The purpose of this research was to apply proper gas-surface collision
rules to achieve an adiabatic solid wall condition and to study the effects of
aspect ratio (AR) and lateral boundary conditions on critical Rayleigh
numbers (Rac) and flow patterns of rarefied gas in a rectangular micro
enclosure. The results showed that the general trend for the onset of
convection went on at nearly the same Rayleigh number (Ra) as the AR
increased for AR greater than 2. Due to the closer confinement of the
sidewalls, the Rac increased gradually with decreasing AR for AR lying
between 2 and 1 and then increased rapidly for AR less than 1. The
prediction of Rac versus AR with three different lateral boundary conditions
showed a similar qualitative trend. The Rac for a lateral boundary with
isotropic scattering conditions (IS-BC) was greater than those with specular
reflection conditions (SR-BC) and a little smaller than partial specular
conditions (PS-BC) in the quantitative aspect. The larger values of Rac can
be attributed to the increased viscous effect of the sidewalls. Also, because
of sidewall friction realized by IS-BC and PS-BC, the flow patterns of even
roll modes were easier to form and convection rolls rotated alternatively in
counterclockwise and clockwise directions from left to right side
boundaries.