Experimental study of upward flame spread over discrete thin fuels

Abstract

Experiments are performed to study upward flame propagation over discrete combustibles separated by air gaps. An array of ten 1 cm-long 5 cm-wide filter papers is uniformly distributed on a vertical sample holder subjected to double-sided burn. The distance between the samples was varied from 0 to 4 cm. After being ignited from the bottom end, the flame spread process is recorded by front and side video cameras. A precision balance with 0.01g resolution is used to monitor the mass loss and deduce the solid burning rate. The results show that both flame spread rate and solid burning rate have a non-monotonic relationship with the gap size. The presence of gaps decreases the fuel load (fuel mass per unit length), which results in an increasing apparent flame spread rate as the gap size increases. The gaps also allow the lateral entrained air to push the flame closer to the sample surface, enhancing the conductive heat input to the samples. This results in an increased solid burning rate and flame spread rate. However, when the gap size is large, the effective heating length of the sample and hence the total burning rate decrease as the gap size increases. Eventually, the flame fails to spread.