Fire Safety Science Digital Archive

Effects of humidity and temperature on downward flame spread over combustible solids have been studied experimentally. The flame spread tests were carried out in a vertical duct mounted on a nozzle of a wind tunnel. The humidity and temperature of the air flowing in the duct were controlled with a humidifier and heater installed in the wind tunnel. The velocity and temperature were adjusted to the values of 10 cm/s, and 20°C or 30°C, respectively, and the humidity was widely varied. The test pieces used were of filter paper as a typical hygroscopic material. It was found that the ratio w / w, of the weight of water absorbed in the paper, to the weight of the paper sheet does not depend on the thickness of the paper sheet, and is closely related not to the absolute humidity but to the relative humidity. The humidity is supposed to affect the flame spread rate both in the gas and condensed phases. From the comparison of these effects in two phases, the effect in the condensed phase, namely, the effect of the higroscopicity of the paper sheet, is inferred to be the decisive factor. The effect in the gas phase is also examined by estimating the heat transfer from the spreading flame into the preheating region of the sheet. The results show that the heat supply from the flame is closely related to the absolute humidity.