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

Abstract

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.

Upward flame spread and self‐induced buoyant blow‐off over two‐sided thin fabric at different inclination angles

“…Such different sensitivities to the inclination angle cause the easier probability of one‐side extinction on a 6 cm sample at 45° inclination. For the sample width larger than 8 cm, the one‐side cannot be found even at a vertically oriented sample in our previous work 25 . It can be explained by the larger blow‐off velocity limit for a wider sample in the related work of Johnston et al 24 Because the wider sample produces a longer flame length (larger flame size), which increases the heat feedback from the flame to the sample 54 .…”

“…The experimental setup is schematically shown in Figure 1, which was developed from our previous works. 22,25,26,38 The test samples were clamped by two pairs of identical sheet holders and mounted onto the support frames. For the convenience from the side view observation, no sidewall was installed on the sample holder.…”

“…For the sample width larger than 8 cm, the one-side cannot be found even at a vertically oriented sample in our previous work. 25 It can be explained by the larger blow-off velocity limit for a wider sample in the related work of Johnston et al 24 Because the wider sample produces a longer flame length (larger flame size), which increases the heat feedback from the flame to the sample. 54 While the heat loss and lateral fuel diffusion could be more effective when the sample width decrease.…”

“…Besides the accelerating of upward flame spread, some instability phenomena, one‐side blow‐off, have been found in the cases of vertical upward flames on two‐sided thin fabrics 24‐26 . The one‐side blow‐off always occurred in the narrow strip fabrics with an adequate flame length, which was explained as the small Damköhler number caused by self‐induced buoyant flow.…”

“…Besides the accelerating of upward flame spread, some instability phenomena, one-side blow-off, have been found in the cases of vertical upward flames on two-sided thin fabrics. [24][25][26] The one-side blowoff always occurred in the narrow strip fabrics with an adequate flame length, which was explained as the small Damköhler number caused by self-induced buoyant flow. The blow-off mechanism of a spreading flame has been widely investigated under downward or opposed force flow configurations [27][28][29][30][31][32] ; while a few findings of blow-off in upward spread flames in air stream due to limitation of sample length.…”