Fire Safety Science Digital Archive

The mechanisms of delayed extinction of flames spreading downward over paper sheets have been explored experimentally. The transient temperature fields near spreading flames were examined using schlieren photography and fine thermocouples. Just after an instantaneous increase of free-stream velocity, the flame was observed to approach the pyrolysis zone and its length increased. At the same time, the temperature of the pyrolysis zone surface started to increase at the rate of several hundreds °C/s. Then, the flame turned away from the pyrolysis zone surface and finally became small to extinction. At a few tens ms after the velocity increase, the temperature of the pyrolysis zone surface reached its maximum of several tens °C higher than that before the free stream velocity increase. Based on the experimental results, the heat transfer process of delayed extinction has been discussed in detail. The phenomena to be considered for the theoretical prediction of rate or limit of flame spread were elucidated.