Transported PDF Modelling of Soot-Radiation Turbulence-Chemistry Interactions

摘要

This thesis concerns modelling of diesel-relevant spray combustion, with a focus on the interconnected aspects of turbulence, chemistry, radiation and pollutant formation, particularly formation of soot. The approach taken to the modelling exploits the composition transported probability density function (TPDF) method, which has particular advantages for the consideration of pollutant formation and radiation that are not straightforwardly available in other methods. In particular, predictions of pollutant formation are facilitated by the chemical reaction source term appearing in closed form in the equations, while for predictions of radiation, emission source terms also appear in closed form. Focussing on a series of experimental target flames of the Engine Combustion Network (ECN) involving spray combustion of n-dodecane in constant volume chambers (known as spray A and its parametrics) the thesis makes contributions in assessing the TPDF methodology in terms of its ability to predict soot, radiation, and effects of multiple injections. In the approach adopted, soot is modelled with a two-equation acetylene precursor model, while where radiation is considered the discrete ordinates method is adopted and compared to an optically thin model. The TPDF mixing model employed is interaction by exchange with the mean. A particular focus is applied on the effects of turbulent fluctuations, known as turbulence-chemistry interactions (TCI) and turbulence-radiation interactions (TRI), which are assessed by comparison to simple well mixed (WM) models which ignore fluctuations. The key aspects of the study are as follows. First, a study of flame structure and soot formation is conducted for spray A (with a single long injection) under various ambient conditions. A wide array of chemical mechanisms is tested and evaluated, with clear conclusions about the advantages and limitations of the available mechanisms being established. For two selected reduced mechanisms, comparisons to experiment for overall flame structure as revealed by 355 nm planar laser-induced fluorescence (PLIF, imaging formaldehyde and polycyclic aromatic hydrocarbons) and shlieren images is reported, with generally good agreement. A detailed analysis of soot formation processes is presented and quantitative comparisons to experimental soot measurements reported. Soot is predicted well in the baseline spray A condition during the quasi-steady phase, but the initial transient phase needs further work and some off-baseline conditions are not predicted particularly well. TCI effects on soot were found in the present study to be relatively minor with the present choices for mixing model and other parameters. Next, the effects of radiation and turbulence-radiation interactions (TRI) are assessed and evaluated in terms of effects on pollutant formation. Radiation and TRI were found to play relatively minor roles in terms of both soot and NO; however, in contrast to soot, NO was significantly affected by TCI. Finally, cases having multiple injections are studied. The results are compared to experimental 355nm PLIF and reveal good agreement in qualitative trends to the experiments, with key differences between single and multiple injection cases being reproduced. The results are analysed further with respect to presumed probability density functions (PDFs). Overall the work progresses the TPDF approach both in terms of incorporating additional physics and by providing significantly enhanced comparisons to experiment compared with previous work for diesel sprays.