237 Proceedings of the Combustion Institute, Volume 29, 2002/pp. 237–243 PROPAGATION AND EXTINCTION MECHANISMS OF OPPOSED-FLOW FLAME SPREAD OVER PMMA YUJI KUDO and AKIHIKO ITO Faculty of Science and Technology Hirosaki University Hirosaki, 036-8561, Japan 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 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 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, Q y , is about 60% of the total heat transfer rate, Q T , in the no imposed flow condition (u  0 m/s). However, the heat transfer rate through the condensed phase, Q x , is over 80% of Q T in the opposed flow near the extinction limit (u ¯  0.65 m/s). The radiative heat loss from the surface, Q R , increases with increasing opposed-flow rate and reaches at most 13% of Q T at u ¯  0.65 m/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. Introduction To understand the propagation and the extinction mechanisms of a spreading flame along a combus- tible solid is important to predict the behavior of the initial stage of building fires. The controlling mech- anism of flame spread appears to differ with the sur- rounding conditions, such as the ambient oxygen condition, gas flow velocity, direction of flame spread, and so on [1–3]. The flame spread rate in air is controlled by the rate of thermal energy feedback from the flame to the unburned fuel surface [4,5]. Our latest thermally thick fuel experiments in open air conditions [3] showed that the heat transfer rate from the gas phase is about 58% of the net total heat transfer rate at downward flame spread and 78% at horizontal flame spread. These experimental results were supported by the numerical analysis results [6–8]. The numerical results [7,8] also show that the ratio of the heat transfer rate from the gas phase to that through the condensed phase changes with air- flow condition. In opposed-flow flame spread, the fraction of the heat transfer rate through the con- densed phase in the total heat transfer rate increases with increasing airflow rate, and heat conduction through the condensed phase is the dominant heat transfer path near the blow-off limit. However, the experimental data for confirming these numerical results have not been obtained. It is still unclear what the dominant heat transfer path is near the blow-off limit condition. It is also questionable that an insufficient heat feedback to the condensed phase is the main reason why the flame is extinguished in such opposed-flow conditions. In order to determine the heat transfer path, we must know the detailed temperature structure both in the gas and condensed phases. Usually, thermo- couples were used for measuring temperature dis- tributions [9–11]. However, it is difficult to deter- mine the temperature gradient accurately due to lack of spatial resolution. We recently applied a ho- lographic interferometry (HI) technique combined with IR thermography for determining the tempera- ture structure within a transparent material [3,12]. Using these techniques carefully, the heat flux dis- tribution can be determined with high spatial reso- lution. In this study, the detailed temperature distribu- tions in a thick slab of polymethyl methacrylate (PMMA) during horizontal flame spread in opposed flow conditions were measured. Also, the flow struc- ture near the flame leading edge was measured using a particle-track laser sheet (PTLS) technique with a high-speed video camera. Based on the temperature structure in the condensed phase and local flow