Experimental study of the non-monotonous moisture effect on upward flame spread over cotton fabric

The present paper studies the moisture effect on upward flame spread over cotton fabric by performing experiments using 0.245 mm thick, 180 cm tall and 10 cm wide sample sheets with moisture content ranging from 0% to 18%. As the moisture content increases, the flame height, pyrolysis height, burnout height, pyrolysis length and spread rate show the same trend, first increasing and then decreasing. The maximum value is observed in the case of the 2% moisture content samples. Furthermore, at an infinite length of fabric fuel, the upward flame propagation would reach an asymptotic steady state with constant pyrolysis length and spread rate. Finally, a higher moisture content corresponds to a lower flame temperature. For upward flame spread, the moisture content has the negative effects of increasing the thermal inertia parameters and reducing the flame temperature and heat feedback; simultaneously, the positive effect is that the moisture can enlarge the flame size and the effective preheating length. The combined effects of positive and negative effects result in the non-monotonous trend in the pyrolysis spread rate.