Laser attenuation measurements of soot volume fractions during reduced-gravity combustion of heptane and heptane/hexadecane droplets

Results from reduced-gravity experiments on combustion of heptane/hexadecane mixture droplets and heptane droplets are presented. Initial droplet diameters were about I mm, and initial hexadecane mass fractions in the droplets were 0.6. The droplets were burned in air at 0.1 MPa. Attenuation of the beam from a diode laser was used to gain information on average soot Volume fractions in the vicinities of the droplets. Results indicate that average soot volume fractions increased over the observation time for heptane droplets. For heptane/hexadecane mixture droplets, however, results indicate that average soot volume fractions increase after ignition, decrease when a flame contraction occurs, and then increase again after the flame contraction has been completed. Interpretation of the data indicates that peak soot volume fractions as high as about 50 ppm existed in these experiments. Simplified theory is developed to predict effects of droplet heating on quasisteady soot shell locations. The theory indicates that sufficiently-large droplet heating rates, which might occur during flame contractions, may cause soot to be deposited onto droplet surfaces as a result of thermophoretic forces.