Turbulent, concurrent, ceiling flame spread: The effect of buoyancy

Abstract

Experiments have been conducted to study the effects of forced air flow velocity and grid-generated turbulence on the flow-assisted flame spread over a flat solid combustible surface in a ceiling configuration. The tests are conducted with thick PMMA sheets as fuel, and air as oxidizer. Flame spread rate, flame length, surface heat flux, and products composition are obtained for air flow velocities ranging from 0.25 to 4.5 m/s and turbulence intensities the 1% to 15%. It is found that for all turbulence intensities the ceiling flame spread rate increases with the flow velocity, and that the flow turbulence retards the flame spread for flow velocities larger than 1 m/s and enhances it at lower velocities. The flame length and the surface heat flux exhibit power law correlations with the fuel pyrolysis length, and the flame spread rate data can be correlated with an expression deduced from a simplified heat transfer analysis of the process. In order to determine the effect of buoyancy on the flame spread processes, data from the ceiling configuration experiments are compared with data from floor tests conducted previously. The experimental results indicate that in ceiling spread, buoyancy has two main competing effects. One is an enhancement of the heat transfer from the flame to the solid surface because the flame stands closer to the surface, the other is an incomplete combustion caused by larger heat losses to the wall and boundary layer stratification. For large flow velocities (larger than 1 m/s), the enhanced heat transfer is found to be dominant and results in a faster flame spread in the ceiling than in the floor. For small flow velocities, the incomplete combustion becomes more important and the opposite result is observed. The species concentration data show that in general the combustion reaction is less complete in ceiling spread than in floor spread, and that significant amounts of CO and unburned hydrocarbons are produced in ceiling flame spread.