Rarefied flow separation in microchannel with bends
Rarefied flow separation in microchannel with bends
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DOI：
10.1017/jfm.2020.585
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发表时间：
2020-10-25
影响因子：
3.7
通讯作者：
Zhang, Yonghao
中科院分区：
文献类型：
--
作者：
Ho, Minh Tuan;Li, Jun;Zhang, Yonghao
基于精确的数值解的动力学方程，使用良好的分辨率的空间和速度网格，稀薄气体在双矩形弯曲的微通道中的分离流动进行了研究在很宽的努森数和雷诺数范围。稀疏效应被发现发挥不同的作用，在流动分离（涡的形成）在凹，凸的角落。只有当努森数分别为和时，才能观察到凹、凸角附近的流动分离。随着Knudsen数的进一步增加，流动分离消失。由于固体壁面处的速度滑移，凹（凸）涡被抑制（增强），这导致稀薄气体流动相对于雷诺数的分离的晚（早）开始。凹涡和凸涡出现的临界雷诺数分别为0.32 × 10 ~（-3）和30.8。发现凹（凸）角附近的滑移速度随努森数的增加而增加（减小）。对于所有的努森数，在凹拐角附近都出现了逆压梯度，而对于凸拐角，逆压梯度只在努森数小于时出现。由于矩形弯管附近存在二次流和逆压梯度，相同长度的弯管和直管之间的质量流量比是努森数的非单调函数。我们的研究结果澄清了文献中报道的关于弯曲微通道中流速增强和涡流形成的多样化且往往相互矛盾的观察结果。
Based on an accurate numerical solution of the kinetic equation using well-resolved spatial and velocity grids, the separation of rarefied gas flow in a microchannel with double rectangular bends is investigated over a wide range of Knudsen and Reynolds numbers. Rarefaction effects are found to play different roles in flow separation (vortex formation) at the concave and convex corners. Flow separations near the concave and convex corners are only observed for a Knudsen number up to and , respectively. With further increase of the Knudsen number, flow separation disappears. Due to the velocity slip at the solid walls, the concave (convex) vortex is suppressed (enhanced), which leads to the late (early) onset of separation of rarefied gas flows with respect to the Reynolds number. The critical Reynolds numbers for the emergence of concave and convex vortices are found to be as low as 0.32 x 10(-3) and 30.8, respectively. The slip velocity near the concave (convex) corner is found to increase (decrease) when the Knudsen number increases. An adverse pressure gradient appears near the concave corner for all the examined Knudsen numbers, while for the convex corner it only occurs when the Knudsen number is less than . Due to the secondary flow and adverse pressure gradient near the rectangular bends, the mass flow rate ratio between the bent and straight channels of the same length is a non-monotonic function of the Knudsen number. Our results clarify the diversified and often contradictory observations reported in the literature about flow rate enhancement and vortex formation in bent microchannels.