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Abstract
The thesis considers computational modelling targeted at spray and combustion phenomena in diesel engines. Accurate predictions of these phenomena are needed to enable timely development of diesel engines having lower emissions and higher efficiency. However, the modelling required stands out as a grand challenge problem, involving coupled phenomena of high Weber number spray, high Reynolds number turbulence, and complex multi-step kinetics, and having multiple combustion modes and regimes occurring in a single flame.
This thesis considers among the first attempts to model diesel spray combustion with the transported probability density function (TPDF) method. The TPDF approach has significant potential to become a useful tool in the automotive industry and address the above challenges. The principal advantage of the TPDF method is that the nonlinear chemical source term appears in closed form, which is expected to be an advantage for the treatment of finite rate chemical processes such as ignition and pollutant formation. Furthermore, it does not require different modelling to be applied between different combustion modes such as ignitions, premixed flames, and nonpremixed flames; and can also, in principle, treat both slow and fast chemistry limits. The TPDF method has been demonstrated, with considerable success, mostly for gaseous atmospheric pressure laboratory flames, but prior to this thesis, demonstration for spray flames at practical diesel engine conditions has been lacking.
In this thesis:
• The TPDF model is demonstrated and validated against experimental spray flame databases in ambient conditions representative of diesel engines - this being among the very first efforts to address this problem. The databases chosen were n-heptane and n-dodecane spray flames in constant volume and constant flow chambers, which were available through an international collaboration known as the Engine Combustion Network.
• Choices for the sub-models involved are recommended, principally the molecular mixing and chemical kinetic models. Excellent results are demonstrated with the TPDF method given appropriate submodel choices.
• The importance of considering interactions of turbulence and chemistry is assessed and found to be significant, particularly in less reactive conditions. This provides guidance on whether or not these interactions should be considered in modelling, and demonstrates how they affect modelling outcomes.
• Basic physical and chemical characteristics of ignition and combustion of diesel spray flames are comprehensively investigated by a detailed analysis of the TPDF results. Several experimentally observed features are reproduced and/or explained, while other new features which have as yet not been observed experimentally are also noted.