Critical Criterion for Flame Spread Acceleration of Concave Surfaces with Different Curvatures

Concave surface is a common geometry in both industrial buildings and natural environments; the flame spread behaviors on this special surface are worth studying, while few studies have been completed yet. In this study, kraft paper, which is a typical charring material, was chosen to investigate the behaviors of concurrent flame spread on concave surfaces. The results showed that there were three stages of the flame spread process on a concave surface: the flame gathering stage, the flame acceleration stage and the flame burnout stage. A peak mass loss rate was found at the end of the flame acceleration stage and then decayed rapidly due to the lack of sample that can maintain the flame spread. An experiential equation to predict the maximum mass loss rate was established. The flame spread showed an obvious acceleration with the increase in curvature, a new dimensionless number was proposed to find out whether the flame spread was accelerated or not. For the accelerated flame spread, the critical value is 0.85. Segmented expressions between dimensionless flame height and dimensionless heat release rate were developed, with good correlation for smaller curvatures. This study’s results will fill the blank of flame propagation on concave surfaces, improve the understanding of fires in special cases, and provide assistance in related fire risk evaluations.