Experimental Verification of the Dusty-Gas Theory for Thermal Transpiration through Porous Media

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Abstract
For complete comparison of the dusty‐gas theory for thermal transpiration through porous media with experimental results, isothermal permeability measurements as well as a knowledge of the geometric properties of the porous media are needed in addition to the thermal transpiration measurements. Such measurements were made for several gases on five porous ceramics and two Millipore filters under a variety of conditions, and steady‐state results of a net flow that equals 0 and results of a net flow that does not equal 0 were compared with the theory. Good agreement was found to exist in the range of a pore diameter to molecular mean‐free‐path ratio d e / λ < 10 . The coefficients appearing in the equations can all be determined from independent experiments; for a quantitative prediction of transpiration results it is, however, necessary to adjust the porosity–tortuosity factor appearing in the coefficient which arises from the termal‐diffusion term. This porosity–tortuosity factor was found to be more closely represented by ε / q 2 rather than ε / q .

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