Experimental study of flame spread transition from chemistry to heat transfer controlled regime at sub-atmospheric pressure: The effect of sample width.

Understanding the flame spread rate (FSR) is controlled by either chemistry or heat transfer is important for industrial process safety. In this study, we study the effect of width on flame spread transition from chemical to thermal regime at sub-atmospheric pressure using thin paper sample with widths from 10 mm to 90 mm. Results show that the transition boundary can be identified using not only reported FSR but also the flame image or radiation, as each of them has significantly different characteristics in different regimes. We find that the orientation significantly affects the flame spread in the thermal regime but has negligible influence in chemical regime. A width dependent characteristic Damkohler number coupled lateral heat and mass transfer has been developed to analyze this transition. The Damkohler number increases as the width increases only if the width is narrow. When the width is higher than a critical value, its effect on the Damkohler number can be neglected. For this reason, the transition pressure first shifts from 25kPa (10 mm and 20 mm width) to 20kPa (30 mm width), and then keeps unchanged at 15kPa (width>50 mm). This work strengthens our understanding on the effect of width on fire risks in aircraft. [ABSTRACT FROM AUTHOR]