Knudsen过渡区中细铁颗粒的着火
The ignition of fine iron particles in the Knudsen transition regime
AI总结：
本文通过考虑Knudsen过渡输运效应和两种氧化动力学模型，研究了细铁颗粒着火温度随颗粒尺寸及初始氧化层厚度的变化规律，揭示了小颗粒极限下热绝缘与氧化层抑制作用的竞争机制，并比较了过渡与连续介质输运模型的预测差异。
摘要：
本文考虑了一个理论模型，用于预测细铁颗粒能够发生热失控——即着火温度——的最低环境气体温度。该模型考虑了Knudsen过渡输运效应，当颗粒尺寸与周围气体的分子平均自由程相当或更小时，这种效应变得显著。分析了两种铁的高温固相氧化动力学模型。第一种模型（抛物线动力学）考虑了颗粒表面氧化铁层对氧化速率的抑制效应，以及一个与气态氧化剂浓度无关的动力学速率。通过考虑氧化层生长的非稳态分析，求解了着火温度随颗粒尺寸和初始氧化层厚度的函数关系。在小颗粒极限下，过渡热输运的热绝缘效应可导致着火温度随颗粒尺寸减小而降低。然而，氧化层的存在减缓了反应动力学，并且其在小颗粒极限下所占比例的增加可导致着火温度随颗粒尺寸减小而升高。对于足够大的初始氧化层厚度，可以观察到这一效应。结果表明，与过渡输运模型相比，连续介质输运模型预测初始直径超过30 μm的铁颗粒的着火温度时，差异为3%（30 K）或更小。第二种动力学模型（一级动力学）考虑了多孔、无阻碍的氧化层，以及氧化动力学速率对气态氧化剂浓度的线性依赖关系。利用过渡输运模型和连续介质输运模型求解了着火温度随颗粒尺寸的变化，并讨论了两种模型预测的着火特性之间的差异。
英文摘要：
A theoretical model is considered to predict the minimum ambient gas temperature at which fine iron particles can undergo thermal runaway--the ignition temperature. The model accounts for Knudsen transition transport effects, which become significant when the particle size is comparable to, or smaller than, the molecular mean free path of the surrounding gas. Two kinetic models for the high-temperature solid-phase oxidation of iron are analyzed. The first model (parabolic kinetics) considers the inhibiting effect of the iron oxide layers at the particle surface on the rate of oxidation, and a kinetic rate independent of the gaseous oxidizer concentration. The ignition temperature is solved as a function of particle size and initial oxide layer thickness with an unsteady analysis considering the growth of the oxide layers. In the small-particle limit, the thermal insulating effect of transition heat transport can lead to a decrease of ignition temperature with decreasing particle size. However, the presence of the oxide layer slows the reaction kinetics and its increasing proportion in the small-particle limit can lead to an increase of ignition temperature with decreasing particle size. This effect is observed for sufficiently large initial oxide layer thicknesses. The continuum transport model is shown to predict the ignition temperature of iron particles exceeding an initial diameter of 30 $μ$m to a difference of 3% (30 K) or less when compared to the transition transport model. The second kinetic model (first-order kinetics) considers a porous, non-hindering oxide layer, and a linear dependence of the kinetic rate of oxidation on the gaseous oxidizer concentration. The ignition temperature is resolved as a function of particle size with the transition and continuum transport models, and the differences between the ignition characteristics predicted by the two models are discussed.
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