FLAME SPREAD IN AN OPPOSED TURBULENT-FLOW

An experimental study is presented of the influence of turbulence on the spread of flames over the surface of a solid combustible in a gas flow opposing the direction of flame propagation. The flow velocity and its turbulent intensity are varied to observe their effects on the flame spread rate over thick PMMA and thin filter paper sheets. Also observed is their effect on the conditions for flame extinction. The results show that for thick PMMA sheets the spread rate first increases and then decreases with increasing turbulent intensity. The presence of a maximum appears to be the result of the flow transition from laminar to turbulent that influences the heat transfer process from flame to fuel. Through interferometric photographs it is shown that the decrease of the spread rate with the turbulence intensity is primarily due to the convective cooling of the region near the flame front in conjunction with other less important mechanisms related to flame and surface radiation. For thin paper sheets the spread rate always decreases as the turbulence increases. The observed differences between the thick PMMA and the thin paper results are probably due to their differing mechanisms of heat transfer ahead of the flame. It is also found that extinction of the flame occurs at lower flow velocities for higher turbulence intensities. This result suggests that convective cooling together with radiation and flame stretching effects, and not turbulent mixing, are the major factors through which flow turbulence affects the extinction of a spreading flame, at least for the present experimental conditions. © 1990.