Experimental and numerical study on the burning and opposed flame spread behaviors over PMMA under different flow conditions

Abstract

Opposed flame spread over PMMA under different flow velocities ($u_{\infty }$) and temperatures ($T_{\infty }$) was studied experimentally and numerically. By comparing the local flame standoff distance and burning rate from simulations with theoretical derivation, it was found that the normalized flame standoff distance varies inversely with the square root of the local Reynolds number. The valley effect on the local burning rate increases with $u_{\infty }$ and an effective B-number is about 0.88. Flame spread rate is less sensitive to $u_{\infty }$ at $u_{\infty }$≤1 m/s due to the counteracting effect of flow velocity on the heat feedback to the preheating region through solid and gas phases. At higher flow velocities, the dominance of reduced gas-phase heat feedback leads to a decreased flame spread rate. In addition, an increase in $T_{\infty }$ increases the thermal penetration depth, resulting in a greater role played by solid-phase heat conduction. As a result, the critical criterion for the thermally-thick decreases at larger flow temperatures. Based on the theoretical analysis, $L_p$ was found to be proportional to $V_f$ and sample thickness with an exponent of 0.66, $L_p$=10,913 ${\left(\frac{{V_f}^2{\delta }^2}{u_{\infty }{\mu }_{\infty }{\rho }_{\infty }}\right)}^{0.66}$. $L_f$ is independent of the flow properties and depends on $L_p$ for laminar flames, $L_f=4.45L_p$. For transient or turbulent flames, $L_f$ shows less dependence on $L_p$, $L_f=0.87{L_p}^{2/3}$.