稀薄大气中水平极细金属丝的自由对流换热关联式
A free convection heat transfer correlation for very thin horizontal wires in rarefied atmospheres
摘要：
近年来，火星和地球上层大气探测任务蓬勃发展，增加了人们对稀薄气体传热的兴趣。在这些低压下，涉及特征长度为气体分子的平均自由程数量级的小几何形状的传热问题引起了新的困难。这在自由对流的情况下尤其严重，其中气体的速度与传热问题密切相关。在定制的真空室中进行了直径为$12.7\ μ\mathrm{m}$和$25\ μ\mathrm{m}$的水平线的实验，该真空室的压力从$0.03$毫巴到环境压力，在线和空气之间达到高达$80$ K的温度差。实验数据包括从$10^{-13}$到$10^{-5}$的瑞利数，其覆盖了从自由分子流到连续区的宽范围。特别是对于过渡区，与克努森数范围在0.1和10之间，实验数据表明，自由对流换热不能表示为单独的瑞利数的函数，但克努森数的依赖性必须引入相关性以及。所呈现的努塞尔数低于瑞利数的$10^{-8}$的结果表现出明显偏离文献中可用的相关性。本文提出了一个新的从过渡区到连续区的水平细金属丝自由对流的经验关联式。目前的工作揭示了当前的热工程问题，其中涉及非常细的电线在其系统中与周围环境的相互作用，具有显着的稀疏水平。
英文摘要：
Mars and upper Earth atmosphere missions are booming in recent years increasing the interest in heat transfer in rarefied gases. At these low pressures, the heat transfer problems involving small geometries whose characteristic length is of the order of the mean free path of the gas molecules, raises new difficulties. This being especially severe in the case of free convection, where the velocity of the gas is closely linked to the heat transfer problem. Experiments with horizontal wires with diameters of $12.7\ μ\mathrm{m}$ and $25\ μ\mathrm{m}$ were conducted in a tailored vacuum chamber with pressures from $0.03$ milibar to ambient pressure, reaching temperature differences of up to $80$ K between the wire and the air. Experimental data is comprised of Rayleigh numbers from $10^{-13}$ to $10^{-5}$ that cover a wide range from free molecular flow to continuum regime. Particularly for the transition regime, with Knudsen numbers ranging between 0.1 and 10, the experimental data shows that the free convection heat transfer cannot be represented solely as a function of the Rayleigh number, but a dependence on Knudsen number must be introduced into the correlation as well. The presented results for the Nusselt number below Rayleigh numbers of $10^{-8}$ exhibit a clear departure from the available correlations in the literature. A new empirical correlation for free convection from horizontal thin wires from the transition to continuum regime is presented. The present work sheds light on current thermal engineering problems which involves very thin wires in their systems which interact with a surrounding environment with a significant level of rarefaction.
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