Construction and analyses of molecular exchange flow for gas mixtures in microchannels

The special flow phenomenon in microflow, molecular exchange flow, can be used for gas separation. The mathematic models to describe principles of molecular exchange flow are established by illustrating a novel gas separator with similar structure to the Knudsen pump based on thermal transpiration effect. The ratio of temperature gradient to pressure gradient is employed to determine the characteristics of molecular exchange flow. Taking binary gas mixtures of He–Ne, He-Ar and Ne–Ar as examples, the construct conditions and influence factors on flow behaviors are investigated on negative/ideal/positive molecular exchange flows. The limiting ratios of temperature gradient to pressure gradient are sensitive to Knudsen number. The intensity of molecular exchange flow is greatly affected by not only mole fractions of gas components but also Knudsen number. The temperature difference and implicitly corresponding pressure difference should be carefully chosen since they are the driving forces for molecular exchange flow phenomena. The intensities and construct conditions of molecular exchange flows are closely related to the properties of species in gas mixtures.