The effects of sooting in droplet combustion under microgravity and normal-gravity conditions

It is recognized that individual droplet behavior is a necessary component for laying the foundation for a better understanding of spray processes. Under the idealized situation producing spherically-symmetric flames, it represents the simplest geometry in which to formulate and solve the governing equations of mass, species and heat transfer for a chemically reacting two-phase flow with phase change. However, most fundamental studies of droplet combustion have not considered sooting effects which have impeded the development of useful strategies for particulate emission control.;As part of this study, the effects of sooting in droplet combustion under microgravity conditions were investigated using the 2.2 second droptower at NASA-Lewis Research Center. The distributions of the soot volume fraction and the flame temperatures were measured using full-field light extinction and the two-wavelength pyrometry techniques, respectively. This represents the first measurements of soot concentrations under microgravity conditions for a spherically-symmetric droplet flame. Results indicate that the soot concentrations for microgravity flames were significantly higher than corresponding values measured for normal-gravity flames. It is believed that this is caused by the longer residence times and thermophoretic effects that are manifested under microgravity condition.;Burning rate and soot volume fraction measurements which were performed for various-sized droplets indicate that the maximum soot volume fraction and the mass of soot contained within the region bounded by the droplet surface and the luminous flame increase significantly as droplet diameter is increased. The presence of the soot particles within this region can modify the effective thermophysical properties. It was also demonstrated that radiative heat losses may account for the observed reductions in the burning rate as the droplet size was increased.;The pressure reduction experiments demonstrated that the soot volume fraction is sensitive to the ambient pressure. At 1 atm, the average soot volume fraction was ;In the normal-gravity experiments, the soot volume fraction for toluene flames was measured using full-field light extinction/tomographic inversion. Experiments performed using simultaneous gravimetric sampling and full-field light extinction indicate a direct relationship between soot volume fraction and the total soot yield. The calibrated optical technique can be used for simultaneously measuring the detailed structure of the sooting region within the flame and for measuring the total soot emitted to the environment in a non-intrusive manner.