Thermal and mutual diffusivities of fuel-related binary liquid mixtures under pre-combustion conditions

In modelling heating and evaporation of fuel droplets, infra-droplet heat and mass transfer has to be considered. This is only possible with the help of reliable data for thermal and mutual diffusivity. At present, research activities in connection with the modelling of the evaporation of fuel droplets are based on theoretical diffusivity data. For their check and validation, there is, however, a lack of reliable experimental data. In this study, a dynamic light scattering apparatus which was especially developed for the simultaneous determination of both thermal and mutual diffusivity for fuel-related mixtures is presented. In the apparatus, fuel-related mixtures can be investigated under defined conditions in the compressed liquid phase close to saturation conditions in macroscopic thermodynamic equilibrium. Model binary mixtures composed of five different substances representative for fuel-related compounds were investigated at temperatures up to 523 K and close to their bubble point line. In detail, the mixtures investigated in the present study include 9 of the 10 possible binary combinations of isopentane, isooctane, toluene, n-decane, and ethanol. The results document that even under extreme conditions, thermal and mutual diffusivities are accessible with average expanded uncertainties (k = 2) of 8% and 6%. For all systems, concentrations, and temperatures, the thermal diffusivity changes only slightly. In contrast, the mutual diffusivity varies by more than two orders of magnitude from about 10(-10) to 10(-8) m(2).s(-1). For mixtures of isooctane and toluene, the concentration dependency of the mutual diffusivity reveals a nonideal behavior which becomes more pronounced with increasing temperature. For all binary mixtures, the temperature dependency of the mutual diffusivity can be correlated well with the vapor pressure of the individual components. The presence of the biofuel ethanol strongly slows down the molecular diffusion process at low temperatures.