The velocity field characteristics of high‐viscosity fluids falling film flow down clamped channels

Abstract

The complexity of falling film flow has been studied in many industrial applications. In this work, the velocity field of high-viscosity fluids falling film flow down clamped channels was investigated numerically and experimentally.The results show that the numerical simulation results are consistent with the experimental results, and the characteristics of the velocity field are related to the fluid properties, operating conditions, and structure of the clamped channels. When the fluid viscosity is greater than or equal to 10 Pa Á s, the type of velocity field changes into I shape, U shape, and V shape. While the fluid viscosity drops to 0.89 Pa Á s, the viscous force cannot resist the inertial force and gravity, resulting in a cardioid velocity field. By adjusting the structure of the clamped channels and operating conditions, the tension of the liquid film can be changed, and the velocity distribution of the liquid film can be manipulated.Significantly, under the fluctuating curtain flow, the liquid film coalesces and breaks frequently, which enlarges the surface area of the liquid film and strengthens the surface renewal frequency. Hence, this form of falling film flow can be applied to process intensification of high-viscosity materials.clamped channels, falling film flow, high-viscosity fluids, velocity field | INTRODUCTIONAs an energy-efficient process for intensifying heat and mass transfer, falling film flows have been widely used in chemical engineering, [1] refrigeration, [2] and seawater desalination. [3] Various flow patterns of falling film flows based on fluid properties, the geometric structure of the supporting wall, the working environment, and other factors have been described in the literature. Many reports found that the flow patterns of liquid film flow down vertical geometric structures could be categorized as drop-like flows, inertially driven flows, gravity-driven flows, solitary wave-like flows, and disturbance flows. [4][5][6][7] Liquid viscosities can vary from low (0.001$1 Pa Á s), [8,9] such as water, glycerin, and aqueous lithium bromide solution, to high (thousands of times higher than the viscosity of low-viscosity fluids), [10][11][12] such as silicone oil, syrup, and melt polymer. The hydrodynamic mechanism of the falling film is strongly dependent on fluid viscosity. The falling film flow of low-viscosity liquid quickly evolves into wave flow driven by inertial force and gravity, [13] while the primary flow pattern of highviscosity fluid is peristaltic laminar flow due to the sizeable viscous resistance. Additionally, specific operating conditions may cause irregular fluctuations in falling film Abbreviations: CFD, computational fluid dynamics; LDA, laser doppler analysis; VOF, volume of fluids.