|Budget Amount *help
¥2,000,000 (Direct Cost : ¥2,000,000)
Fiscal Year 1996 : ¥300,000 (Direct Cost : ¥300,000)
Fiscal Year 1995 : ¥1,700,000 (Direct Cost : ¥1,700,000)
A theoretical model describing ignition and transition to flame spread over an inflammable solid material in a microgravity environment with and/or without a slow ventilation wind is developed for application to fire safety in a spacecraft. This study treats a thermally thin cellulosic paper accompanied with both solid and gas phase reactions. A process of ignition and subsequent flame spread on the solid material is complicated by strong coupling between chemical reactions and transport processes not only in the gas phase but also in the solid phase. The objectives of this study are to be able to obtain a more definitive understanding of the ignition and flame propagation mechanisms under a microgravity environment, and also to develop fire safety applications in a spacecraft. In fire safety applications, the transition from ignition to flame spread is crucial to determine whether a fire will be limited to a localized, temporary burn or will transit into a growth mode with a potential
to become a large fire.
Axisymmetric heat and mass transport processes near the heated solid surface accompanied with solid and gaseous reactions were numerically calculated in a quiescent microgravity environment. 3-dimensional unsteady numerical computation was also performed in order to investigate the effects of the ambient wind and oxygen concentration on phenomena of ignition and subsequent flame spread over the surface, since even a slow ambient flow along the sample surface similar to a ventilation flow in a spacecraft has significant effects on flame propagation velocity, and might affect the mechanisms of smoldering and also gaseous combustion. A one-step global gas phase oxidation reaction and three global degradation reactions for the solid phase are used in the present model. The equations for temperature and concentrations are solved using an operator-splitting technique to prevent the solutions from diverging.
A higher oxygen concentration increases the gaseous temperature due to an increase in gas phase reaction rate and thus increases the energy feedback rate to the sample material, which increases the supply rate of combustible gaseous degradation products. The transition from autoignition to the flame spread is controlled by the energy feedback rate. Shortly after the ignition, the flame has an umbrella-like shape. The center portion of the flame becomes weaker with time and eventually disappears. Then, a ring-shape flame propagates radially outward. The flame near the propagation front is considered to be a premixed flame followed by a diffusion flame at its tail.