Propagation and extinction mechanisms of opposed-flow flame spread over PMMA

Abstract

To understand the propagation and the extinction mechanisms of spreading flame along a solid fuel, opposed-flow flame spread along a thick polymethyl methacrylate slab was experimentally investigated for several airflow rates from 0 to 0.67 m/s. The detailed temperature structure during flame spread near the flame leading edge was measured using holographic interferometry and IR thermography. The particletract laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that the flame spread rate with a constant speed is proportional to the net total heat transfer rate. The heat transfer rate from the gas phase, Qy, is about 60% of the total heat transfer rate, QT, in the no imposed flow condition (u=0m/s). However, the heat transfer rate through the condensed phase, Qx, is over 80% of QT in the opposed flow near the extinction limit (u¯≈0.65m/s). The radiative heat loss from the surface, QR, increases with increasing opposed-flow rate and reaches at most 13% of QT at u¯≈0.65m/s. In spite of enough heat feedback to the condensed phase near the flame leading edge, the flame spread rate rapidly decreases close to the extinction limit. In order to interpret the extinction mechanism, we introduce the burning rate at the finite flame sheet with finite chemical reaction rate. The PTLS result shows that the local entrainment velocity at the flame leading edge increases with increasing opposed-flow rate. When the local entrainment velocity exceeds the burning rate at the leading edge, the flame may not be sustained. The flame retreats and finally it is extinguished.