Recently, a large number of experiments have been conducted to examine the applicability of Navier-Stokes equations to predict the friction factor for the laminar flow of polar fluids in microchannels. However, significant discrepancies still exist between various results. In order to investigate the effect of electric double layer on the pressure-driven flow of polar fluid in smooth rectangular microchannels and to reveal whether continuum model can still be applied, numerical investigations are conducted in this paper. The simulated microchannels are made of silicon engraved substrate with Pyrex cover, and the measured relative roughness of channels is less than 0.5%. Deionized water and tap water with different ion concentration and electrical conductivity are used as the working fluid. The governing equations include the two-dimensional, non-linear Poisson-Boltzmann equation, the modified N-S equation and the electric field Ez equation. The steady state electric field Ez equation is coupled with the momentum equation. The FVM (finite volume method) is adopted to discretize the governing equations. The non-uniform grid systems 152×102 is applied and the Reynolds number ranges from 0.1 to 300. Good agreements are achieved between the numerical results and the experimental data available in the literature. The effects of electrical potential, fluid ion concentration, electrical conductivity and the channel dimensions on the EDL profile, and the electroviscous effect and the friction coefficient are presented in detail. The simulation results reveal that the Debye thickness depends on ion concentration greatly. Only when the ratio of Dh/δ (the ratio of channel hydraulic diameter to the Debye thickness) is low, the electroviscous effect should be considered. According to the experimental conditions, the numerical results show that if the ratio of Dh/δ is greater than 15, the predicted friction factors agree well with the macroscale classical law, and the disparities are less than 4%.

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