Modelling of the heating and evaporation of fuel droplets

Abstract

The results of a comparative analysis of liquid and gas phase models for fuel droplets heating and evaporation, suitable for implementation into computational fluid dynamics (CFD) codes, are presented. Among liquid phase models, the analysis is focused on the model based on the assumption that the liquid thermal conductivity is infinitely large, and the so called effective thermal conductivity model. Seven gas phase models are compared. These are six semi-theoretical models, based on various assumptions, and a model based solely on the approximation to experimental data. It is pointed out that the gas phase model, taking into account the finite thickness of the thermal boundary layer around the droplet, predicts the evaporation time closest to the one based on the approximation to experimental data. The values of the absorption coefficients of gasoline fuel (BP Pump Grade 95 RON ULG), 2,2,4-trimethylpentane (CH3)2CHCH2C(CH3)3 (iso-octane) and 3-pentanone CH3CH2COCH2(CH3)3 have been measured experimentally in the range of wavelengths between 0.2 μm and 4 μm. The values of the average absorption efficiency factor for all fuels have been approximated by a power function aRdb, where Rd is the droplet radius. a and b in turn have been approximated by piecewise quadratic functions of the radiation temperature, with the coefficients calculated separately in the ranges 2 - 5 μm, 5 - 50 μm, 50 - 100 μm and 100 - 200 μm for all fuels. This new approximation is shown to be more accurate compared with the case when a and b are approximated by quadratic functions or fourth power polynomials of the radiation temperature, with the coefficients calculated in the full range of 2 - 200 μm. Results of experimental studies of heating and evaporation of monodisperse ethanol and acetone droplets in two regimes are compared with the results of modelling. It is pointed out that for relatively small droplets the experimentally measured droplet temperatures are close to the predicted average droplet temperatures, while for larger droplets the experimentally measured droplet temperatures are close to the temperatures predicted at the centre of droplets. All the developed models have been implemented into the KIVA-2 CFD code and validated against available in-house experimental data referring to spray penetration and ignition delay in Diesel engines.

Date of Award	Oct 2007
Original language	English
Awarding Institution	University of Brighton