Diode effect of a rarefied binary gas mixture flowing through a long conical capillary

For gas mixtures flowing through conical capillaries with small feature sizes, intermolecular collisions compete with ballistic transport. This leads to discrepancies between real flow rates and predicted results by conventional hydraulic methods. A model is established to calculate the flow rates of rarefied gas flowing through long conical capillaries at arbitrary pressure and concentration drop. The applicability of the model extends from binary gas mixtures to reduced single-species gases. The research results show that the diode effect exists for either binary gas mixtures or single-species gases. The maximum and minimum of the diode effect is obtained in the intermediate flow regime, where the intermolecular collisions and the ballistic transport contribute comparably. The diode-effect curve associated with the heavy species exhibits the same nonmonotonic dependence on the mean rarefaction parameter as observed in the single-species gas case. Interestingly, the diode effect for the light species is exactly the opposite, presenting a trend of first decreasing and then increasing. This abnormal diode effect can be heuristically interpreted, within the framework of the present long-capillary approximation, as resulting from the combined influences of local gas-surface interactions, cross collisions between binary species molecules, and gas separation.