Study on Natural Convection in Differentially Heated Micro-Enclosure Systems Using DSMC Method

• Main Authors: Liu, Ming-ho, 劉明和
• Other Authors: Tzeng, Pei-yuan
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/06488903634741537005

博士 === 國防大學中正理工學院 === 國防科學研究所 === 93 === Fluid motion driven by thermal gradients is a common and important phenomenon in nature, as well as in numerous engineering applications such as meteorology, geophysics, astrophysics, solar energy collectors, crystal growth for microprocessors, cryogenic storage, heat exchangers, as well as multilayer walls and double windows in buildings. In addition to these applications, it can also be employed to analyze fluid hydrodynamic instability, bifurcation, self-organization, and chaotic behavior in fluids. The field of microelectromechanical systems (MEMS) has rapidly developed in the last years. MEMS applications range from consumer products to national defense industry. The thermal and flow behaviors of fluids in micro-scale systems or components are different from those obtained by traditional heat transfer and hydrodynamics in large scale systems; therefore, to understand the flow and heat transfer characteristics of the micro-scale thermally driven flow will be helpful in micro-systems design and fabrication. The natural convection in two-dimensional differentially heated inclined micro-enclosures, also called Rayleigh-Bénard convection, is investigated numerically by using the direct simulation Monte Carlo (DSMC) method in this dissertation. The major concerns of this study are convective flow instability, mode-transition, hysteresis phenomena, and influences of initial simulation conditions on convective flow patterns in the micro-enclosure. In the study of thermally driven flow in horizontal enclosures, the macroscopic heat conduction and convection states and the transition between these two states are simulated at the molecular level. Besides, the flow behaviors under various initial conditions are simulated systematically. It is found that the onset critical Rayleigh number predicted by the DSMC agreed with that of the linear stability theory. The results prove that the DSMC method is valid for studying the rarefied gas flow behavior driven by temperature gradients in micro-scale systems, and the Boussinesq approximation is not valid. Furthermore, the convective flow patterns are very sensitive to the initial conditions. The changes in the initial wall temperatures, enclosure scales, number of simulated particles, or cell sizes may cause the multiple solutions for the convective rolls. The boundary conditions for modeling adiabatic side walls are developed successfully in this study. The results show that the temperature and velocity distributions near the wall meet the adiabatic and non-slip conditions, and the critical Rayleigh number predicted by the simulations with these conditions are close to that obtained by linear stability analysis. Accordingly, they can be used for simulating the thermally driven flow in practical enclosures. In the study of natural convection in an inclined micro-enclosure, the macroscopic convective flow mode-transitions and hysteresis phenomena for inclination angle increasing and decreasing are observed at molecular level. Furthermore, the influences of hot wall temperatures, miniaturization, and side wall boundary conditions are also discussed. The results show that the inclination angles of mode-transition obtained by DSMC method in the micro-enclosure are much smaller than those resolved by CFD method with continuum model in a macro-scale system. Furthermore, the hysteresis phenomena can be observed but not obvious, and the differences between the solutions obtained by simulations with various side walls boundary conditions are very small. In the inclined micro-enclosure, the center of uni-roll structure is closer to the lower region of enclosure ( ), as hot wall temperature increases or enclosure miniaturizes. The results indicate that the macroscopic flow behaviors obtained by molecular model are different from those obtained by continuum model. The natural convection flow characteristics in micro-enclosures are discussed in depth in this study. The results can provide useful information for practical design and improvement of the related micro-devices and for convective flow instability investigations in other micro-scale systems.