Direct numerical simulation of the combustion of a suspended droplet in normal gravity.

DropletSMOKE++ is a multiphase CFD framework based on OpenFOAM, originally developed and validated for suspended droplets evaporation in a gravity field and convective conditions. In this work the solver is further extended to account for gas-phase combustion introducing: (i) an operator-splitting methodology to efficiently solve the gas-phase chemistry with large kinetic mechanisms, (ii) a model for the flame radiation and (iii) a double vaporization model to account for possible boiling. This allows to simulate the combustion of suspended fuel droplets in normal gravity with a detailed description of the gas-phase chemistry, representing the novelty and the main objective of this work. The numerical model is applied to simulate the vaporization, ignition and combustion of a methanol droplet suspended on a quartz fiber at different oxygen concentrations. The numerical results are compared with recent experimental data, showing a satisfactory agreement in terms of diameter decay, radial temperature profiles and sensitivity to the oxygen concentration in the gas-phase. In particular, the burning rate is found to be significantly affected by thermal conduction from the fiber, due to its relatively large size and the high temperatures involved in the combustion process. On the other hand the fiber perturbs the flame itself, providing a partial quenching close to its surface. The droplet combustion behavior has been compared to the one predicted in microgravity conditions, evidencing a lower standoff ratio, a higher flame temperature and an intense internal circulation. The gas-phase chemistry has been analyzed in terms of distribution of the main species in the gas phase, showing a local accumulation of (i) intermediate oxidation products at the fiber (due to the quenching) and (ii) water at the surface, which partially condenses on the droplet surface affecting the vaporization rate.