气体在纳米孔内流动与换热的实验与分子动力学模拟研究

Gas flow and heat transfer at nanoscale show unconventional phenomena. It's very necessary to master the variation law of the gas's flow resistance coefficient and convective heat transfer coefficient at nanoscale. On the other hand, our preliminary studies have found that at nanoscale the gas molecule's energy and momentum transfer characteristics, which play an important role in the gas's flow and heat transfer, change compared with that at macroscale. We believe that it's much necessary to make an in-depth study of gas flow and heat transfer characteristics at nanoscale from the perspective of the gas molecule's energy and momentum transfer characteristics as well as mastering the variation laws of flow resistance coefficient and convective heat transfer coefficient. The present project chooses the nanopore which is the most common and most easily being processed as the nanoscale space form to be studied. First, the experimental studies will be carried out to get the equations describing the flow resistance coefficient and convective heat transfer coefficient and the relationships between the two coefficients. At the same time, using the molecular dynamics simulations the present project will study the gas molecule's momentum and energy transfer characteristics in nanopores and their corresponding effects on the flow and heat transfer, study whether the analogies between the momentum and energy transfer still exist at nanoscale and study the interaction between the gas flow and heat transfer in nanopores. Based on the laws of conservation and combing with the gas molecule's energy and momentum transfer characteristics in nanopores, the present project will study and analyze the differences and correlations between the gas flow and heat transfer in nanopores and the corresponding macroscale pipes, which would lay theoretical foundations to solving the problems of gas flow and heat transfer existing in the engineering at nanoscale such as NEMS.

纳米尺度下气体流动与换热表现出非常规现象，掌握其流动阻力系数与对流换热系数变化规律很有必要。另一方面，项目前期研究发现对流动与换热起重要作用的气体分子间动量能量传递特性在纳米尺度下有所改变。我们认为在掌握流动阻力系数与对流换热系数变化规律的同时，更需从气体分子间动量、能量传递特性角度深入研究纳米尺度下气体流动与换热特性。本项目选择纳米孔这一最常见最易加工的纳米空间形式，先实验研究描述纳米孔内气体流动阻力系数、对流换热系数的准则关联式及两系数间的联系。同时，本项目采用分子动力学模拟研究纳米孔内气体分子间动量、能量传递特性及其对流动与换热的影响，研究其动量、能量传递间是否存在类比关系，及流动与换热间的相互作用。在守恒定律基础上，结合纳米孔内气体分子间动量、能量传递特性研究其流动和换热规律与宏观尺度下管内对应规律的区别与联系，为NEMS等纳米尺度下工程中所遇气体流动与换热问题的解决奠定理论基础。

人们已认识到气体在纳米尺度下表现出一些异于宏观尺度下的流动与换热现象，但这些现象背后的物理机理还有待揭示。研究气体在纳米尺度下的流动与换热特性对流体力学、传热学理论发展及NMES等纳米尺度下工程中所遇气体流动与换热问题的解决具有重要意义。. 本项目对气体在纳米孔内的流动与传热特性进行研究，主要内容包括：实验与分子动力学模拟研究气体在纳米孔内的流动特性，研究气体在纳米孔内流动摩擦阻力系数，研究气体在纳米孔内分子平均自由程及粘性变化规律，研究气体在纳米孔内的动量交换特性；分子动力学模拟研究了气体在纳米孔内的换热特性，包括对流换热系数，温度分布，能量交换特性。我们的研究结果表明气体在纳米孔内流动时摩擦阻力系数大大减小，并受纳米孔大小和气固间作用强弱的影响。对气体在纳米孔内分子平均自由程及粘性系数的研究结果表明，气体在纳米孔内的分子平均自由程及粘性系数都比宏观尺度下小，并且表现出尺度效应及气固间作用强弱依赖效应。气体分子在纳米孔内的运动表现出明显的各向异性，轴向气体分子平均自由程大于径向气体分子平均自由程。重要的是气体在纳米孔内的分子平均自由程及粘性系数并非均匀一致，而是呈现出明显的分布特征。在靠近固体壁面处，气体的分子平均自由程及粘性系数最小，这为揭示气体在纳米孔内流动摩擦阻力系数减小奠定了理论基础。本项目对气体在纳米孔内的动量、能量交换特性研究表明，气体在纳米孔内的动量、能量交换分为两个区域，一个区域失去动量或能量，而另一区域获得动量或能量。靠近固体壁面0.5nm范围内是流动摩擦阻力及与固体壁面进行热交换的主要区域，故该区域内气体的行为特征对其流动与换热起着决定性的作用。本项目对气体在纳米孔内对流换热的分子动力学模拟研究表明气体在纳米孔内的对流换热系数比经典传热学理论值小，同样表现出尺度效应及气固间作用强弱依赖效应。当气体与定壁温纳米孔进行对流换热时，其最高温度出现在靠近壁面处，而不是经典传热学理论所给出的孔中心。同时，在紧靠近固体壁面处存在很大的温度梯度。本项目的这些研究成果为人们认识和控制气体在纳米孔内的流动与换热提供了物理机理上的解释与理论支持。