Internal temperature distributions of interacting and vaporizing droplets

A line of mono-sized and periodically spaced droplets is moving in the diffusion flame sustained by the droplet fuel evaporation. The temperature field within the droplets is measured with the help of the two-color laser-induced fluorescence technique. Experiments are undertaken on droplets made of different fuels including acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane which have different physical properties such as their volatility and their viscosity. In some cases, the isotherms appear circular and concentric suggesting that only thermal conduction occurs in the droplet. In other cases, measurements show rather significant temperature differences between the leading and the trailing edges of the moving droplets. A simplified model of the heat transfer within the droplet is developed, taking into account both heat conduction and heat advection by the droplet internal fluid circulation. Heat and mass transfer are described in a quasi-steady approach within the framework of the film theory. The internal velocity field is assumed to correspond to the spherical Hill vortex solution, so that the velocity can be related to the stress exerted on the droplet surface. Comparisons between the measurements and the simulations reveal that the heat convection inside interacting droplets is strongly reduced, compared to the model of the isolated droplet.