The friction characteristics of gaseous slip flow in microtubes

The present work studies the friction characteristics of gaseous slip flow in microtubes. The reduced diameter of microtubes has significant influence s on the flow. The degree of influence depends on the Knudsen number. If th e Knudsen number is in the range of 10-(3) to 0.1, the fluid can be assumed to be a continuum but a slip boundary condition at the tube wall has to be employed to account for the incomplete tangential momentum and energy exch anges between the gas molecules and the wall. Although slip flow in microtu bes can be investigated by solving numerically the compressible Navier-Stok es equations, the hyperbolic-parabolic character of the equations makes it very inefficient. The very large length to diameter ratio of microtube flow s suggests that they can be predicted accurately as well as efficiently by solving the compressible boundary-layer equations. The parabolic character of the boundary-layer equations renders the present method a very efficient and accurate tool in studying slip flows. The results confirm the findings of earlier investigators that the product off - Re is smaller for laminar microtube flows than that predicted by the conventional theory when the flo w is in the slip region.