Impact of Particle Size Variation on Smoke Transport and Wall Deposition

Accurate prediction of smoke transport and wall deposition is critical to fire safety design. This study systematically analyzed the effects of smoke particle size variation on fire dynamics simulator (FDS)-based smoke transport and wall deposition predictions. Experimentally, the particle size distribution (PSD) generated during the combustion of acrylonitrile-butadiene-styrene (ABS) was measured using a high-performance particle counter (GRIMM 11-D). Measurements were performed in the near-flame region (5 cm) and the upper plume region (50 cm), with the mass mean aerodynamic diameter (MMAD) measured at 1.03 and 10.04 <i>μ</i>m, respectively. The measured MMAD values were applied in the FDS numerical model that simulated the transmission cell (TC), a key component of the gravimetric and simultaneous light extinction (GSLE) experimental device, to perform simulation. The flow conditions inside TC were fixed at an air/smoke mixture inflow rate of 6.6 L/min and a wall temperature of 25 °C. Smoke particle transport and bottom wall deposition behavior was then compared and analyzed. Simulation results showed that under the MMAD 10.04 <i>μ</i>m condition, the average smoke concentration inside TC (<i>ρ</i>Y<sub>s</sub>) decreased rapidly owing to gravitational settling, whereas the MMAD 1.03 <i>μ</i>m condition maintained more stable concentration characteristics. In addition, the cumulative mass of the smoke particles deposited on the bottom wall of TC was significantly higher under the large particle condition (5.28 mg) than that under the small particle condition (0.14 mg). Sectional analysis of deposition revealed that larger particles tend to accumulate near the TC inlet. This study confirmed that incorporating measurement-based PSD variation into the FDS model causes significant differences in smoke transport and wall deposition, even under the same conditions. This indicates that significant errors may occur in smoke concentration and deposition predictions during actual fire situations if particle size is entered as a fixed value in FDS simulation. Therefore, input that reflects smoke particle size variation is essential to enhance the reliability of fire simulation.