W.A. Abdelghaffar

Radiative Heating of Semi-Transparent Diesel Fuel Droplets

Absorption spectra of four types of diesel fuel are studied experimentally in the range between 0.2 μm and 6 μm. The ageing process of fuels is simulated by prolonged boiling. The average absorption efficiency factor of droplets is assumed to be proportional to ar b d, where r d is the droplet radius, and a and b are polynomial functions of external gas temperature. Explicit expressions for a and b are derived for diesel fuel droplets in various realistic droplet radii and external gas temperature ranges for all four types of fuel.

Approximate Analysis of Thermal Radiation Absorption in Fuel Droplets

The values of absorption coefficients of gasoline fuel (BP Pump Grade 95 RON ULG (research octane number unleaded gasoline)), 2,2,4-trimethylpentane (CH 3 ) 2 CHCH 2 C(CH 3 ) 3 (iso-octane) and 3-pentanone CH 3 CH 2 COCH 2 CH 3 have been measured experimentally in the range of wavelengths between 0.2 μm and 4 μm. The values of the indices of absorption, calculated based on these coefficients, have been compared with those previously obtained for low sulphur ESSO AF1313 diesel fuel. These values are generally lower for pure substances (e.g., iso-octane and 3-pentanone) than for diesel and gasoline fuels. The values of the average absorption efficiency factor for all fuels are approximated by a power function aR b d , where R d is the droplet radius a and b in turn are approximated by piecewise quadratic functions of the radiation temperature, with the coefficients calculated separately in the ranges of droplet radii 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 whole range 2-200 μm. This difference in the approximations of a and b, however, is shown to have little effect on modeling of fuel droplet heating and evaporation in conditions typical for internal combustion engines, especially in the case of diesel fuel and 3-pentanone.

Fuel Droplet Heating and Evaporation: Analysis of Liquid and Gas Phase Models

Recently developed liquid and gas phase models for fuel droplet heating and evaporation, suitable for implementation into computational fluid dynamics (CFD) codes, are reviewed. The analysis is focused on the liquid phase model based on the assumption that the liquid thermal conductivity is infinitely large (infinite thermal conductivity (ITC) model), and the so called effective thermal conductivity (ETC) model. Seven gas phase models are compared. 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 of experimental data. In most cases, the droplet evaporation time depends strongly on the choice of the gas phase model. The dependence of this time on the choice of the liquid phase model, however, is weak if the droplet break-up processes are not taken into account. Corrections to Newtons law for droplet transient heating are discussed. For the values of parameters relevant to diesel engines, the values of these corrections were shown to be significant. Recent kinetic models for droplet evaporation into a high pressure background gas are reviewed. It is recommended that the kinetic effects are taken into account when accurate analysis of diesel fuel droplet evaporation is essential. A new dynamic decomposition technique for a system of ordinary differential equations is reviewed.