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

为应用于航天器的防火安全，建立了一个理论模型，该模型描述了在有和/或没有缓慢通风风的微重力环境中可燃固体材料上的点火和向火焰蔓延的过渡。本研究处理一热薄纤维素纸伴随着固相和气相反应。点燃和随后的火焰在固体材料上传播的过程由于不仅在气相中而且在固相中的化学反应和传输过程之间的强耦合而变得复杂。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，并开发航天器中的消防安全应用。在消防安全应用中，从点火到火焰蔓延的转变对于确定火灾是否将限于局部的、暂时的燃烧或将转变成具有潜在的燃烧风险的增长模式是至关重要的。 ...更多信息 在静止微重力环境下，数值计算了加热固体表面附近的轴对称热质输运过程，并伴有固、气反应。3-为了研究环境风和氧气浓度对点火现象和随后的火焰在表面上传播的影响，还进行了三维非定常数值计算，因为即使是类似于航天器中的通风流的沿样品表面的缓慢环境流沿着也对火焰传播速度有显著影响，并且可能影响阴燃和气体燃烧的机理。在本模型中使用了一个一步全局气相氧化反应和三个全局降解反应的固相。采用算子分裂技术求解温度和浓度方程，以防止解的发散;较高的氧气浓度会由于气相反应速率的增加而提高气相温度，从而增加对样品材料的能量反馈速率，从而增加可燃气体降解产物的供应速率。从自燃到火焰蔓延的过渡由能量反馈速率控制。在点火后不久，火焰具有伞状形状。随着时间的推移，火焰的中心部分变得越来越弱，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一个预混火焰，然后在其尾部的扩散火焰。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