Computational Studies on the Effects of Non-Linear Temperature Functions in Thermal Creep Membranes of Radiantly Driven Knudsen Compressors

Employing rarefied gas phenomenon of thermal creep (also known as thermal transpiration), Knudsen Compressor is a micro/meso-scale gas compressor/pump without moving parts. Driven by a temperature difference, gas molecules moved from the cold side of the thermal creep channel, which has a size less than the molecular mean free path, to the hot side of the channel. To utilize its low thermal conductivity and nanometer range size pores, carbon opacified aerogel membranes, treated as a bundle of thermal creep channels, were used in prior experimental studies of radiantly driven Knudsen Compressors. By absorbing the radiation energy, a temperature gradient will develop inside of a carbon opacified aerogel membrane to drive thermal creep flows. Analytical studies of the radiation energy absorbed by a carbon opacified aerogel membrane were performed and the resulting non-linear temperature distribution function within the carbon opacified aerogel thermal creep membrane was identified previously. This paper presents DSMC (Direct Simulation Monte Carlo) simulation studies that incorporate the previously reported non-linear temperature distribution function to investigate the performance of the radiantly driven Knudsen Compressor with a carbon opacified aerogel membrane. Cases with different connector temperatures for a closed system Knudsen Compressor were studied to observe the maximum pressure differences. Comparison of results indicates that radiantly driven Knudsen Compressor with a carbon opacified aerogel membrane could achieve a larger pressure gradient than what is predicted by the theoretical model reported by Muntz et al.

Multiscale Simulation of Knudsen-Pump Channel Flows

This paper describes the development and application of an efficient hybrid continuum-molecular approach for simulating non-isothermal, low-speed, internal rarefied gas flows, and its application to flows in Knudsen compressors. The method is an extension of the hybrid approach presented by Patronis et al. (2013) [J. Comp. Phys., 255, pp 558–571], which is based on the framework originally proposed by Borg et al. (2013) [J. Comp. Phys., 233, pp 400–413] for the simulation of micro/nano flows of high-aspect-ratio. The efficiency of the multiscale method allows the investigation of alternative Knudsen-compressor configurations to be undertaken. We characterise the effectiveness of the single-stage Knudsen-compressor channel by the pressure differential that can be achieved between two connected reservoirs, for a given temperature difference. Our multiscale simulations indicate that the efficiency of the single-stage Knudsen compressor is robust to modifications of the streamwise temperature variation.

Performance Model for Optically Driven Micropumps With Carbon Opacified Aerogel Membranes

Aerogel, a highly porous material with less than several percent of solids, has been utilized in applications requiring high precision thermal managements due to its extremely low thermal conductivity. Combining the advantages of high porosities and low thermal conductivities, aerogels were used as thermal creep membranes in Knudsen Compressors, micro/meso-scale pumps/compressors with no moving parts. Heating one side of the thermal creep membrane to create a temperature gradient, a Knudsen Compressor is operated based on the rarefied gas phenomenon of thermal creep to create flows and to induce a pressure gradient from the cold side to the hot side of the membrane. Adding carbon particles in silica aerogels creates an optically thick, opacified carbon aerogel that can absorb radiation energies to heat up one side of the aerogel membrane in a Knudsen Compressor to create thermal creep flows. An analytical model was developed to predict the temperature profile inside of the carbon opacified aerogel thermal creep membrane for the Knudsen Compressor. Applying this temperature model, pressure ratios achieved by the optically heated Knudsen Compressors for given operating conditions were also studied and correlations between the membrane thickness and the maximum pressure increase were determined.

Characterization of a Radiantly Driven Multistage Knudsen Compressor

Knudsen Compressors are meso/micro scale gas compressors/pumps based on thermal transpiration or thermal creep. The design of radiantly driven Knudsen Compressors is discussed, along with a model that was developed to understand their performance. Experimental pumping performances for Knudsen Compressors with one, two, five, and fifteen stage, radiantly driven cascades are also discussed. Temperature measurements across the transpiration membranes, for various pressures of Nitrogen, were obtained and compared to those predicted by the performance model. The results agree with the model to within 15% consistently under predicting the measured hot side temperature of the transpiration membrane. The pump-down curves, steady-state maximum pressure differences, and maximum flow rates produced by a single stage Knudsen Compressor were obtained. A variety of configurations were studied at pressures from 500 mTorr to atmospheric pressure. The experimental results agreed with the performance model’s predictions to within 20%.

The Knudsen Compressor As an Energy Efficient Micro-Scale Vacuum Pump

The low-pressure pumping limit of the MEMS Knudsen Compressor, a thermal transpiration pump, is identified and discussed. The practical low-pressure limit is due to a requirement for an approach to continuum flow in the connector section, it is roughly 10mTorr. A previously developed transitional flow model was used to size several Knudsen Compressor cascades that operate down to the low-pressure limit. Designs based on previous experimental Knudsen Compressors do not provide the necessary pumping efficiency. A design, employing carefully sized capillaries etched in aerogel transpiration membranes, is shown to result in a viable device. A cascade incorporating this design provides a gas flow rate of 3E16 mol/sec, while pumping from a pressure of 10mTorr to 1 atm. It requires a volume of 73 cm3 and 2.0 W. Design considerations are outlined for MEMS Knudsen Compressors operating at their lower pressure limit. A primary concern, efficiently transitioning from the capillary section to the connector at constant temperature is discussed.