Numerical investigation of soot formation from microgravity droplet combustion using heterogeneous chemistry

The use of isolated droplets as idealized systems is an established practice to get an insight on the physics of combustion, and an optimal test field to verify physical submodels. In this context, this work examines the dynamics of soot formation from the combustion of hydrocarbon liquid fuels in such conditions. A detailed, heterogeneous kinetic mechanism, describing aerosol and particle behavior through a discrete sectional approach is incorporated. The developed 1-dimensional model accounts for (i) non-luminous and luminous radiative heat losses, and (ii) incomplete thermal accommodation in the calculation of the thermophoretic flux. The combustion of droplets of n-heptane, i.e., the simplest representative species of real fuels, was investigated as test case; an upstream skeletal reduction of the kinetic mechanism was carried out to limit calculation times. After checking the performance of the reduced mechanism against gas-phase experimental data, the transient evolution of the system was analyzed through a comprehensive study, including fiber-suspended (D0 < 1 mm) as well as free (D0 > 1 mm) droplets. The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches.