Improving the mixing characteristics of coiled configurations by early flow inversion

Abstract

This work considers numerical investigations of liquid–liquid mixing in helical pipes and coiled flow inverters (CFI). Since mixing is particularly efficient in the developing region, a higher mixing efficiency is obtained by inverting the flow direction in the entrance region. Starting from a conventional, 2-turn helical pipe, the inversion angle α was varied from 0∘ (standard design) to 140∘. Additionally, 6 different inlet interface angles ϕ between the liquids were investigated. The Reynolds number (Re) was systematically varied from Re = 5 to Re = 5000, covering a wide laminar range and corresponding to the Dean number range of De = 1.5–1455. To check the diffusion impact, two Schmidt number values, Sc = 100 and Sc = 1000, were considered. The numerical model was first validated against experimental data from the literature, showing excellent agreement. The results show that mixing can be significantly improved by early flow inversion, compared to a conventional helical pipe. Mixing improvement is most noticeable between Re = 40 and Re = 1400, when the secondary flow is strong. When the inlet liquid interface is parallel to the coil axis (ϕ=90∘), mixing is very high in all cases, since the interface is optimally aligned with the secondary vortices. However, for inclined or perpendicular interfaces, early flow inversion is required to promote mixing. The range of inversion angles between α=90∘ and α=120∘ leads to optimal mixing, independently from the inlet interface direction, and is thus recommended for practical applications. All the coiled configurations show similar pressure drops, since a single flow inversion is considered, so that the needed pumping power is not negatively impacted.