This paper explores the applicability of the Direct Simulation Monte Carlo (DSMC) method to the uid and thermal analysis of microelectromechanical systems (MEMS). Flows in two-dimensional microchannels are investigated because they represent basic geometrical components of MEMS. Supersonic, subsonic, and pressure-driven, low-speed ows are simulated by DSMC in microchannels of varying aspect ratios for a range of continuum to transitional regime rare ed ows. Both hot and ambient wall temperature cases are presented. The results are strongly dependent on Knudsen number and channel aspect ratio. They are in qualitative agreement with other computational and experimental results for longer microchannels. Near the continuum limit, they show the same trends as classical theories, such as Fanno/Rayleigh ow and boundary-layer interaction with shocks. This investigation establishes DSMC as an ef cient method for the analysis of MEMS: all simulations are carried out on a personal computer. Nomenclature h = channel height L = channel length P = pressure T = temperature U = streamwise velocity x = length coordinate along channel wall = mean free path Subscripts i = inlet o = outlet w = wall = freestream
A series of numerical simulation has been carried out to study thermo-hydraulic characteristics of a primary surface type heat exchanger, which is designed for the evaporator and condenser of a geothermal ORC. Working fluid is geothermal water at hot side and R-245fa, which is a refrigerant designed for ORC, at cold side. Amplitude ratio of the channel and Reynolds number are considered as design parameters. Nusselt number is presented for the Reynolds number ranging from 50 to 150 and compared to analytic solutions. The result shows that higher amplitude ratio channel gives better heat transfer performance within the range of investigation.
Key words ORC(유기랭킨사이클), Primary surface heat exchanger(주전열면 열교환기), Amplitude ratio(채널주름비),Friction factor(마찰계수), Nu(누셀트수) †Corresponding author, E-mail: jahn@kookmin.ac.kr
Conjugate heat transfer analysis for an ethylene furnace was carried out based on numerical simulation. Detailed distributions of velocity vectors, chemical species, and temperature inside the furnace are presented and discussed. Von Mises stress and heat flux at the tube surface were also evaluated to elucidate mechanisms regarding failure of the tube. Maximum stress was found at the upstream elbow of the tube, which did not coincide with the location of maximum heat flux. This implies that thermal stress at a steady state would not be a dominant component of the stress. Degradation of the material, as well as the system arrangement, should be considered in order to accurately predict the lifetime of the tube material in the furnace.
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