Modelling of biofuel droplets dispersion and evaporation

Alternative renewable fuels are more and more important due to increasing oil prices, environmental concerns and their potential to preserve the agricultural activity. For the case of biofuels derived from agricultural crops, several possibilities can be considered, such as raw oil, oil-derived methyl esters, bioethanol or mixtures with conventional fuels (diesel fuel or gasoline). The straight use of vegetable oils and their derivates poses some problems on the mixture formation process and droplet evaporation, that are essentially derived from their much higher viscosity (up to five times the viscosity of the diesel fuel) and higher boiling temperature. The present paper presents a numerical study on evaporating biofuel droplets injected through a turbulent cross-stream. This study uses an Eulerian/Lagragian approach to account for turbulent transport, dispersion, evaporation and coupling between both processes in practical spray injection systems, which usually include air flows in the combustion chamber-like swirl, tumble and squish in I.C. engines or cross-flow in boilers and gas turbines. An array of evaporating biofuel droplets through a cross-flow is studied, and a comparison of the droplet fuel dispersion and evaporation with conventional fuels is performed. The results obtained with DF-2 and RME showed that an homogeneous mixture can only be obtained with very high levels of pre-heating, and the use of ethanol (obtained from sugar or starch crops) may be a better alternative in ignition-assisted combustion systems, while RME can be successfully used as an alternative fuel in applications that utilize diffusion flames.