To bridge rarefied gas flow phenomena between macro- and microscale - Model development and experimental validation
Final Report Abstract
The flow behavior of so-called rarefied gases and their interaction with surfaces is of great interest in scientific fields such as catalysis, gas separation membranes and space technology. Rarefied gas flows occur when the pressure or the pores or channels through which the gas flows are small enough for the measured mass flow rate to be significantly higher than predicted by the classical Poiseuille equation, which is known as slip flow. The questions what slip is and how it depends on either the rarefaction, the characteristic length scale of confinements, or also on surface functionalisation remained unanswered. Despite several attempts to explain this effect phenomenologically by momentum accommodation (boundary conditions for models derived from kinetic theory), they remained incomplete as they neglected the forces imposed on the gas molecules by the channel surfaces. Furthermore, complex numerical simulations have been required to describe rarefied gas flows, as simple analytical models have so far either only covered very specific cases or require empirical parameters, which significantly limited their predictive power. In addition, all these modelling approaches have ignored functionalized surfaces, for which a clear influence on the diluted gas flow is known for pore diameters below 20 nm.In order to show whether such effects can also be observed for larger geometries with the same dominance of gas-wall interactions, i.e. similar Knudsen number, experiments were carried out at the TRANSFLOW facility in Karlsruhe and at the IUSTI facility in Marseille. Furthermore, an analytical model was developed that can describe the flow of dilute gas in straight microchannels for all dilution regimes by determining an effective gas molecule diameter. The general finding here is that it is possible to analytically describe rarefied gas flow in straight channels without empirical, geometry-dependent parameters, rendering the approach predictive. The simulation results of this model and complex numerical simulations were compared with the experimental flow data measured on planar and functionalized microchannels. This comparison made it possible to shed new light on the gas-surface interaction of dilute gases also with regard to the gas molecule diameter and to show that, surprisingly, even under the conditions of dilute gases, where gas-wall interactions dominate, wall functionalization does not influence the gas flow in microchannels.
Publications
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Model, plots and data for rarefied gas flow in straight channels
Kunze, S.
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Molecular diameters of rarefied gases. Scientific Reports, 12(1).
Kunze, S.; Groll, R.; Besser, B. & Thöming, J.
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Predicting Rarefied Gas Flow Through Surface Functionalized Channels [Oral presentation]. 32nd International Symposium on Rarefied Gas Dynamics, Seoul, South Korea, July 4-8
Kunze, S., Perrier, P., Groll, R., Besser, B., Varoutis, S., Graur, I. & Thöming, J.
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Rarefied Gas Flow in Functionalized Micro Channels [Oral presentation]. 4th European Conference on Non-Equilibrium Gas Flows, Eindhoven, Netherlands, March 29-31
Kunze, S., Perrier, P., Besser, B., Varoutis, S., Graur, I. & Thöming, J.
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Rarefied Gases and Functionalized Surfaces. PhD thesis. University of Bremen
Kunze, S.
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Raw data of rarefied gas flow through plain and functionalized rectangular micro channels
Kunze, S.
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Rarefied gas flow in functionalized microchannels. Scientific Reports, 14(1).
Kunze, Simon; Perrier, Pierre; Groll, Rodion; Besser, Benjamin; Varoutis, Stylianos; Lüttge, Andreas; Graur, Irina & Thöming, Jorg
