Modelling of fuel droplet heating in diesel engines

Abstract

This work focuses on the modelling of diesel fuel heating process taking into account temperature distribution inside droplets, and their semitransparency with a view to potential application to diesel engines. Suggested approaches to numerical modeling of droplet heating are based on new analytical solutions of the heat conduction equation inside a spherical droplet. They are obtained for constant, almost constant and arbitrary convection heat transfer coefficients (h) in the presence of thermal radiation. To take into account the semitransparency of diesel fuel droplets, the absorption spectra of four types of diesel fuel are studied experimentally in the range between 0.2 μm and 6 μm. It is shown that the solution based on the assumption of constant heat transfer coefficient is the most computer efficient for implementation into numerical codes. This approach is shown to be more effective than the approach based on the numerical solution of the discretised heat conduction equation inside the droplet, and more accurate than the solution based on the parabolic temperature profile model. Droplet evaporation and the effects of time dependent gas temperature and convection heat transfer coefficient are taken in the limit of slow evaporation. The relatively small contribution of thermal radiation to droplet heating and evaporation allows us to adopt a simplified model, which does not consider the variation of radiation absorption inside droplets. The suggested models for temperature gradient inside semitransparent droplets are applied to the investigation of the effects on droplet evaporation, break-up and the ignition of fuel vapour/ air mixture based on a zero-dimensional code. This code takes into account the heat and mass exchange between liquid and gas phases and describes the autoignition process based on the Shell model. Even in the absence of break-up the effect of temperature gradient inside droplets leads to a noticeable decrease of the total ignition delay. In the presence of the break-up process the temperature gradient inside droplets can lead to substantial decrease of the droplet evaporation time and ignition delay. The numerical algorithm based on the analytical solution of the heat conduction equation inside droplets for constant h is implemented into KIV A2 CFO code. The predictions of the new model were validated against available experimental data. It is recommended that both the effects of temperature gradient inside droplets and their semitransparencyare taken into account in computational fluid dynamics codes not only in diesel sprays but also in a wide range of spray applications.

Date of Award	Jun 2005
Original language	English