the doubly conditioned moment closure equations. However, solution of this set of equations is computationally expensive and key parameters describing the rate of dissipation of the progress variable, which is a reactive scalar, are not yet fully understood. By considering conditional fluctuations of the progress variable, applying simple relationships for scalar dissipation and using a pre-computed functional dependence of conditional moments on the progress variable, the effect of double conditioning on the chemical source term and on the overall chemistry predictions can be examined. The methodology is tested for its capability to predict the turbulent. piloted flames of the Sandia D-F series. These laboratory flames show an increasing degree of local extinction and reignition due to varying turbulence levels. Hence they provide an ideal benchmark for the study of models trying to predict these phenomena.

The energy penalty for post‐combustion carbon dioxide capture from fossil‐fired power plants can be greatly reduced ‐ independently of the intrinsic heat of regeneration of the solvent used ‐ by effective thermodynamic integration with the power cycle. Yet expected changes in electricity generation mix and the current immaturity of post‐combustion capture technology are likely to make effective thermodynamic integration throughout the operating life of such plants a challenging objective to achieve because of a requirement for extensive part‐load operation and also for matching to future technology improvements. Most previous published studies have, however, focused on base‐load operation of the power cycle and the carbon dioxide capture plant and with the assumption of a fixed technology. For carbon dioxide capture‐ready plants the characteristics of the capture plant are also not known when the plant is designed. The plant must operate initially without capture at a similar efficiency to ‘standard’ plants to be competitive. Capture‐ready plants then also need to be able to be retrofitted with unknown improved solvents and to be capable of integration with a range of future solvents. This study shows that future upgradability for post‐combustion capture systems can be facilitated by appropriate steam turbine and steam cycle designs. In addition fossil‐fired power plants with postcombustion capture may need to be able to operate throughout their load range with the capture unit by‐passed, or with intermediate solvent storage to avoid the additional emissions occurring when the absorption column is by‐passed. Steam cycles with flexible steam turbines can be adequately designed to accommodate for part‐load operation with these novel operating conditions and with rapid ramp rates. Several approaches for effective capture‐ready pulverised coal and natural gas plants are also described. These achieve identical performance before retrofit to a conventional plant with the same steam conditions, but have the potential to perform well after capture retrofit with a wide range of solvents, at the expense of only a small efficiency penalty compared to hypothetical plants built with perfect foreknowledge of the solvent energy requirements. For existing plants that were not made capture‐ready, and provided sufficient space is available and other physical limits are not too constraining, ways to achieve effective thermodynamic integration are also discussed.

This research concerns the Large Eddy Simulation (LES) of turbulent combustion in both the premixed and the non-premixed regime. Non-premixed hybrid bluff-body/swirl flames are simulated by means of a steady flamelet model (Flamelet-LES) based on detailed chemical kinetics. LES of lean premixed twin flames stabilised on a turbulent opposed jet (TOJ) burner are carried out using an algebraic Flame Surface Density model (FSD-LES) and a newly developed model based on Linear Eddy Mixing (LEM-LES). Isothermal swirling flow at a medium Reynolds and swirl number is simulated first and the LES model is shown to accurately predict the velocity statistics and the complex flow field governed by vortex breakdown and two recirculation zones. The Flamelet- LES model is subsequently used to simulate a low speed swirling methane flame and the capability of the model to predict downstream recirculation, vortex breakdown and central jet precession in the presence of heat release is demonstrated. The simulation of two high speed hydrogen/methane swirl flames with the Flamelet-LES model shows that some quantitative predictions of this challenging test case for combustion simulation can be achieved, while the overall predictions are not satisfactory. The flamelet approach is found sensitive to minor errors in the mixing field which strongly affect the simulation results due to the highly non-linear mixture fraction/density relationship. Non-reacting simulations of turbulent opposed jet flows at moderate Reynolds number are performed and compared to experimental reference data. The inclusion of the flow field inside the nozzles into the computational domain is shown to yield accurate predictions of the velocity statistics between the nozzles. For these predictions the detailed knowledge of the initial jet development region near the turbulence generating plates is vital and provided by PIV measurements inside a glass nozzle. FSD-LES of the twin premixed TOJ flames show that the velocity statistics, both along the burner axis and the stagnation plane, can be predicted to high accuracy. However, the algebraic flame surface density model employed in the present study requires the adjustment of a model parameter and as a result, predictions of the turbulent burning velocity cannot be achieved. The comparison of two different interpretations of the FSD model show that a formulation using an additional diffusion term allows for a better resolution of the premixed flames in LES than the original formulation without diffusion. A complex LEM combustion model is first developed as a Stand-Alone approach to simulate premixed combustion and subsequently coupled to LES. The LEM-LES model requires a number of sub-models to represent the effects of sub-grid stirring, finite-rate chemistry, sub-grid expansion, 3D convection (splicing) and flame propagation which are described in detail. The LEM-LES model is – to the author’s knowledge – the first attempt to simulate premixed flames with finite-rate chemistry in incompressible turbulent flow. Preliminary results from the application of LEM-LES to the premixed twin TOJ flames are reported and show a high sensitivity to the 3D convection model and the requirement to improve the splicing procedure for premixed flames in anisotropic turbulent flow. The difficulty to accurately resolve the turbulent flow field by LES while simultaneously accommodating a premixed flame of finite thickness on the LEM sub-grid is found to be a challenge for the LEM-LES of premixed TOJ flames.

