Abstract
Gas flow and heat transfer in confined geometries at micro-and nanoscales differ considerably from those at macro-scales, mainly due to nonequilibrium effects such as velocity slip and temperature jump. Nonequilibrium effects increase with a decrease in the characteristic length-scale of the fluid flow or the gas density, leading to the failure of the standard Navier–Stokes–Fourier (NSF) equations in predicting thermal and fluid flow fields. The direct simulation Monte Carlo (DSMC) method is employed in the present work to investigate pressure-driven nitrogen flow in divergent microchannels with various divergence angles and isothermal walls. The thermal fields obtained from numerical simulations are analysed for different inlet-to-outlet pressure ratios (1.5≤Π≤2.5), tangential momentum accommodation coefficients, and Knudsen numbers (0.05≤Kn≤12.5), covering slip to free-molecular rarefaction regimes. The thermal field in the microchannel is predicted, heat-lines are visualised, and the physics of heat transfer in the microchannel is discussed. Due to the rarefaction effects, the direction of heat flow is largely opposite to that of the mass flow. However, the interplay between thermal and pressure gradients, which are affected by geometrical configurations of the microchannel and the applied boundary conditions, determines the net heat flow direction. Additionally, the occurrence of thermal separation and cold-to-hot heat transfer (also known as anti-Fourier heat transfer) in divergent microchannels is explained.
Highlights
IntroductionUnderstanding heat and fluid flow in microchannels is essential to engineer novel microelectromechanical systems (MEMS) [7,8,9]
Gas flow in micro- and nano-channels with nonuniform cross-sections offers opportunities to develop small devices with novel applications.Understanding heat and fluid flow in microchannels is essential to engineer novel microelectromechanical systems (MEMS) [7,8,9]
This is a challenging task since the rate of collisions between the gas molecules and solid walls reduces with decreasing characteristic length-scale of the fluid flow or the gas density, affecting the random movement of molecules, which is commonly known as nonequilibrium effects [10,11]
Summary
Understanding heat and fluid flow in microchannels is essential to engineer novel microelectromechanical systems (MEMS) [7,8,9]. This is a challenging task since the rate of collisions between the gas molecules and solid walls reduces with decreasing characteristic length-scale of the fluid flow or the gas density, affecting the random movement of molecules, which is commonly known as nonequilibrium effects [10,11]. The Knudsen number (Kn), which is the ratio of the molecular mean free path λ to a characteristic length scale L , is often employed as an indicator of deviation from the equilibrium condition. Four different rarefaction regimes have been defined based on the Knudsen number [14], and are commonly named the free molecular (Kn > 10), transition (10−1 ≤ Kn ≤ 10), slip (10−3 ≤ Kn ≤ 10−1 ), and continuum (Kn ≤ 10−3 ) regimes
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