Experimental study and CFD simulation on the falling film characteristics of viscous fluid outside a vertical tube at very low Reynolds numbers

Abstract

The removal of volatiles from polymers is of great importance. In this paper, the falling film outside the vertical tube at very low Reynolds number (Re＜1) is studied by both experimental and simulation methods. Experimental data were used to establish a relationship between film thickness, fluid viscosity, and flow rate at low Reynolds numbers. The CFD simulation showed good agreement with the experimental results. Simulations also explored the residence time distribution density ($E\left(t\right)$), showing that higher flow rates or lower fluid viscosities correspond to narrower $E\left(t\right)$ distributions. The minimal differences in dimensionless residence time distribution density ($E\left(\theta \right)$) across different inlet flow rates indicate good consistency in the film flow. As fluid viscosity increases, $E\left(\theta \right)$ shifts slightly leftward and broadens, which enhances mass transfer efficiency during devolatilization. Three special-shaped tube models were established, the simulations revealed that the velocity distribution within the liquid films of elliptical and semicircular tubes follows the Nusselt semi-parabolic distribution. Additionally, all three special-shaped tubes can increase the surface renewal frequency of the gas-liquid interface to reduce the coagulation, which provide that the special-shaped tube has a good application prospect in the vertical falling film devolatilization production of high-viscosity and uniform-quality materials.