The present research focuses on studying an isothermal spray to understand the mechanism of interaction between droplets and turbulent air flow, and an evaporative spray to evaluate the group evaporation of droplets as opposed to single droplet evaporation. The thesis describes the development and application of two novel experimental techniques for simultaneous characterization of droplet and gaseous phases in isothermal and evaporative sprays respectively. Both approaches use the out-of-focus imaging technique, Interferometric Laser Imaging for Droplet Sizing (ILIDS), for planar measurements of droplet size and velocity. The in-focus imaging techniques Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) are respectively combined with ILIDS for simultaneous measurement of gas flow characteristics in an isothermal spray and vapour concentration distribution in an evaporative spray. Combination of either of the two optical arrangements results in a discrepancy in the location of the centre of a droplet leading to erroneous identification of the droplets in the PIV/PLIF images. This issue has been addressed and a method is proposed to reduce the droplet positioning error. The coupling between the droplet and gas phases in the isothermal spray is explained by evaluating several statistical quantities, the most important being the spatial correlation coefficients of the droplet-gas velocity fluctuations obtained conditional on droplet size classes. The effect of anisotropy and gravity on the momentum transfer between the two phases are studied. The gas flow eddy structures are extracted by applying Proper Orthogonal Decomposition (POD) on the instantaneous gas velocity data and the selective influence of the large scale eddy structures of the gas phase flow on the droplet-gas flow interaction are examined. In order to study the effect of inter droplet spacing on the droplet evaporation rate, experiments are first performed for the mono-sized droplet stream. The smaller inter droplet spacing of the larger droplet sizes causes the vapour to surround the droplet stream leading to droplet group evaporation. The smaller magnitude of the mean group evaporation number, evaluated at different radial locations in the evaporative acetone spray, suggests the mode of evaporation is in the range of regimes of individual to group evaporation. It is shown that the assumption of uniform droplet spacing in the theoretical expressions for the evaluation of the group evaporation number always leads to overestimation of the group evaporation number.

No description available.

Carbon dioxide (CO2) capture and geological storage (CCS) is recognised as one of themain options in the portfolio of greenhouse gas (GHG) mitigation technologies beingdeveloped worldwide. The CO2 capture and storage technologies require significantamounts of energy during their implementation and also change the environmentalprofile of power generation. The holistic perspective offered by Life Cycle Assessment(LCA) enables decision makers to quantify the trade-offs inherent in any change to thepower production systems and helps to ensure that a reduction in GHG emissions doesnot result in significant increases in other environmental impacts. Early LCA studies ofpower generation with CCS report a wide range of results, as they focus on specific CO2capture cases only. Furthermore, previous work and commercial LCA software have arigid approach to system boundaries and do not recognise the importance of the level ofdetail that should be included in the Life Cycle Inventory (LCI) data. This research developed a complete LCA framework for the ?cradle-to-grave?assessment of alternative CCS technologies in carbon-containing fuel power generation. A comprehensive and quantitative Life Cycle Inventory (LCI) database, which modelsinputs/outputs of processes at high level of detail, accounts for technical and geographicdifferences, generates LCI data in a consistent and transparent manner was developedand arranged and flexible structure. The developed LCI models were successfully applied to power plants with alternativepost-combustion chemical absorption capture and oxy-fuel combustion capture. Theresults demonstrate that most environmental impacts come from power generation withCCS and the upstream process of coal production at a life-cycle perspective. LCAresults are sensitive to the type of coal used and the CO2 capture options chosen. Moreover, the models developed successfully trace the fate of elements (including tracemetals) of concern throughout the power generation, CO2 capture, transport andinjection chain. Monte Carlo simulation method combined with the LCI models wasapplied to quantify the uncertainty of emissions of concern. A novel analytical framework for the LCA of CO2 storage was also developed andapplied to a saline aquifer storage field case. The potential CO2 leakage rates werequantified and the operational and geological parameters that determine the ratio of CO2leakage total volume of CO2 injected were identified.

