Fire Smoke Particle Size Distribution Measurement And Evolution Simulation

The dynamic changes of smoke particle size and concentration are important parameters to fire detector response. It affects the carrying and transmission of fire toxic gas. In large volume building case, the increase of smoke particle size and mass control the smoke aerosol settling and arising height, so the validity of fire detector and the design of fire detection system in high space are affected. Smoke particle size distribution evolved under coagulation and nucleation mechanism, etc.Using scanning mobility particle sizer, measuring smoke aerosol of four national standard test fire and three European standard test fire, recording and analyzing time evolution of smoke particle size distribution, geometry mean particle size and total number particle concentration. The results indicate that: 1) mostly smoke particle size are smaller than 1 μm, and aerosol particle of test fire six is smaller than 400nm, and smoke particle of flaming fire is larger than smoldering fire and pyrolysis; 2) Smoke particle size distribution is bimodal, and fitted to log-normal distribution for particles size which larger 50nm; 3) Except TF6 smoke aerosol particle which distribution is self-preserving, size distribution center of other test fires shifts to right with combustion and time. The geometry means particle size increases and distribution is broadened. So it indicates that smoke particles coagulate and the coagulation rate is varying with combustion parameters.Considering that coagulation mechanism is the key role in changing smoke particle size and concentration in earlier fire, the dynamic equation of smoke particle size evolution is simplified, and the Moment Method is used to solve the Brownian coagulation equation of agglomerate smoke particle. Based on the simulation for smoke particle concentration change at fixed coagulation rate of Baum and Farouk, combining Fire Dynamic Simulator embedded large eddy simulation technique and the Moment solution of coagulation equation, the time evolution of smoke particle size distribution parameters can be predicted when the initial distribution and heat release rate is known. And the varying smoke coagulation rate with smoke dynamic viscosity coefficient and temperature and concentration is derived.The smoke particle size distribution parameters of national standard test fire SH3 and SH4 are simulated using FDS-Moment solution. The simulated change and decay of smoke particle number concentration is near to measuring results, and is more agreed with the real combustion process than the results with constant coagulation rate and the case that coagulation mechanism didn't considered. The simulated geometry mean particle size and log-standard variance change little during combustion arising and varying quickly after combustion reducing or end. The coagulation rate increases with time, and it is sensitive to particle morphology. The coagulation rate increase with the fractal dimension of smoke particle decreasing.The calculation of smoke detector response to SH3 and SH4 smoke indicates that the response of ionization smoke detector is over-estimated and photo-electrical scattering smoke detector is under-estimated if the coagulation effect is ignored. The FDS-Moment solution of fire smoke particle size distribution offered a effective method to estimate the smoke detector and access the validity of building fire detection system in performance-based building design. And this solution is useful to smoke transport simulation of large space building.