Mixing characterization in different helically coiled configurations by laser-induced fluorescence

P Kováts,D Thévenin,C Velten,K Zähringer,M Mansour

doi:10.1007/s00348-020-03035-0

P Kováts, D Thévenin + Show 3 more

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https://doi.org/10.1007/s00348-020-03035-0

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Abstract

Flow Mixing of two miscible liquids has been characterized experimentally in three different helically coiled reactor configurations of two different lengths in the laminar flow regime at Re = 50…1000. A straight helical coil, a coiled flow inverter, and a new coiled flow reverser have been built, each in a 3-turn and a 6-turn configuration. Laser-induced fluorescence of resorufin has been used to visualize and quantify mixing in cross-sections throughout the reactors. A mixing coefficient is derived from the fluorescence images to allow for a quantitative measure and comparison of the six configurations. It becomes obvious from these experimental results, that an early flow redirection in the helical configuration is beneficial to mixing. The 3-turn reactors achieve nearly the same mixing coefficients as the 6-turn reactors with the double length. This can be explained by the stabilizing effect of the Dean vortices in the helix, which develop during the first two turns. After that, the liquid is trapped inside the vortices and further mixing is inhibited. Accordingly, the coiled flow inverter and coiled flow reverser configurations lead to much higher mixing coefficients than the straight helical coil. The results of these measurements are now used for validation of numerical simulations, which reproduce the geometrical and flow conditions of the experiments. Some exemplary results of these calculations are also shown in this article.Graphic abstractMass fractions of tracer fluid at Re = 500 in the six examined helix configurations.

Highlights

Coiled helical reactors are used in industrial processes for static mixing or heat and mass transfer to intensify these operations compared to straight tubular reactors (Vashisth et al 2008)
It was shown that the performances of the coiled flow inverter (CFI) and the coiled flow reverser (CFR) are comparable and obviously higher than those of the straight coil, for the perpendicular interface at the inlet, that leads to very bad mixing in the straight coil
Page 5 of 17 203 that one main application of coiled mixers lies in the laminar flow regime, this study only considers Reynolds numbers from 50 ≤ Re ≤ 1000

Summary

Introduction

Coiled helical reactors are used in industrial processes for static mixing or heat and mass transfer to intensify these operations compared to straight tubular reactors (Vashisth et al 2008). The development of secondary flows (Dean vortices) in these helical tubes enhances the radial mixing, while keeping a low axial back-mixing behaviour This increases heat and mass transfer and leads to narrower residence time distributions. The group of Nigam (Kumar et al 2006; Sharma et al 2017) investigated experimentally and numerically the influence of Reynolds number Re = (u ⋅ d ⋅ )∕ , Schmidt number Sc = ∕( ⋅ ab) , and curvature ratio on the mixing performances of different coiled geometries, by determination of the residence time In these equations, u is the mean velocity in the tube, is the fluid density, is the dynamic viscosity, the curvature ratio is defined as the ratio of the tube diameter d to the coil diameter D , ab is the mass diffusivity from the first fluid into the second fluid. They conclude that mixing increases with the Reynolds number, while Schmidt number only plays a role for rather low Reynolds-numbers, Re < 20

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