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The thesis comprises of a thorough assessment of turbulent non-premixed combustion modelling techniques, emphasising the fundamental issue of turbulence-chemistry interaction. The combustion models studied are the flame-sheet, equilibrium, eddy breakup and laminar flamelet models. An in-house CFD code is developed and all the combustion models are implemented. Fundamental numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD. The combustion models are evaluated for a number of fuels ranging from simple H2/CO and CO/H2/N2 to more complex Cl4/H2 burning in bluff body stabilised burners at different inlet fuel velocities. The bluff body burner with its complex recirculation zone provides a suitable model problem for industrial flows. The initial and boundary conditions are simple and well-defined. The bluff body burner also provides a controlled environment for the study of turbulence-chemistry interaction at the neck zone. The high quality experimental database available from the University of Sydney and other reported measurements are used for the validation and evaluation of combustion models. The present calculations show that all the combustion models provide good predictions for near equilibrium flames for temperature and major species. Although the equilibrium chemistry model is capable of predicting minor species, the predictive accuracy is found to be inadequate when compared to the experimental data. The laminae flamelet model is the only model which has yielded good predictions for the minor species. For flames at higher velocities. the laminar flamelet model again has provided better predictions compared to predictions of other models considered. With different fuels, the laminar flamelet model predictions for CO/H2/N2 fuel are better than those for CH4/H2 fuel. The reasons for this discrepancy are discussed in detail. The effects of differential diffusion are studied in the laminar flamelet modelling strategy. The flamelet with unity Lewis number is found to give a better representation of the transport of species. The laminar flamelet model has yielded reasonably good predictions for NO mass fraction. The predictions of NO mass fraction are found to be very sensitive to differential diffusion effects. This study has also considered the issue of inclusion of radiative heat transfer in the laminar flamelet model. The radiation effects are found to be important only where the temperature is very high. The study undertaken and reported in this thesis shows that the presently available laminar flamelet modelling concepts are capable of predicting species concentrations and temperature fields with an adequate degree of accuracy. The flamelet model is also well suited for the prediction of NO emissions. The inclusion of radiation heat transfer has enhanced the predictive capability of the laminar flamelet model.

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Automotive catalysts are the most effective short-term answer to air pollution from automobiles. Since strict control of exhaust emissions is, or will be,covered by legislation in most developed countries in the world, catalytic devices will be increasingly fitted to cars. There is consequently an urgent need for the development of catalysts that will not compete for scarce precious metal resources. A number of problems have already been identified in connection with base metal catalysts but quantitative investigations are lacking. The base metal reduction catalysts developed by Imperial Chemical Industries Limited, catalysts and Chemical Group, in collaboration with the Air Pollution Control Laboratory, B L Cars Limited for automotive emission control, are susceptible to de-activation by three major mechanisms. These are: physical loss of the wash-coat (a high surface area coating which supports the active species), aggregation of the active species and poisoning by fuel and engine oil additives. This thesis is especially concerned with the first two of these and attempts to indicate the relative magnitude .of their effect on the activity of. the catalysts. Aggregation of the active species or sintering, as it is loosely called, was studied by using impregnated granules to overcome effects due to the loss of the wash-coat. Samples were aged in a synthetic exhaust gas, free from poisons, and metal crystallite sizes were measured by scanning-electron microscopy. The increase in particle size was correlated with the loss in catalytic activity. In order to maintain a link with the real conditions of service a number of monolithic catalysts were tested in an engine-dynamometer and several previously tested endurance catalysts were examined. A mechanism is proposed for the break-up and subsequent 10s.5 of the wash-coat and suggestions for improved resistance to loss of the' coating and active species are proposed.

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The diesel engine currently accounts for 32 per cent of the new passenger car sales in Europe. In the US, diesel-power is responsible for 94 per cent of all freight movement. Comparing European Stage III standard petrol and diesel passenger car emissions, diesel NOx emissions are still considered a concern. This thesis investigates the mechanisms by which oxides of nitrogen are formed during diesel combustion. It reviews the current methods of controlling NOx emissions, such as retarding fuel injection timing, exhaust gas re-circulation (EGR), water injection and exhaust after-treatment. Modelling using a phenomenological model, is used to demonstrate the extended Zeldovich mechanism and formation trends, the effects of EGR and the significance of the Zeldovich mechanism rate constants. Modified Zeldovich rate constants are proposed to improve the correlation to measured data. Clearly, EGR is currently the most effective method of reducing NOx emissions from passenger car diesel engines. The way EGR works in suppressing NOx formation is reviewed in detail. Experimentation on a 1.8 litre inline 4-cylinder 4-valve per cylinder DI diesel with a variable nozzle turbine (VNT) turbocharger was used to demonstrate the concept of "additional" EGR on this small automotive engine. "Additional" EGR is the concept whereby a proportion of the EGR is added to the total charge, so that the volumetric efficiency increases as EGR is introduced. By using "additional" EGR, the benefits of lower NOx emissions combined with reduced particulates emissions and improved fuel consumption were clearly demonstrated at two test conditions. The reasons for achieving lower NOx emissions when using a VNT turbocharger and EGR have been explained. Finally, several methods of calculating EGR proportion were used and compared against true mass flow. The use of a CO2 balance was found to be the most accurate method.

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No description available.