The Width Effect in Upward Flame Spread Modelling

[[abstract]]火災大多起因於小火，若周圍持續供應可燃性物質，小火則會持續延燒成大火。延燒可在地面上或任何可燃物質上發生，由於加熱範圍及強度的不同，又以火焰垂直向上延燒的速度最為快速。為了評估此種現象的危害程度，許多理論及火災模擬程式已逐漸被發展。然而，先前研究指出，火焰較寬時火焰較厚，所產生的輻射熱也較大，而寬度不同的火焰是否會對火焰延燒造成影響，目前尚未出現系統性的研究討論，本研究即以實驗及電腦模擬來探討寬度效應。 本研究使用未具試體延伸板及具試體延伸板兩項裝置進行牆面延燒，於未具試體延伸板採用1m高、厚6 mm及20 mm且寬為100 mm、300 mm、500 mm、700 mm及900 mm之PMMA，具試體延伸板之實驗採用1m高、18 mm厚，寬為100 mm、200 mm、300 mm、500 mm及700 mm之PMMA進行實驗。 實驗結果證實寬度效應確實會影響火焰延燒行為，牆面火焰延燒速率隨火焰寬度增加而增加，且寬度對火焰高度及加熱強度均有造成影響。模擬部份，FDS結果顯示，寬度影響加熱強度及延燒速率，但其數值及趨勢均與實驗不符，無法模擬牆面延燒及寬度效應，後續可藉由本次實驗結果進行Tsai and Drysdale模擬程式，討論寬度效應。[[abstract]]Previous work has demonstrated upward flame spread on vertical surfaces to be one of the most hazardous fire scenarios. To assess the risk of this scenario, several models have been developed to predict the flame spread rate, relying on empirical correlations of flame height and heat feedback to unburned surface ahead of pyrolysis region. However, the width effect was not regarded in those models but to influence flame thickness, causing the variation of radiation. In this study, experiment was designed to access the width effect. In addition, to understand the effect of extension on the top of the samples, two sets of tests were carried out. In the tests without extension, samples used were 6 and 20 mm thick clear PMMA with height of 1000 mm and widths of 100, 300, 500, 700 and 900 mm. In the tests with extension, samples used were 18 mm thick clear PMMA with height of 1000 mm and widths of 100, 200, 300, 500, and 700 mm. Our data showed that the width effect was significant for upward flame rate in both sets of tests. In addition, the width effect was obvious in flame height correlation and total heat flux distribution. As to the radiant heat flux distribution, our measurements were much lower than recognized in previous studies