Alternatives to petroleum-derived fuels are receiving significant interest in order to reduce dependence on finite resources of fossil fuels and to lower fossil-derived CO2 emissions. The present study addresses the production of bio-oil from biomass pyrolysis, one of the potential renewable substitutes to petroleum-derived fuels. The first objective of this work was to investigate the effect of pyrolysis operating parameters, i.e. temperature, heating rate and pyrolysis time, on product distributions in a wire-mesh reactor (WMR) which was designed to minimise secondary reactions. It has been found that high heating rate promotes melting of biomass and this facilitates volatile ejection, thereby resulting in high yield of large bio-oil molecules and high combustion reactivity of residual char. Maximum bio-oil yield is obtained at 500 °C for both rice husk and beech wood whereas a relatively low pyrolysis temperature, e.g. 350 °C, does not allow complete pyrolysis to take place. Chars produced from long holding time and high temperature tests show a decrease in the TGA combustion reactivity which is due to thermal annealing. The comparison between bio-oils obtained from the WMR and Gray-King retort demonstrates the impact of reactor configuration on the variation of bio-oil properties. The unstable nature of bio-oils provided the second objective of this work. The ageing behaviour of bio-oil and the use of organic solvents to improve the bio-oil properties have been investigated. Polymerisation plays a key role in bio-oil ageing and is enhanced by high temperature. Only slight changes in functional groups have been observed by 13C-NMR and FT-IR. UV-F results suggest that phenolic resin formation is one of the polymerisation reactions occurring during bio-oil ageing. With the addition of methanol and acetone to bio-oil, the extent of polymerisation decreases and NMR results indicate the formation of hemiacetals/acetals.

No description available.

A three dimensional, coupled computational fluid dynamics and finite element model of a single, anode supported solid oxide fuel cell has been developed in order to predict the probability of failure of the ceramic components subjected to an idealised operating duty cycle. The duty cycle represents cooling from sintering, warming to a uniform temperature of 800°C where anode chemical reduction takes place, operation at low, medium and high power and finally cooling to room temperature. The Star CDTM computational fluid dynamics code provided the platform to determine the temperature distribution throughout the operating fuel cell by solving the conservation equations for energy, mass and momentum, with additional subroutines written to account for species transport, electrochemical reactions and heat generation. An Abaqus™ finite element model used the temperature distribution predicted by the computational fluid dynamics model at low, medium and high power to solve for the thermal stress distribution for individual cases and throughout the duty cycle. The finite element model included the effects of thermal expansion, residual stress from manufacture, material properties changes due to chemical reduction of the anode and viscoplastic creep. The maximum principal stress in the anode support layer at 800°C and low, medium and high power was found to be 5.0, 26.5, 33.2 and 39.8 MPa respectively. The stress analysis results were used to determine the time independent and time-dependent (accounting for sub-critical crack growth) probability of failure, and showed that over the duty cycle sub-critical crack growth significantly increased the predicted probability of failure in the anode support layer from less than 1 x 10⁻¹² to 0.54, and in the cathode layer from 1.28 x 10⁻⁵ to 1.24 x 10⁻³. The probability of failure of SOFC ceramic components is thus shown to be both time and history dependent.

The potential of algal biofuels has been technically and experimentally confirmed with laboratory- and pilot-scale studies in past literature. However, the most important factor now is to confirm that algal cultivation for biofuels and other end-products is economically feasible on the large, commercial scale. The ALGADISK project aimed to produce a novel biofilm-based photo-bioreactor with the aims of CO2 capture and making valuable products such as biofuel, economically viable. This thesis aimed to investigate and provide substrates in which algae biofilm is stimulated and increased. Polyelectrolyte (PE) coatings adsorbed onto cost-effective polymers were investigated, based on the strategy of electrostatic attraction. It was found that the algae species charge density and cell wall functional groups composition affected attachment onto charged PE coatings. Two coatings labeled C1 and C3 were selected due to their promising growth results with the strains C.sorokiniana, C.vulgaris and S.obliquus. Harvesting growth results showed inconsistent regrowth due to the lack of textured structure. Sandpapering the surface with certain grades was found to improve regrowth and consistency. Surface roughness did not show correlation to initial attachment of algae or strength of attachment. It was shown instead that surface roughness improved long-term growth As part of the aims of the ALGADISK project, the coatings large scale potential and cost was optimized. It was found that airbrushing rather than dip-coating, reduced the amount of PE solution needed drastically. Furthermore, photo-cross- linking with UV exposure enhanced the strength of C1 according to scratch and wear data. Lastly, the physico-chemical properties of both algae and substrates were examined in order to examine the thermodynamic model for algae adhesion prediction. It was found that the two thermodynamic approaches tested did not predict algae adhesion results with good accuracy. However, it was revealed that there could be a possible link between the substrate physico-chemistry and lipid content found in the biofilm attached. It was found that the less favorable the predicted thermodynamic conditions the higher the lipid content.