在连续、自由分子和过渡传热状态下铝粒子点火的热理论
Thermal theory of aluminum particle ignition in continuum, free-molecular, and transition heat transfer regimes
摘要：
目前对纳米和微米级铝粒子点火的研究主要集中在粒子内部的点火过程.本文采用不同的传热模型：连续介质模型、自由分子模型和Fuchs模型，对空气中铝粒子的热点火进行了数值模拟。在纳米到毫米直径的颗粒尺寸范围内，假设单一的抛物线氧化规律。当颗粒达到氧化物熔点时，认为其被点燃。定义每个模型的有效性限度的标准是连续和自由分子传热速率的比率。着火温度$T_i$对颗粒尺寸的依赖性与实验趋势定性一致：对于纳米颗粒，由于自由分子热传递的主要贡献，$T_i$可以具有700- 1500 K范围内的值，并且粒径在1-100 $μ$ m范围内的$T_i$的急剧增长是由于热传递的过渡特性。对于较小的调节系数，点火可能发生在临界点火模式与热失控。结果表明，在纳米和微米尺寸的颗粒的点火中，热传递，特别是能量容纳的重要性。
英文摘要：
Most studies on nano- and micro- sized aluminum particle ignition have been focused on the processes occuring inside particles. In the current paper, thermal ignition of an aluminum particle in the air is simulated with different heat transfer models: continuum, free-molecular and Fuchs model. A single parabolic oxidation law is assumed in the particle size range from nano- to millimeter diameters. A particle is considered ignited when it reaches the oxide melting point. The criterion defining the limits of validity for each model is the ratio of continuum and free-molecular heat transfer rates. The dependence of ignition temperature $T_i$ on particle size is in a qualitative agreement with the experimental trends: $T_i$ can have values in the range 700--1500 K for nanoparticles due to the dominating contribution of a free-molecular heat transfer, and sharp growth of $T_i$ with the particle size in the range of 1-100 $μ$m diameter is due to the transitional character of heat transfer. For small values of accommodation coefficient, ignition may occur in the critical ignition mode with the thermal runaway. The results suggest the importance of heat transfer and, in particular, energy accommodation in ignition of nano- and micro- sized particles.
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