Effects of Bulk Densities and Inlet Airflow Velocities on Forward Smoldering Propagation Properties of Flexible Polyurethane Foam

Smoldering processes of flexible polyurethane foam (FPUF) with densities of 20.0, 30.0 and 35.0 kg/m3 were simulated in a self-designed mesoscale generator with dimensions of 60.0 cm × 25.0 cm × 25.0 cm. Characteristic parameters like smoldering establishment time, average smoldering rate, smoldering temperature, and high temperature duration were proposed and analyzed at five different air flow rates of 1.0, 2.0, 3.0, 5.0 and 7.0 L/min. Scanning Electron Microscopy (SEM) was used to investigate the morphological evolution of bulk materials before establishment and propagation of smoldering. Air was forced in the direction of approximately one-dimensional forward propagation. Results show that FPUF (20.0 kg/m3) has the shortest average smoldering establishment time of 1828.0 s at five airflow rates, FPUF (30.0 kg/m3) has the longest average smoldering establishment time of 2646.0 s. The average smoldering rate of FPUF (35.0 kg/m3) is higher than that of FPUF (30.0 kg/m). Smoldering of FPUF (20.0 kg/m3) cannot be self-sustained, due to largest porosity and insufficient heat accumulation. The average smoldering temperature and duration of high temperature values of FPUF (30.0 kg/m3) are higher than those of FPUF (35.0 kg/m3), and the highest average smoldering temperature of FPUF (30.0 kg/m3) is 410.0 ℃. Since smoldering is derived from chemical reactions of oxygen transported and pyrolyzed products, the bulk material with low density shows higher porosity and contains more oxygen, it burns more fully and thus smoldering establishment time is less. In the process of smoldering, the oxidation and exothermic effects of porous carbon-based chars and oxygen is the main source of energy for the self-sustaining propagation of smoldering. The porosity of FPUF with high density (35.0 kg/m3) is lower, the burning produces a porous carbonate residue and limits the heat loss on the surface, so the smoldering rate of FPUF (35.0 kg/m3) is faster.