Combustion of n-propylbenzene: Experiments and numerical modeling

The burning characteristics of n-propylbenzene (nP) were examined from the perspective of droplet flames burning in the standard atmosphere under conditions that promote a one-dimensional gas transport and spherical symmetry. The parameter was the initial droplet diameter (D-0) in the range of 0.6 mm to about 6 mm and the droplet diameters, soot shell diameters and flame diameters were measured during droplet combustion. Experiments were carried out on the International Space Station (ISS) for D-0>1 mm while ground based experiments in a drop tower were performed for D-0=0.6 mm. Numerical simulations incorporated unsteady liquid and gas phase transport, a model for soot formation, radiation including both nonluminous and luminous components, a detailed nP kinetic mechanism that included formation of soot precursor species and a model for soot formation that incorporated processes of nucleation and aggregation to form macroscopic soot particles and ultimately a soot 'shell'. Results showed simulations that generally agreed with measurements of the evolution of droplet, flame, and soot shell diameters. Burning rates decreased with increasing D-0 and were well predicted by the simulations. Flame extinction was observed for the large droplets, but cool flames were not observed under the conditions examined in keeping with the chemical structure of nP. Soot shells corresponded to the location of simulated maximum soot volume fraction. The importance of including a soot formation model in the simulations was demonstrated by simulations where the soot model was removed from the chemical mechanism, in which case the simulations were completely different from the experiments. The evolution of droplet diameter showed a very early extinction process not seen with the full model and with poor agreement of measurements compared to when soot was included.