Droplet Evaporation Characteristics of Diesel and Biodiesel Blends on Hot Surface

Abstract The worldwide drive to move towards renewable energy sources has made biodiesels an attractive proposition for development in replacement of diesel as a fossil fuel for the internal combustion engine. This study explores droplet evaporation characteristics for both diesel and three biodiesels: namely, sunflower-, corn- and palm-oil derived fuels; when tested on hot stainless steel and aluminium surfaces. Experimental work was carried out across a surface temperature spectrum of between 100 and 460 °C and with droplets of between 100 and 500 µm in diameter, and assessed the impact of type of surface, droplet size and fuel used upon time to evaporation and change between evaporation regime. The distinctive evaporation regimes recorded were film evaporation, nucleate boiling, transition boiling, and the spheroidal or Leidenfrost evaporation, with each presenting a different interaction mode between droplets and surfaces. Regime mapping was conducted in relation to surface temperature, and the results point to an extended evaporation time for biodiesels in comparison with diesel, based on their greater boiling point and more viscous nature. The longest evaporation time was identified for palm biodiesel, with sunflower biodiesel evaporating comparatively more quickly than the other biodiesels. A significant impact was observed from surface material, with droplets evaporating in a shorter time than on the more thermally conductive aluminium compared to stainless steel. The findings also show that decreased droplet size shortened time to reaching a critical regime and shorter overall evaporation time. Initiation of spheroidal evaporation differed by fuel, with biodiesels in general reaching this stage at a higher temperature. The results, contribute to detailed understandings of thermal properties and regime-change behaviours among biodiesels, and can be used to inform efforts to reduce emissions, increase efficiency in spray combustion, and optimize heat transfer in biodiesel applications.