Viscous and Knudsen gas flow through dry porous cometary analogue material

According to current theories of the formation of stellar systems, comets belong to the oldest and most pristine class of bodies to be found around a star. When approaching the Sun, the nucleus shows increasing activity and a pressure increase inside the material causes sublimated and trapped gas molecules to stream away from their regions of origin towards the surface. The present work studies two essential mechanisms of gas transport through a porous layer, namely the Darcy and the Knudsen flow. Gas flow measurements are performed in the laboratory with several analogue materials, which are mimicking dry cometary surface properties. In this first series of measurements, the aim was to separate gas transport properties from internal sources like local sublimation or release of trapped gases. Therefore, only dry granular materials were used and maintaining a low temperature environment was unnecessary. The gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained, thereby representing the relative importance of the respective flow mechanism. The experiments performed with air at a stable room temperature show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. The larger the grains, the bigger the permeability and the Knudsen diffusion coefficient. From the latter we estimated effective pore diameters. Finally, we explain how these parameters can be adapted to obtain the gas flow properties of the investigated analogue materials under the conditions to be expected on the comet.

Ten Stage Knudsen Gas Pump

A Knudsen gas pump is driven by the thermal transpiration effect. In this study, a Knudsen gas pump was fabricated which is powered by using a thermoelectric module. By adding current to the thermoelectric module, each module’s ceramic plate is heated/cooled causing a temperature gradient when positioned properly across the nano-porous membranes. Thermal transpiration occurs, causing a pressure gradient across the channels of the membrane which then induces a gas flow across the membrane. During the study, each single pump was initially tested for pressure performance by powering the thermoelectric at 6 and 15 Watts. Loaded and unloaded flow rates were also tested for a single pump. Five and 10 pumps were then connected into series and the pressure performance was tested at 6 and 15 Watts. The flow rate under loaded and unloaded conditions was also tested. It has been demonstrated that the 10 stage series Knudsen pump creates a significantly larger pressure than a 5 stage series or a single stage Knudsen pump.