Soot and gaseous species formation in a water-in-liquid fuel emulsion spray—a mathematical approach

Abstract

The use of water-in-fuel emulsions as the combustible matter in various combustion systems has been advocated as a means of achieving cleaner combustion. Several experimental investigations have shown that adoption of this technique will result in marked reductions in NOx emissions and in the levels of smoke and soot particulate produced from spray-based combustion systems. Little effort has been directed towards modelling the combustion of water-in-oil emulsion, though understanding the underlying reasons for the aforementioned improvements will be enhanced by modelling the processes involved. This paper attempts to provide an understanding of the effects of the presence of water in the fuel on spray combustion and soot and gaseous product formation by modelling the relevant processes.The simplified model described below is capable of predicting soot and gaseous species concentrations in a burning water-in-liquid fuel emulsion. It is based on the Adler and Lyn treatment of an evaporating spray in a co-flowing air stream where the spray domain is considered to be a continuous medium allowing the conservation equations of continuity, momentum and energy to be represented in a classical differential form. The evaporating droplet is modelled through a quasi-steady gas phase approach enabling the lifetime of the droplet to be determined. The presence of water inside the emulsified fuel droplet is accounted for and the onset of micro-explosions is predicted by the droplet model. The spray analysis incorporates the droplet model and makes use of a probability function to determine to what extent the micro-explosion predicted by the droplet analysis affects the spray development. The computer model is organized in distinct subroutines facilitating easy use and the possibility of replacing one sub-model by another (such as soot formation sub-model). This allows for parametric studies and for prediction of the burning spray behaviour at various conditions of water content, pressure, temperature and input air–fuel ratio. A collection of results obtained using the model are presented and discussed in the paper. One particular set is compared with experimental results obtained on a specially designed combustion chamber running at conditions similar to the ones used in the model.