Experimental Characterization of Gas Flow Properties of Dry Refractory Materials

In the previous years, the CoPhyLab* has established an experimental environment that makes it possible to simulate the activity of comets in the best way possible. Nevertheless, it is vital to know all parameters that have an impact on the outcome of such experiments, as well as parallelly developed models. Therefore, several smaller experiments provide important knowledge, especially about the used comet analogue, the CoPhyLab-dust.

With the gas-flow experiment we conduct in Graz it is possible to learn about the permeability and effusion of gas through porous media in a dry environment. In this presentation we will present how that works and what challenges we needed to overcome to achieve good scientific outcomes.
With the results of this experiment a special case of the dusty-gas model, established by Evans et al. 1961 [1], will be tested in high vacuum.

A vacuum chamber with two compartments, which are connected via a tube that holds the tested sample, is used for the experiment (see Figure 1). In both compartments pressure sensors are mounted onto the chamber. For our measurements we use two different gas-flow controllers with different gas-flow ranges to cover a wide pressure range. As this is a continuing work, the theoretical scheme was established by Schweighart et al. 2021 [2]. According to Equation 10 from their work, knowledge of the upstream and downstream pressure as well as the mass flux flowing through the sample, allows us to measure the Knudsen diffusion Dk as well as the permeability B of the sample.

Figure 1: Experiment Setup

Because gas-flow must be known precisely, a leakage of gas into the chamber, which is always there could have an influence on the evaluation of the results. Therefore, a bleed-up test was performed. To work with porous media is a difficult task. Therefore, measurements with round glass beads, with a predictable packing behavior, were performed.

We found that the results deviate from the regression using the dusty gas model at lower pressures or smaller gas flows (see Figure 2). There is a systematic decrease in pressure at smallest gas flows. We are currently investigating the possible causes for this behavior.
On the one hand, we consider if the deviation could be explained by set up related properties, like a leak in the chamber or a systematic offset on the gas flow. On the other hand, an outgassing of volatiles or a temperature dependence are possibilities we are looking into. Nevertheless, we already gain high confidence results in the measured pressure regime. In future we will investigate how those results compare to theoretical models, which consider the porosity and the tortuosity of such porous samples (e.g., Asaeda et al. 1974 [3]).

Figure 2: Experimental results for an asteroid analogue sample,
with the apparent Diffusion D_app depending on the mean pressure within the tested sample.

Acknowledgements:

This work is carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (DFG GU 1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36).

References:

[1] Evans III, R. B., G. M. Watson, and E. A. Mason. "Gaseous diffusion in porous media at uniform pressure." The journal of chemical physics 35.6 (1961): 2076-2083.

[2] Schweighart, M., et al. "Viscous and Knudsen gas flow through dry porous cometary analogue material." Monthly Notices of the Royal Astronomical Society 504.4 (2021): 5513-5527.

[3] Asaeda, Masashi, Shigeyuki Yoneda, and Ryozo Toei. "Flow of rarefied gases through packed beds of particles." Journal of Chemical Engineering of Japan 7.2 (1974): 93-98.

*The CoPhyLab (Comet Physics Laboratory): https://www.cophylab.space/index.php?id=home