The Influence of Transport Properties on Droplet Burning

The combustion of spherical droplets burning in a quiescent oxidizing atmosphere is considered. Chemical reactions are localized in a flame sheet at which the water-gas shift reaction is taken to be in equilibrium. Since nitrogen exists in relatively large concentrations throughout the flowfield, the diffusive properties of each species are described in terms of its binary coefficient with nitrogen. The chemistry model results in molecular hydrogen being present throughout the flow with the consequence that this description of diffusion is expected to be considerably more accurate than one based on a single diffusion coefficient. The final equations are solved by finite difference methods with the flame sheet location and the burning rate as eigenvalues. The predictions for the droplet burning rate for a variety of fuels are found to be in excellent agreement with available experimental data obtained under several ambient conditions. Higher ambient temperature and higher oxygen concentrations are found to increase the burning rate but to have a more complex influence on flame sheet location. Comparison of these locations with experiment is difficult because of unsteadiness in the only relevant experiments, namely those from drop tower tests. However, the agreement between prediction and the limited data is good. The distributions of key flow variables are calculated and compared with the results of a previous theory involving the same chemical model but simplified molecular transport. The most significant difference in the two calculations relates to the flame sheet location. As a consequence of this difference the distributions of the principal flow variables are greatly altered by the more accurate description of the transport properties afforded by the present theory. However, the gradients of species and temperature at the droplet surface are not significantly different in the two calculations so that the burning rates are nearly the same.