Smoke Production and Fractal Structure Properties of Soot from n-Heptane Pool Fires Under Low Pressures

The purpose of this work is to investigate the influence of low air pressure on the smoke production and fractal structure properties of soot aerosols formed in fires. A series of experiments of n-heptane pool fires were conducted in a pressure chamber that could simulate ambient pressures from 40.4 kPa to 101 kPa. Soot aerosols were collected by carbon-supported copper grids and then examined by scanning electron microscopy to determine the soot morphology. A smoke meter was employed to detect the extinction coefficient of smoke in the chamber. The results indicate that the mean primary particle diameter, the mean number of primary particles per aggregate and the mean aggregate radius of gyration were reduced with decreasing air pressure. The Mueller matrix element S11 and extinction efficiency factor were calculated using the discrete dipole approximation method. For the soot formed at lower pressure, values of S11 of the overall scattering angles are much less than those formed under higher pressure. The scattering asymmetry factors indicate that forward scattering is dominant for soot particles of n-heptane pool fires. The ratio of forward scattering intensity to backward scattering intensity along with the extinction efficiency factor of single soot particle and the extinction coefficient of smoke decrease with decreasing pressure; the mean extinction coefficient of smoke is proportional to pressure to the power 3.3. These results may be useful for engineers to develop smoke detectors or other related technologies applied in cargo compartments or other regions under low pressure.