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 (CH₃)₂CHCH₂C (CH₃)₃ (iso-octane) and 3-pentanone CH₃CH₂COCH₂CH₃ 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_d^b, 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.