The effect of fire-wall distance on the flame attachment characteristics and flame height of near-wall fires

Abstract

Near-wall fires happen frequently in buildings and may ignite the solids on the wall, leading to a rapid spread of fire. A series of experiments and numerical simulations were conducted on near-wall and free (the same as that of near-wall fire source after mirroring) fires using propane and methane as the fuel to investigate the effect of the wall on flame attachment properties and flame height. The presence of a wall decreases the degree of turbulence for downstream flame and influences the velocity and temperature evolutions due to the no-slip and impermeability conditions. Due to the asymmetric evolution of vortices, intermittent flame attachment emerges at a certain downstream location and is intensified with decreasing fire-wall distance. The upper attachment limit increases with the heat release rate, however, the dependence of the lower attachment limit on the heat release rate is related to the fire-wall distance. The flame attachment probability increases with the aspect ratio of the gap between the fire source and the wall. Grashof number is a suitable parameter to correlate the flame attachment parameters of near-wall fires, including flame attachment limit, length and probability. The flame height increases with decreasing fire-wall distance and is 17.8%∼46.5% higher than that of free fires. By defining a dimensionless heat release rate incorporating the flame attachment probability, a unified model was built to predict the flame height of near-wall and free fires. The data and models are expected to be useful for evaluating the risk of early-stage near-wall fires.