Effects of sample width on flame spread over horizontal charring solid surfaces on a plateau

Abstract

Experiments were conducted on the Tibetan Plateau to investigate the effects of sample width (W) on flame spread over horizontal charring solid surfaces. The results show that the rate of spread is influenced by the combined effects of convection heat transfer and flame radiation. Moreover, two different regimes depending on sample width were found in flame spread behavior. The rate of spread decreased with width in the convective regime (W≤3 cm) but increased in the radiative regime (W>3 cm). Similar results were observed in comparative experiments conducted on a plain in Hefei, with the rate of spread reaching its minimum value at W=4 cm rather than 3 cm. Flame spread over solid surfaces is a common phenomenon in fire accidents. Studying flame spread behavior is useful in predicting fire growth. Many studies have been conducted to explore the characteristics of flame spread. However, most research on flame spread over wood surfaces has been conducted on plains; few studies on the characteristics of flame spread on high plateaus have been reported, especially at altitudes over 3000 m. Recent research indicated that ignition times of wood at altitude might be shorter [1] than at sea level and the combustion temperature of the flame might be higher [2], which means that fire accidents on plateaus might be potentially more hazardous. Conducting further research on the characteristics of flame spread over wood surfaces on high plateaus could provide basic data and ultimately benefit fire control and prevention at higher altitudes. Much of the surface flame spread experiments were conducted on a laboratory scale owing to limitations in experimental conditions. To apply these small-scale experimental results to real large-scale fire situations, scale effects *Corresponding author (email: sunjh@ustc.edu.cn) (e.g. sample width effects) on surface flame spread need to be explored. Recent research [3][4][5][6] has reported that sample width below a certain threshold value has an important role in flame spread. For surface flame spread, the spread rate is influenced by flame convection and radiation. The convective heat transfer decreases with width while radiation increases with scale. Therefore, when the sample is narrow, convective effects are dominant and flame spread rate should drop. In contrast, in a radiative dominant regime, this rate should rise when sample width is large. For upward flame spread with finite sample width, flame radiation is dominant and the rate of spread increases with sample width [3][4][5]. However, for horizontal flame spread, opinions as to width effects on spread rate are wide and no consensus has been reached. Flame spread rates over cellulosic samples at small widths of 2−5 cm were studied in numerical simulations by Mell and Kashiwagi [7], and they found that rates of spread decreased with width. Li et al.[2] performed flame spread experiments over wood surfaces with a larger width range of 2−11 cm, with a width interval of 3 cm, and found the rate of spread increased with sample width....

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