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高温气体热化学反应的DSMC微观模型分析

ANALYSIS OF DSMC REACTION MODELS FOR HIGH TEMPERATURE GAS SIMULATION 1)

  • 摘要: 热化学耦合的非平衡现象一直是高温气体热化学问题研究的难点, 制约了诸如爆轰波胞格结构、低温点火速率等现象的分析. 本文以高温氮气离解和氢氧燃烧中的链式置换反应为例, 从微观反应概率、振动态指定的反应速率、热力学非平衡态的宏观反应速率、碰撞后的能量再分配等角度, 分析了直接蒙特卡罗模拟中的典型化学反应模型(TCE, VFD, QK模型)的微观动力学性质. 研究发现, 无论是高活化能的高温离解反应还是低活化能的链式置换反应, 实际参与反应的分子的振动能概率分布都偏离了平衡态的Boltzmann分布, 包含较强振动能额外影响的VFD模型可以很好地模拟高温离解反应, 而TCE (VFD的一个特例)和QK模型对活化能较低的链式置换反应的预测效果相对更好. 此外, 化学反应碰撞后的能量再分配应遵循微观细致平衡原理, 细微的偏差都可能造成平动能和振动能难以达到最终的平衡状态. 直接蒙特卡罗模拟的应用评估结果表明, 化学反应的振动倾向对热化学耦合过程产生了明显的影响, 特别是由于高振动能分子更多地参与了化学反应, 气体平均振动能的下降将影响后续化学反应的进行.

     

    Abstract: The non-equilibrium phenomenon of thermochemical coupling has been a difficult problem in high temperature aerothermal dynamics, and hinders to analyze phenomena such as cell structure of detonation wave and ignition speed of low temperature combustion. In this paper, typical chemical reaction models (TCE, VFD, QK models) employed in the direct simulation Monte Carlo (DSMC) simulation are analyzed using two examples (namely, N 2 dissociation at high temperature, and chain displacement reaction in H 2? O 2 combustion) from microscopic reaction probability, vibrational state specific reaction rates, total reaction rate under thermal nonequilibrium condition, and post-collision redistribution of internal energy. It is found that the probability distribution of vibrational energy of reacted molecules deviates from the equilibrium Boltzmann distribution for both the high temperature dissociation reaction having high activation energy and the chain displacement reaction having low activation energy. The VFD model with strong vibrational favored contribution can predict well the high temperature dissociation reaction, whereas the TCE model (a special case of VFD model) and QK model are better for the chain displacement reaction. Besides, the post-collision redistribution of internal energy should follow the principle of detailed balance, as small deviations may cause inequality between the translational and vibrational energy under final equilibrium state. The DSMC simulation results also show that the vibrational favor of chemical reactions has an obvious effect on the thermochemical coupling process. Particularly, because molecules having high vibrational energy are more easily to have chemical reactions, the decrease of the average vibrational energy of the gas will affect the subsequent chemical reactions.

     

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