Fire Safety Science Digital Archive

Effects of humidity on downward flame spread over combustible solids have been studied experimentally. The experiments were performed using a wind tunnel with heater and humidifier. The temperature and velocity of the air flowing in the tunnel were controlled to be 20°C and 10 cm/s, respectively, and the moisture content was varied. The test pieces used were of filter paper and PMMA, typical hygroscopic and non-hygroscopic materials, respectively. Although both the flame spread rates over these materials decreases as the moisture content increases, the decreasing rate for filter paper is much larger than that for PMMA. It is shown that filter paper is influenced by moisture more strongly than PMMA. This result indicates that flame spread in an air stream with moisture depends strongly on the hygroscopicity of the combustible material. When the moisture content in the air is high, local blow offs are observed repeatedly at various parts on the leading flame edge spreading over a test piece of filter paper, while the flame spread over PMMA seems stable. Amount of heat transferred from the spreading flame to unburned material at preheat zone is calculated on the basis of experimental results. When the moisture content increases, the heat input for both materials decreases though the decreasing rate for filter paper is larger than that for PMMA. This result shows that the moisture in the air stream influences not only the phenomena in the solid phase but also in the gas phase. The effects of the moisture in the air stream on the flame spread is inferred to be attributable to the reduction of chemical reaction rate in the gas phase caused by water vapor ejected from the solid surface.