Theoretical Efficiency of a Microfabricated Knudsen Pump

This paper describes the derivation of the maximum theoretical efficiency for a Knudsen pump. The Knudsen pump functions on the principle of thermal transpiration in rarified gas conditions. The pump requires only a temperature difference between the inlet and the outlet to operate. The rarified gas conditions are established by having the pump channel's characteristic dimensions, i.e. the diameter or height, smaller than the mean free path of the gas being pumped. The efficiency is shown to follow all of the trends imposed by the principle of thermal transpiration. The maximum obtainable efficiency for any gas approaches the ratio of the ideal gas constant divided by the specific heat capacity, which is achieved at a very high temperature ratio.