Experimental Analysis of Limited Distance Effects on Self Induced Blow Off and Heat Transfer in Upward Flame Spread Over
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Experimental Analysis of Limited Distance Effects on Self Induced Blow Off and Heat Transfer in Upward Flame Spread Over Thin Fabric Fuels Yunji Gao *, Yuchun Zhang, Chunjie Mou, Xiaolong Yang and Liang Gong, Department of Fire Protection Engineering, Southwest Jiaotong University, Chengdu 611756 Sichuan, People’s Republic of China Received: 7 December 2019/Accepted: 10 September 2020
Abstract. Most previous work concentrated on the one-sided upward flame spread under limited conditions. However, few investigations have studied the effects of limited distance between wall and fuel surface on two-sided upward flame spreading. In this paper, a series of upward flame spread experiments were conducted using 0.65 mm thick, 130 cm tall and 4.0 cm wide flax fabric sheets with various limited distance of 1.0 cm–3.5 cm. The flame shape, flame length, flame standoff distance, pyrolysis spread rate and surface temperature were measured and analyzed. An unexpected but interesting behavior of one-sided flame blow off was observed in upward flame spread. When limited distance is no more than 1.5 cm, the one-sided flame blow off behavior occurs on unlimited side, whereas this behavior occurs on limited side for 2.0 cm–3.5 cm limited distance. This result could be qualitatively explained by Damkohler number Da, which could be characterized by two parameters of induced flow velocity and heat flux received by virgin surface. Moreover, the maximum flame length, steady flame length, maximum spread rate and steady spread rate first increase and then decrease as the limited distance. This non-monotonicity trend is determined by heat flux profiles in the preheating zone, which is due to the combined limited distance effects of restricting oxygen supply, chimney effect and wall radiation. The surface temperature profile ahead of pyrolysis front is used to qualitatively describe the heat flux received by virgin surface. The results in this paper are essential for better understanding of self induced flame blow off mechanism and upward flame spread under limited distance conditions. Keywords: Upward flame spread, Flame blow off, Limited distance, Heat transfer, Pyrolysis spread rate
1. Introduction The concurrent flame spread over vertical configuration is defined as the most hazardous scenario due to the same direction of flame propagation and buoyant air flow, which has been investigated widely as the classical problem [1]. The * Correspondence should be addressed to: Yunji Gao, E-mail: [email protected]
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Fire Technology 2020 effects of sample dimension [2–7], inclination angle [1, 7–11], ambient pressure [12–16], oxygen concentration [7, 17–19] and moisture content [20, 21] on flame spread over solid fuel surface have been studied experimentally and theoretically. In a real fire, the burning solid fuels are not completely attached to the objects or freely exposed both sides to the air, but some distance exists between the burning fuels and objects. The distance between burning fuels and objects is defined as the limited distance in this paper, which could significantly
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