Li Haiwang, Huang Binghuan, Wu Min
National Key Laboratory of Science and Technology on Aero Engines Aero-thermodynamics, The Collaborative Innovation Middle for Advanced Aero-Engine of China, School of Energy and Power Engineering, Beihang University, Beijing 100191, China.
Micromachines (Basel). 2019 May 11;10(5):317. doi: 10.3390/mi10050317.
Flow characteristics within entrance regions in microchannels are important due to their effect on heat and mass transfer. However, relevant research is limited and some conclusions are controversial. In order to reveal flow characteristics within entrance regions and to provide empiric correlation estimating hydrodynamic entrance length, experimental and numerical investigations were conducted in microchannels with square cross-sections. The inlet configuration was elaborately designed in a more common pattern for microdevices to diminish errors caused by separation flow near the inlet and fabrication faults so that conclusions which were more applicable to microchannels could be drawn. Three different microchannels with hydraulic diameters of 100 μm, 150 μm, and 200 μm were investigated with Reynolds (Re) number ranging from 0.5 to 50. For the experiment, deionized water was chosen as the working fluid and microscopic particle image velocimetry (micro-PIV) was adopted to record and analyze velocity profiles. For numerical simulation, the test-sections were modeled and incompressible laminar Navier-Stokes equations were solved with commercial software. Strong agreement was achieved between the experimental data and the simulated data. According to the results of both the experiments and the simulations, new correlations were proposed to estimate entrance length. Re numbers ranging from 12.5 to 15 was considered as the transition region where the relationship between entrance length and Re number converted. For the microchannels and the Reynolds number range investigated compared with correlations for conventional channels, noticeable deviation was observed for lower Re numbers ( < 12.5) and strong agreement was found for higher Re numbers ( > 15).
微通道入口区域内的流动特性因其对传热和传质的影响而十分重要。然而，相关研究有限，且一些结论存在争议。为了揭示入口区域内的流动特性并提供估算流体动力入口长度的经验关联式，对具有方形横截面的微通道进行了实验和数值研究。入口结构按照微器件更常见的模式精心设计，以减少入口附近分离流和制造缺陷引起的误差，从而得出更适用于微通道的结论。研究了三种不同水力直径分别为100μm、150μm和200μm的微通道，雷诺数（Re）范围为0.5至50。实验中，选择去离子水作为工作流体，并采用微观粒子图像测速技术（micro-PIV）记录和分析速度剖面。数值模拟方面，对测试段进行建模，并使用商业软件求解不可压缩层流纳维-斯托克斯方程。实验数据和模拟数据之间取得了高度一致。根据实验和模拟结果，提出了估算入口长度的新关联式。12.5至15的雷诺数范围被视为过渡区域，在此区域入口长度与雷诺数之间的关系发生转变。与传统通道的关联式相比，在所研究的微通道和雷诺数范围内，对于较低雷诺数（<12.5）观察到明显偏差，而对于较高雷诺数（>15）则发现高度一致。
