To bridge rarefied gas flow phenomena between macro- and microscale - Model development and experimental validation

对于稀薄气体流动，连续介质理论不成立，因为分子与管道壁的碰撞比分子之间的碰撞更频繁。在低压下或在非常小的管道中测量的质量流量明显高于经典泊泽伊方程的预测-一种传统上称为滑移流的效应。在过去的一个世纪里，人们设想了几种方法，通过动量调节（从动力学理论推导出的模型的边界条件）从现象学上解释这种效应，但由于忽略了管道表面施加在气体分子上的力，这种方法是不完整的。滑移是什么，以及它如何依赖于裂缝的稀缺性或裂缝的特征长度尺度，这些问题仍然没有答案。随着稀薄度的增加和特征长度的减小，气壁相互作用主导了流动，在相同的稀薄度下，不同的科学团体对流动的描述不同：“稀薄气体团体”（真空）使用动力学理论，而“多孔介质团体”（包括催化和膜研究）通过叠加扩散机制（对流、扩散）来描述气体流动。不同现象在宏观尺度和微/纳米尺度之间的桥梁还没有建立起来。这导致了这样一个事实，即在整个气体稀薄和尺寸尺度范围内对流动的可靠预测绝对超出了目前的技术水平。在提出的工作中，我们希望通过实验检测和理论描述在稀薄条件下发生的现象，但在稀薄变量，逆压力和逆特征长度都在中等范围内的情况下。上述两种方法都不能很好地描述该地区的气体流动现象。理论工作包括进一步发展初步开发的表面扩散模型，以便在吸附性质和表面扩散率从MD模拟或相应的实验中已知时，获得整个气体稀薄范围和所有尺寸尺度的预测方法。这项工作的第二个支柱是相应的实验活动。我们想要研究几何上彼此相似的钢制宏观和微通道，并将在不同温度和功能化通道表面下进行无因次质量流量与Knudsen数的比较。作为副产品，这项广泛的参数化工作将为稀薄气体动力学领域的未来研究提供前所未有的模型验证数据库。

For rarefied gas flows the continuum theory fails since molecules collide more frequently with the duct walls than with each other. Themass flow rate measured under low pressure or in very small ducts is significantly higher than predicted by the classical Poiseuille equation - an effect traditionally referred to as slip flow. In the past century several approaches were conceived to phenomenologically explain this effect by means of momentum accommodation (boundary conditions for models derived from kinetic theory) which is incomplete since forces imposed on gas molecules by the duct surfaces are neglected. The questions what slip is and how it depends on either the rarefaction or also on the characteristic length scale of confinements remain unanswered. With increasing rarefaction anddecreasing characteristic length, gas-wall interactions dominate the flow which, at identical rarefaction, is differently described by different scientific communities: the "Rarefied Gas Community" (vacuum) uses kinetic theory and the "Porous Media Community" (that comprises, e.g., catalysis and membrane research) describes gaseous flows by means of superimposed diffusion mechanism (convection, diffusion). The bridge between different phenomena at macro-scale and at micro/nano-scale is not built up yet. This results in the fact that the reliable prediction of flows over the whole range of gaseous rarefaction and size scales is definitely beyond the current state-ofart. In the proposed work we want to experimentally detect and theoretically describe phenomena that occur under rarefied conditions but in situations, in which both rarefaction variables, the inverse pressure and the inverse characteristic length, are in a moderate range. Gas flow phenomena in this area cannot be described properly by neither of the two approaches mentioned above. The theoretical work includes further development of the preliminary developed surface diffusion model in order to obtain a predictive approach for the whole range of gaseous rarefaction and all size scales when adsorption properties and surface diffusivity are known from, e.g., MD simulations or corresponding experiments. The second pillar of this work is given by the corresponding experimental activities. We want to investigate steel-made macro- and micro-channels, which are geometrically similar to each other and comparisons of dimensionless mass flow rate versus Knudsen number will be carried out for different temperatures and functionalized channel surfaces. As a byproduct, this broad parametric work will produce an unprecedented data base for model validation for future research in the field of rarefied gas dynamics.