An analysis of width effects on flame spread in conjunction with concurrent forced flow using a variable B-number

Width effects on concurrent forced flow flame spread over thermally-thick PMMA slabs were studied. Different flame spread behaviors were observed depending on the sample width. For wider slabs (width ≥ 10 cm), the flame spread was width-independent. The total burning rate decreased with the streamwise location to the −0.45 power, which was very close to that predicted by the laminar boundary layer theory. For narrower slabs (2.5 and 3 cm), the flame length and flame spread rate decreased with the width. The total burning rate was nearly constant along the streamwise direction, suggesting that turbulence set in at some point. It was found that width effects changed from heat and fuel loss-dominated to air entrainment-dominated as the flame grew. Width-dependent B-numbers were calculated from the measured standoff distances. The B-number was width-independent for wider slabs but decreased with the decreasing sample width for narrower slabs as a result of lateral fuel diffusion and heat loss. The lateral fuel diffusion rate decreased with the increase of sample width, streamwise location and flow velocity. The flow velocities in this study had a slight effect on the B-number. The good agreement between predicted and experimental data suggested that classical flame spread theories under the infinite width assumption can apply to the finite width case with a modified B-number.