Experimental study of the width effects on self-induced buoyant blow off in upward flame spread over thin fabric fuels

In this paper, a series of upward flame spreading experiments were conducted on thin flax fabric with various widths ranging from 3.0 to 8.0 cm and length of 1.6 m. Symmetric ignition at the entire bottom edge of samples led to two-sided upward flame growth initially. A very interesting behavior of flame blown off was observed in upward flame spreading and an explanation was provided based on the increased buoyancy-induced velocity at the flame base. When the sample width is 6 cm or less, the flame length increases to a critical value and, correspondingly, the buoyancy-induced velocity reaches the blow off velocity, which results in a flame being blown off on one side. The remaining flame on the other side would shrink in length and propagate to the end of the sample with an asymptotically constant length and steady spread rate. For samples wider than 6 cm, the two-sided flame continues to spread to the end of samples and the self-induced blow off phenomenon is not observed. Moreover, the width effects on the flame height, flame thickness and flame spread rate are analyzed and explained in this paper. The results of this study may help advance better understanding of flame blow off behaviors over solid surfaces and have implications concerning fire control of flame spread over solid fuels.