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Rapid advances in microfluidic devices have induced interest in the study of the microscale flow mechanism. However, the experimental results of microscale flow often deviate from the classical theory, and we attribute this deviation to the changing liquid viscosity in the microchannels. Because of the effect of the solid–liquid intermolecular force, the viscosity of the liquid near the walls is different from the bulk viscosity. Based on molecular theory and wetting theory, we propose a modified apparent viscosity model. The apparent viscosity of the liquid in microchannels increases with the increase in wettability and decreases with the increase in distance from the wall and the increase in drive pressure. The apparent viscosity near the hydrophilic wall is higher than the bulk viscosity, which increases the flow friction in the microchannels. To validate this model, we experimentally investigate the frictional characteristic of a deionized water flow in smooth parallel-plate microchannels with different wettabilities and heights of approximately 20 and 50 μ m . The results indicate that the friction factor is higher than that predicted by the classical theory. Such a difference increases with increasing wettability and decreases with increasing hydraulic diameter and pressure drop, which is consistent with the results of theoretical analysis. The apparent viscosity calculated by the apparent viscosity model notably fit the experimental results, with a relative difference of less than ± 2.1%. [ABSTRACT FROM AUTHOR]

Present paper investigates the influences of the fluid viscosity and acoustic impedance of the wall on the isentropic acoustic wave propagating along a shear flow contained in a circular pipeline. On the assumption of an axisymmetric acoustic wave, mathematical formulation with respect to the acoustic velocity is deduced from the conservation of mass and momentum. Numerical calculation is processed through an iterative procedure based on the Fourier–Bessel theory. Comparisons of the wave attenuation and measurement performance of an ultrasonic flow meter are addressed among the rigid, steel and aluminum walls. Meanwhile, the differences between the laminar and turbulent flow are provided. In conclusion, parametric analyses of the influence of the acoustic impedance on the wave attenuation and ultrasonic measurement performance are given. [ABSTRACT FROM AUTHOR]

Isentropic wave propagation in a viscous fluid with a uniform mean flow confined by a rigid-walled circular pipeline is considered. A method based on the Fourier-Bessel theory, which is complete and orthogonal in Lebesgue space, is introduced to solve the convected acoustic equations. After validating the method's convergence, the cut-off frequency of wave modes is addressed. Furthermore, the effect of flow profile on wave attenuation is analyzed. Meanwhile, measurement performance of an ultrasonic flow meter based on wave propagation is numerically accounted. [ABSTRACT FROM AUTHOR]

Abstract: Ultrasonic flow meter with non-invasive no-moving-parts construction has good prospective application for space on-orbit fluid gauging. In traditional pulse transit time flow meter, inconsistency of ultrasonic transducers leads to measurement error and plane wave theory, bases of transit time flow meter, is valuable only for low-frequency wave propagation in inviscid fluid and will lose feasibility when fluid viscosity is considered. In this paper, based on the hydrodynamics of viscous fluid, wave propagation with uniform flow profile is mathematically formulated and a novel solution for viscous fluid using potential theory is firstly presented. Then a novel design methodology of continuous ultrasonic flow meter is proposed, where high measurement rangeability and accuracy are guaranteed individually by solving the integral ambiguity using multi-tone wide laning strategy and the fractional phase shift using phase lock loop tracking method. A comparison with transit time ultrasonic flow meter shows the advantage of proposed methodology. In the end, parametric analysis of viscosity on wave propagation and ultrasonic flow meter is compressively investigated. [Copyright &y& Elsevier]