Investigation of Temperature Driven Gas Flows in 4 nm Channels for Applications of Micro-Scale Compressors at Above Atmospheric Pressure

Based on the rarefied flow phenomenon of thermal creep (or thermal transpiration), the Knudsen Compressor is an unconventional micro/meso-scale compressor or pump. Optimization studies have shown that a Knudsen Compressor operates most efficiently when its membrane’s flow channels are at the transitional flow regime, between continuum and molecular flows; simultaneously it provides a desired mass flow and pressure ratio. At higher pressures (> 1 atm), to maintain membrane channel Knudsen numbers in the transitional regime (Kn ∼ 1), the corresponding membrane channel size needs to be less than about 50 nm. More specifically, at 10 atm, the membrane channel size should be as small as 5 nm to provide the most efficient Knudsen Compressor operation. Prior to this work, there has been no documented experimental investigation of thermal creep measurements through channels less than 5 nm. Phenomena that could be associated with such flows are briefly discussed, and possible selection criteria for thermal creep membranes are included in this study. Apparatus design is discussed. Experimental results are provided for thermal creep flows, within a single stage Knudsen Compressor with 4 nm diameter membrane channels. The maximum pressure increases across the Knudsen Compressor’s thermal creep membrane were measured, over a range of operating pressures from 1 atm to 1.1 atm with Helium or Argon as the working gas. Results showed apparent thermal creep effects across the porous glass membrane, and possibly significant force field effects within the nano-scale channels.Copyright © 2008 by ASME