Numerical investigation of flame splitting phenomenon in upward flame spread over solids with a two-stage pyrolysis model

ABSTARCTUpward flame spread over thin solids is numerically studied using an in-house three-dimensional transient numerical model with a two-stage solid pyrolysis. The simulation results reveal fundamentally different flame behaviors in different pressure conditions. At low pressures, conventional continuous flame spread is observed. Upon ignition, the flame grows and spreads upward until it reaches the end of the sample. At high pressures, the flame splits into two flames shortly after ignition. The two flames spread upward separately at different spread rates. In a transitional pressure region, cyclic flame splitting was observed. The flame splits periodically during the upward spreading process. In each splitting cycle, the bottom flame extinguishes shortly after splitting while the top flame continuously spreads upward and starts a new splitting cycle. This flame splitting phenomenon was previously observed experimentally, but to the authors' knowledge is numerically captured for the first time in this study. The splitting process is presented in detail and compared with the continuous flame spread process. The numerical results indicate that the multi-stage pyrolysis is responsible for the flame splitting.