Numerical study of liquid-liquid mixing in helical pipes

Abstract

The flow characteristics and the mixing performance of two miscible liquids in helical pipes have been studied numerically by computational fluid dynamics (CFD). The scalar transport technique is employed to quantify species mixing between the two fluids. The focus of the present study is set on investigating the optimal mixing behavior as a function of different parameters. The study is carried out for a wide range of relevant Schmidt and Reynolds numbers for laminar flow conditions. The Reynolds number (Re) and Schmidt number (Sc) have been varied from 5 to 104, and from 10 to 105, respectively. The model is first validated against experimental data from the literature. The effect of the inlet configuration is then examined; a vertical liquid interface at the inlet lead to the highest mixing efficiency. For low values of the Reynolds number, the results show that the mixing efficiency is reduced with increasing Schmidt number, until an asymptotic behavior is reached for very high Sc. For high values of the Reynolds number, increasing Schmidt number is observed to have only a minor influence on the mixing coefficient. The Reynolds number is found to have a more complex impact on mixing efficiency. Nevertheless, two optimal values of the Reynolds number can be found that lead to best mixing conditions in the laminar regime for a given length of the helical pipe. Though both values depend on the available length of the helix, they are typically found around Re≈50 and Re≈1000.