Fire Safety Science Digital Archive

Thistledown has been used as a tracer to measure the flow of air towards ethyl alcohol and wood fires 91 em in diameter and a smaller town gas fire. The total quantity of air below the mean flame height is approximately one order times the stoichiometric requirements, a substantial part of the air flowing upwards around the flame. The total flow also exceeds that estimated from entrainment theory and measurements of flame tip veIocity. The mean concentration of oxygen on the flame axis and the heat transfer back towards the fuel surface have also been measured. The convection transfer at 1-2 cm above the fuel surface was found to be about 1/ 5 of the total heat transfer at the centre increasing to about 1/2 at the edge. The measured mean axial temperature rise at the mean flame height was about 300-350 degC for wood and alcohol and 500 degC for town gas. The average period of the eddies outside the flame and the corresponding length were about 0.7 sec and 15 cm respectively. No variation was found with height above the base. Elementary considerations of entrainment and the motion of flames have been applied to the merging of the flames from two nearby rectilinear fuel beds and there is reasonable agreement between theory and experiment.