Applicability of a Low-Pressure Environment to Investigate Smoldering Behavior under Microgravity

This paper provides alternative methodology to reproduce the smoldering of a 2-mm dia biomass stick in reduced gravity by utilizing a reduced-pressure approach. Since smoldering has been known as an initial process of fire hazard, a deep understanding of its characteristics is an important task to improve fire safety strategy in space. The aim of the present work is to examine the possibility to reproduce the smoldering in microgravity by utilizing the low pressure. The smoldering behavior and the temperature profile inside the specimen are studied experimentally for both vertical and horizontal orientations to modify the gravity effect. The considered range of pressure is 1.0 kPa to 100 kPa and adopted oxygen concentration are 0.23 and 0.38 in mass fraction. It is found that the identical smoldering rates for both orientations using small-scale specimen for all pressure conditions studied. However, the measured thermal structure shows the slight difference depending on the sample orientation, especially when the pressure is applied above 80 kPa. The difference in thermal structure is less-pronounced when less than 80 kPa pressure is employed. The simple analysis confirms that the smoldering temperature becomes lower when the total pressure is reduced through the reduction of surface oxidative reaction rate. Therefore, combination of (1) applying small-scale specimen together with low pressure technique to suppress the contribution of buoyancy-induced transport and (2) adopting higher oxygen environment to compensate the reduction of heat release at low pressure could be one of methodologies to mimic the smoldering of reduced gravity environment.