Evaporation of free falling droplets of binary alkane-ethanol blends

The admixture of ethanol to gasoline has become a key factor to reduce the reliance on fossil fuels. In this context, the distinct occurrence of component-by-component evaporation compromises the efficient and the low-emission operation of internal combustion engines due to inhomogeneous mixture formation. In order to meet these challenges, a more precise understanding of the mechanisms causing the component-by-component evaporation is inevitable. Therefore, in the present study the evaporation behaviour of binary alkane-ethanol blends has been experimentally investigated. A high-resolution Raman-setup is combined with a fast trigger system to quantify the substance composition of the gaseous phase directly behind free falling droplets. Considerable preparatory work has been carried out to achieve controlled and consistent measurement conditions and to achieve a quantitative and a substance related evaluation under cold start conditions. Binary blends of ethanol and n-hexane, iso-octane or n-decane are studied. A systematic variation of the droplet temperature, the falling distance, the droplet generating frequency and the relative ratios of the applied substances are accomplished. The evaluation demonstrates partially strong component-by-component evaporation dependent on the measurement conditions. Blends of ethanol and hexane in a mixture of 85/15 vol-Wo and of ethanol and decane with 10/90 vol-% show a distinct influenceability of the varied conditions. Furthermore, the results indicate a depletion of the more volatile component at the droplet surface. With increasing temperature, the depletion and the component-by-component evaporation are promoted. Finally, a decisive influence of the droplet spacing, the droplet temperature and the falling distance is described.