The influence of thermal radiation absorption on the interaction of transient transfer processes for a water droplet evaporating in a high-temperature gas flow

Abstract

This work presents a study of transient transfer processes of water droplets evaporating in flue gas, including spectral radiation absorption and droplet slip. The results of numerical modelling of condensation, transitional and equilibrium evaporation regimes of water droplets in flue gas at temperatures ranging from 633 to 1833 K are presented. The modelled radiative transfer is based on geometrical optics theory. Convective heating and evaporation of the droplet are defined by the empirical Clift correlation for Reynolds numbers Re < 400. The interaction between transient processes and the dependence of the physical and optical spectral properties of a warming droplet on temperature are taken into account. The numerical iterative scheme defining the average instantaneous temperature of the droplet’s surface and working according to the fastest descent method is based on the balance of heat fluxes. The influence of Stefan flow, evaluated by the Spalding heat- and mass-transfer parameters, is made universal for different droplet phase change regimes by carefully assessing the dynamics of the temperature gradient within the droplet. It was verified that the competing effects of droplet slipping and the absorption of radiation are essential for the interaction between droplet transfer processes. The internal heat transfer in a droplet is affected by absorbed radiation and influenced by water circulation. Due to the effect of absorbed radiation, the evaporation rate of a large water droplet more than doubles in flue gas at 818 K and increases up to fourfold at 1133 K. These simulation results agree with the experimental data.