Propagation and extinction mechanisms of opposed-flow flame spread over PMMA for different sample orientations

To understand the propagation and the extinction mechanisms of flame spreading along a combustible solid, opposed-flow flame spread along a thick slab of PMMA was experimentally investigated for four different sample orientation angles from 0° (horizontal) to −180° (ceiling flame spread) in several airflow rates. The detailed temperature distribution in the condensed phase during flame spread was measured using holographic interferometry (HI) and infrared thermography (IR). The particle-track laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that a 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, under the no-imposed-flow horizontal flame spread condition (u¯=0m/s). However, the heat transfer rate through the condensed phase, Qx, increases with increasing airflow above 80% of QT near the extinction limit. The radiative heat loss from the surface, QR, increases with increasing opposed-flow rate and reaches a maximum of 13% of QT at u¯=0.65m/s, while the heat transfer rate in ceiling flame spread is different than in horizontal or downward flame spread under nonopposed and slow-opposed flow conditions. The Qy in nonopposed flow ceiling flame spread is double that in horizontal flame spread, and is 85% of QT. Despite adequate heat feedback to the condensed phase near the flame leading edge, the flame spread rate decreases rapidly as it approaches the extinction limit. In order to interpret the absence of flame spread or the extinction mechanism, we introduce the Damköhler number including the local air entrainment velocity and the burning rate at finite flame thickness. PTLS showed that the local entrainment velocity at the flame leading edge increased the opposed-flow rate. This leads to a decrease in the Damköhler number. When the Damköhler number reaches a critical value (at most <1), the flame cannot spread, retreats, and then is finally extinguished.