Soot Production And Its Scattering Properties Of A N-heptane Pool Fire Under Low Pressures

Fire hazards on the plateau or in the aircraft, where the pressure is lower than the ordinary pressure, are getting more serious because of the particularity of burning and fire detection. Understanding the effects of the low pressure on soot properties is an significant aspect of fire safety, and can provide essential data and a theoretical basis for fire detection technology enhancing the fire prevention in aircraft or other low-pressure environments. In order to examine the impact of pressure on soot generation, experimental and theoretical analysis of parameters of combustion, the morphology characteristics and scattering properties of soot emitted from n-heptane pool fires was conducted. The influence of a low pressure on the performance of smoke detector was also revealed. The primary conclusions obtained from experiments and simulations in our research are as follows:Results indicate the low pressure has a significant influence on the burning rate of pool fire as m" (?) P0.63±0.03. The flame height is also affected by the pressure. The mean flame height at 60.6 kPa is about 1.4 times of that at 101 kPa. With lower pressure, the maximum temperature appears at higher location in the plume. The maximum temperature of flame at 60.6 or 70.7 kPa is about 1.6 times of that at 101 kPa. The emissivity of the flame is smalller and there is less radiation of the flame under low pressure. There is no obvious relationship between CO generation and pressure when pressure is equal to or higher than 80.8 kPa; when pressure is equal to or less than 70.7 kPa, CO concentration increases rapidly with the decreasing pressure.The impact of low pressure on the soot’s microcosmic morphology were conducted. A series of experiments about N-heptane pool fires, mixed aviation kerosene pooling fires, paper and cotton smoldering were conducted in a pressure chamber which simulated pressures of 101,90.9,80.8,70.7,60.6,50.5 and 40.4 kPa. Soot was sampled by carbon-supported copper grids and then examined by SEM. By analyzing SEM images of flaming soot, it has indicated primary particle’s diameter, the number of primary particles per aggregate and the radius of gyration of an aggregate/Rg are decreased when air pressure is lower. The fractal dimension and the fractal prefactor show no obvious relationship with pressure. The diameter of the soot for smoldering fire is not related to pressure and is determined by the type of fuel.The impact of low air pressure on the number concentration and size distribution of soot aerosols were investigated. A fast particulate analyzer was employed to analyze the number concentration and size distribution of gas samples. Results imply both the geometric mean diameter and count median diameter of soot particles increased with pressure. A second-order polynomial equation with correlation coefficient of 0.99 was fitted to the geometric mean diameter of particles against pressure. The particle number concentration is significantly decreased when pressure is reduced according to a linear relationship with correlation coefficient of 0.92.To study influence of pressure on the light density of fire smoke, experiments were conducted in a simulated forward cargo compartment of aircraft B737-700. Soot generation rate was found to have an exponential relationship with pressure. The extinction coefficients for both of flaming and smoldering fires were increasing with pressure with a linear relationship. The effect of pressure on the specific extinction coefficient was not obvious and even could be ignored. Besides, a new method of calculating soot number concentration was proposed. The concentration of the soot particles can be deduced from the total cross section by a single particle and the absorbance of light of soot.The physical models of soot particles for typical flaming fires and smoldering fires were built and light scattering properties of soot particles were calculated using DDA method. At lower pressure, values of S11 for overall scattering angles are much less than those under higher pressure for a single average particle. Based on the scattering asymmetry factors, forward scattering is dominant for soot particles of n-heptane pool fires. The ratio of forward scattering intensity to backward scattering intensity is lower at lower pressure. The extinction efficiency factor of a single average particle also increases with air pressure throughout the entire pressure range. Light absorption cross section takes the dominate contribution to the light extinction coefficient.The influence of pressure on the peformance of the photoelectric scattering smoke detector were analyzed using classic Mie and Rayleigh scattering theory and DDA method and recommendations for smoke detectors were made. The sensitivity of smoke detector should be adjusted to a higher lever under low pressure. The laser diode will be a better choice than the LED. The wavelength of the smoke detector should be smaller rather than the infrared light. The scattering angle should be smaller. The smoke detector should enhance their ability to resist the fog droplets. Combining the CO sensor, temperature sensor or the heat flux sensor will enhance its ability to detect fire under low pressure.