Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Implementation of DSMC Method to Nano Knudsen Compressors
Nevsan engil
EDA Ltd., Silikon Blok,No:22, ODTÜ Teknokent,06531, Ankara, Türkiye
Abstract- If density is low or characteristic length is micro/nano scale, it can be said that gas is rarefied. In rarefied gas conditions, gas starts
flowing slowly from cold to hot. This phenomenon is called thermal creep or transpiration. Using thermal creep phenomenon, Knudsen
compressors are built. In this study various properties of a nano scale Knudsen compressors are analyzed with direct simulation Monte Carlo
(DSMC) method.
Lately, a number of Micro/Nano Electro Mechanical
Systems (MEMS/NEMS) have been developed. These
devices sometimes include mechanical systems work with
the fluids such as micro/nano size gas compressors. These
compressors have much potential in the area of
chromatography, spectroscopy, micro plasma
manufacturing and chemical sensors [1,2].
If two gas reservoirs with different pressures and
temperatures are connected with a channel, gas starts
flowing from high-pressure side to low-pressure side.
When the reservoir pressures get equal, gas flow stops
even if reservoir temperatures are different. In micro/nano
scale lengths, gas is rarefied even if pressure is
atmospheric. If gas is rarefied and a temperature gradient
exists, gas flows slowly from cold region to hot region. It
is called thermal creep or transpiration phenomenon [3].
Using this phenomenon, it is possible to construct various
micro/nano size Knudsen compressors.
The theoretical efficiencies of Knudsen pumps are high
compared to conventional vacuum pumps. Besides they
are very reliable because they include no moving parts.
Recent technological development in the area of thermal
isolation on MEMS/NEMS, enable to use high temperature
gradients to obtain high volume rates [1].
Gas flows related with the MEMS/NEMS devices have
high Knudsen numbers (Kn) similar to rarefied gases of
high atmosphere flights. Rarefied gas flows with high
Knudsen number ( Kn 0.1) depart from local thermal
equilibrium because of the inadequate molecule collisions.
Consequently, the linear relations between not only shear
stress and velocity gradient but also heat conduction and
temperature gradient are lost. As a result continuum based
Navier-Stokes and Euler equations cannot be used because
these equations use linear constitutive equations [4].
In rarefied gas flows with high Knudsen number
( Kn 0.1) , both continuum equations with high order
non-linear constitutive equations, like Burnett equations,
and molecular based methods can be used. Burnett
equations are not used widely because these equations are
difficult to solve and have both stability and complicated
boundary condition problems. In rarefied gas flows,
generally molecular methods are preferred. Molecular
methods are based on the Boltzmann equation, which is a
mathematical model and difficult to solve both analytically
and numerically. Only its simplified versions can be
solved. Molecular dynamic (MD) is the best-known
physical molecular method [3]. MD is generally used to
analyze liquid and dense gas flows. Because of the huge
number of the molecules, only very small flow volumes
can be analyzed for very small time durations. Direct
simulation Monte Carlo (DSMC) is another physical
molecular model. In this method molecule movements and
collisions are decoupled and one DSMC molecule
represents many physical molecules [5]. DSMC consists of
four main steps. The first step is “molecule movement”
step. In this step, molecules move inside the flow area. The
second step is “molecule indexing” step. Molecules are
indexed based on their cell information. The third step is
“molecule collisions” step. Here molecules in the same
cells undergo collisions with each other. The fourth step is
“calculation of macroscopic properties” step. In this step,
using microscopic molecule information, macroscopic
values in each cell are calculated.
In this study one stage and multi-stage Knudsen
compressors are analyzed with DSMC method. Pumping
speeds and maximum pressure ratios of Knudsen
compressors will be reported together with boundary
conditions used.
Figure 1. Reservoir pressure decreases with thermal
transpiration.
sengiln@itu.edu.tr
[1] S. McNamara and Y.B. Gianchandani, J., 2005. On-Chip
Vacuum Generated by Micro Machined Knudsen Pump,
Microelectromech. Syst. 14, 4:741-745.
[2] E.P. Muntz and S.E. Vargo, 2002. Microscale Vacuum
Pumps in The MEMS Handbook, M. Gad-el-Hak, Ed. Boca
Raton, FL: CRC.
[3] G.E. Karniadakis and A. Beskok, 2002. Micro Flows
Fundementals and Simulation, Springer-Verlag, New York.
[4] S. Chapman and T.G. Cowling, 1970. The Mathematical
Theory of Non-Uniform Gases, Cambridge University Press,
New York.
[5] G.A. Bird, 1994. Molecular Gas Dynamics and the Direct
Simulation of Gas Flows, Clarendon Press, Oxford.
6th Nanoscience and Nanotechnology Conference, zmir, 2010 688