Experimental study and analysis on flame lengths induced by wall-attached fire impinging upon an inclined ceiling

Experiments were conducted to investigate the flame lengths induced by a wall-attached fire impinging upon an inclined ceiling, which has not been quantified previously. The flame lengths beneath the ceiling were measured in the directions both normal- (x direction) and parallel (y direction) to the intersection line of the wall and ceiling for 576 experimental conditions, involving various heat release rates, source-ceiling heights and ceiling inclination angles (from −20° to 20°). The measured values were found to be essentially different from previous data and correlations for fire impingement without wall, and at the same time, have a complex variation nature with inclination angle. The flame lengths in the x direction increased, whereas that in the y direction decreased with increasing of the ceiling inclination angle. The measured flame lengths were then analyzed regarding the buoyancy effect (gravitational component along the ceiling) in two aspects: one is due to the change of the impinged flow velocity field distribution over the ceiling and the other is due to the formation of corner vortex rotating flow. The flame lengths in either direction normalized by the ceiling height (H) was found to be proportional to (Hwall,f-H)/H (reflects the fraction of the unburned fuel after impingement) where Hwall,f is the free flame height for the wall-attached fire. The effect of the angle was then further accounted for by multiplying (Hwall,f-H)/H with (1 + sinθ)/2 for the flame lengths in the x-direction, and dividing (Hwall,f-H)/H by (1 + sinθ)/2 for the flame lengths in the y-direction. New models were finally obtained to correlate the impinging flame lengths in the two directions. This study provides quantitative data and basic understanding of the flame length beneath an inclined ceiling induced by wall-attached fire impingement, which has a ceiling flow structure essentially different from other impinging configurations reported previously.