Abstract
We conducted systematic numerical investigations of the flow characteristics within the entrance region of rectangular microchannels. The effects of the geometrical aspect ratio and roughness on entrance lengths were analyzed. The incompressible laminar Navier–Stokes equations were solved using finite volume method (FVM). In the simulation, hydraulic diameters () ranging from 50 to 200 µm were studied, and aspect ratios of 1, 1.25, 1.5, 1.75, and 2 were considered as well. The working fluid was set as water, and the Reynolds number ranged from 0.5 to 100. The results showed a good agreement with the conducted experiment. Correlations are proposed to predict the entrance lengths of microchannels with respect to different aspect ratios. Compared with other correlations, these new correlations are more reliable because a more practical inlet condition was considered in our investigations. Instead of considering the influence of the width and height of the microchannels, in our investigation we proved that the critical role is played by the aspect ratio, representing the combination of the aforementioned parameters. Furthermore, the existence of rough elements obviously shortens the entrance region, and this effect became more pronounced with increasing relative roughness and Reynolds number. A similar effect could be seen by shortening the roughness spacing. An asymmetric distribution of rough elements decreased the entrance length compared with a symmetric distribution, which can be extrapolated to other irregularly distributed forms.
Highlights
Due to the rapid development of micro-electro-mechanical systems (MEMS), the flow mechanisms in microchannels have been a hot research topic worldwide [1,2,3,4,5,6]
The results showed that the previous studies that calculated the entrance lengths based on fully developed flow were inaccurate, and the entrance length was a function of Reynolds number and geometric parameters
The hydraulic diameter kept constant at 200 μm (D = 200 μm) and the aspect ratios were 1, 1.25, 1.5, 1.75, and 2, respectively, with the Reynolds numbers ranging from 0.5 to 100
Summary
Due to the rapid development of micro-electro-mechanical systems (MEMS), the flow mechanisms in microchannels have been a hot research topic worldwide [1,2,3,4,5,6]. Chen et al [10] studied the effect of the aspect ratio on laminar flow bifurcations in curved rectangular tubes driven by pressure gradients, and derived the ranges of stable flow solutions. In their further study [11], they investigated the effects of the aspect ratio on multiple flow solutions in a two-sided parallel motion lid-driven cavity. Unlike in conventional devices, in many micro-size channels, the length of the channel is always not sufficient to give rise to fully developed flows [12]. It is of great significance to estimate the entrance length of microchannels, especially under relatively lower Reynolds numbers
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