Modeling and Numerical Simulation on Ignition and Transition to Flame Spread under a Microgravity Environment

针对航天器着火安全问题，建立了在微重力环境下，有或无缓慢通风风时，着火和火焰在易燃固体材料上蔓延的理论模型。这项研究处理了一种热薄的纤维素纸，它同时伴随着固相和气相反应。由于化学反应和传输过程之间的强耦合，不仅在气相中，而且在固相中，着火和随后的火焰在固体材料上的传播过程是复杂的。这项研究的目的是能够更明确地了解微重力环境下的点火和火焰传播机制，并开发在航天器上的火灾安全应用。在消防安全应用中，从点火到火焰传播的转变对于确定火灾是限于局部、临时燃烧还是将过渡到具有潜在…的增长模式是至关重要的数值计算了静止微重力环境下固体受热表面附近的轴对称传热传质过程，以及伴随着固相和气相反应的过程。为了研究环境风和氧气浓度对点火现象和后续火焰在表面传播的影响，还进行了三维非定常数值计算，因为即使是沿着样品表面的类似于航天器内通风流动的缓慢环境气流也会对火焰传播速度产生显著影响，并可能影响阴燃和气体燃烧的机理。模型中采用了一步全局气相氧化反应和三步固相全局降解反应。温度和浓度方程的求解采用算符分裂技术，以防止溶液发散。较高的氧气浓度由于气相反应速率的增加而增加了气体温度，从而增加了对样品材料的能量反馈速率，从而增加了可燃气体降解产物的供应速率。从自燃到火焰传播的转变由能量反馈速率控制。点火后不久，火焰呈伞状。火焰的中心部分随着时间的推移而变得更弱，最终消失。然后，环形火焰径向向外传播。传播前沿附近的火焰被认为是预混火焰，其尾部是扩散火焰。较少

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 … More 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. Less