Low Pressure Performance of Micro/Meso Scale Knudsen Compressors by Dr. Y.-L. Han

Yen-Lin Han Department of Aerospace & Mechanical Engineering University of Southern California Los Angeles, CA Abstract:Continuing advances in MEMS fabrication capabilities and strategies are beginning to facilitate significant progress in miniaturizing the functionality of many conventional and unconventional thermo/mechanical machines. A significant number of these evolving devices require micro/meso-scale gas pumps or compressors in order to create complete, miniaturized systems. One option is the Knudsen Compressor, named in 1994 by Pham-van-Diep et al [1]. Knudsen Compressors are solid-state, micro/meso-scale gas pumps or compressors with no moving parts. Based on the rarefied flow phenomenon of thermal creep, Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membranes [2]. Studies have shown that a Knudsen Compressor with an aerogel membrane, can be operated efficiently by either resistive or radiant heating techniques over a pressure range from about ten atmospheres down to 250 Torr [3][4]. Employing different 'membrane' configurations, Sone and Sugimoto have recently studied several meso/macro-scale thermal creep (thermal transpiration) pumps at pressures of around 2 Torr and lower.[5][6] At low pressures (< 1Torr), relevant issues encountered for providing efficient operation of micro/meso-scale Knudsen Compressors include; large membrane channel sizes, required because of relatively large molecular mean free paths; and "reverse" thermal creep in the connector sections due to finite connector channel to membrane channel size ratios. Mechanically machined aerogel membranes with circular channels have already been studied; results have shown that they are attractive candidates as Knudsen Compressor membranes at low pressures.[7] The performance of these membranes has also been found limited by rarefaction effects in the connector section such as reverse thermal creep flow, and related induced internal flows.[8] The principal goal of this study was to investigate in greater detail than previously reported[8] the fundamental limitations encountered in reducing a micro/meso-scale Knudsen Compressor's operating inlet pressures to as low as 10 mTorr (10-5 atm). Both experiments and simulations were employed in this investigation. For the experimental studies[9] aerogel membranes, incorporating mechanically machined 500 mm high, 0.5 mm long, and 1cm wide rectangular, supplementary flow-channels, were used for the investigation of a Knudsen Compressor stage's performance at low pressures. For connector section Knudsen numbers greater than about 0.1, the pressure ratio gain through an entire stage was seriously impacted by the connector section's reverse thermal creep flow. This finding is consistent with earlier circular channel results [8][9]. Direct Simulation Monte Carlo (DSMC) technique codes were constructed for further investigations of the reverse thermal creep flow in connector sections [9]. A two-dimensional simulated domain was adopted to mimic a simplified, rectangular channel, single stage Knudsen Compressor. The effects of the reverse thermal creep flow in simulated connector sections, for several connector to membrane channel size ratios and several wall temperature distributions, have been obtained in the simulations. The simulation results were in good agreement with appropriate theoretical predictions based on available flow coefficients [10]. This investigation quantifies, using the results of both experiments and simulations, the importance of reverse thermal creep induced flows
in the connector sections of low pressure, single stage micro/meso-scale Knudsen Compressors. As the connector section Knudsen number rises above about 0.1, the performance of Knudsen Compressors, with either rectangular or circular channels, will be progressively decreased by reverse thermal creep induced flows in the connector sections

Location: John Stauffer Science Lecture Hall (SLH) - , Rm 100

Audiences: Everyone Is Invited

Contact: April Mundy