Effects of Rectangular Obstacles on the Flow Characteristics of Ultrafine Dry Powder Fire Extinguishing Agent in Confined Spaces

This study establishes a transient numerical model for the gas–solid two-phase flow of ultrafine dry powder fire extinguishing agents released in the confined space based on the Fluent software platform. The model investigates the spatial flow and diffusion characteristics of ultrafine dry powder under different obstacle volumes and relative positions. The results show that when an obstacle is present, two recirculation zones are formed by the upper surface of the obstacle, and a low-concentration unfavorable region is created around the obstacle. The concentration difference in ultrafine dry powder between the upper and lower recirculation zones increases monotonically with the obstacle volume. When the obstacle volume increases from 8 dm3 to 15.7 dm3, the concentration difference between the upper and lower zones increases by 5 times at 3.2 s. The time required for the average concentration in both zones to reach the minimum extinguishing concentration, as well as the reduction rate of the normalized characteristic dimension of the adverse region, follows an approximately exponential trend with changes in obstacle volume. The concentration difference between the upper and lower recirculation zones decreases as the distance between the obstacle and the nozzle increases. Compared to the upper position, the concentration difference decreases by 89% and 108% when the obstacle is positioned in the middle and lower parts at 3.2 s, respectively. And the reduction rate of the normalized characteristic dimension increases by 12% and 60% when the obstacle is positioned in the middle and lower parts, respectively.