Effect of thermo- and diffusiophoretic forces on the motion of flame-generated particles in the neighbourhood of burning droplets in microgravity conditions

Numerical analysis of the effect of thermo- and diffusiophoretic forces on the motion of moderately large (0.01 ≲ Kn ≲ 0.3) combustion-generated (soot) particles and on the formation of soot-shell structure in the buoyancy-free spherical droplet flames is performed. Transient evaporation, ignition and combustion of a single sooting-fuel droplet immersed into a quiescent hot environment are considered, taking into account the effects of radiative heat losses, variable transport properties and the dependence of the droplet surface temperature on time. Results of numerical calculations are compared with available experimental data and recent theoretical models. We calculated thermo- and diffusiophoretic and total velocities of combustion-generated particles and showed that soot particles form a size-segregated soot-shell structure. Within a soot shell the particles with smaller radii are located closer to the droplet surface, while larger particles are located closer to the flame front. Numerical calculations performed for moderately large soot particles showed that there are two equilibrium locations, where the total velocity is equal to zero, between the droplet and the flame front. It is found that one of the equilibrium locations is the point of unstable equilibrium and the other position is the point of stable equilibrium. We numerically determined the dependence of soot-shell-to-droplet-diameter ratios on the particle radius.