Internal temperature distributions of droplets vaporizing in high-temperature convective flows

Abstract

Transient internal temperature distributions of vaporizing droplets have been carefully measured, using fine thermocouples at 1 atm. and 1000 K. Droplet diameters are fixed at 2000 ± 50 μm with Reynolds numbers being 17, 60 or 100. Fuels tested are JP-10, n-decane and JP-10 thickened with polystyrene. The effects of Reynolds number and liquid viscosity on internal temperature distribution and heating mechanism have been examined. Experimental results indicate that liquid viscosity or circulation intensity strongly affects the temperature distribution and heating mechanism. In contrast, the temperature distributions associated with the three different Reynolds numbers have shown little difference for both low- and high-viscosity cases. For the low-viscosity JP-10 droplets at Reynolds numbers up to 100, where the vortex model of Sirignano and coworkers (Prakash & Sirignano 1978; Tong & Sirignano 1983) has been claimed to be applicable, the vortex model appears qualitatively correct but quantitatively inaccurate. Physical reasons for the deviation have been discussed. Solutions of the full Navier-Stokes equations appear to accord better with the experimental temperature distributions. Circulative heat transport decreases progressively as liquid viscosity increases. A semi-empirical effective conductivity model for high-viscosity cases yields a very good simulation of the experimental temperature distributions at all the Reynolds numbers when proper effective conductivity factors are chosen. A discussion on internal droplet dynamics and heating mechanisms in physical terms has been provided.