The development of helical vortex pairs in oscillatory flows – A numerical and experimental study

• Published in: Chemical engineering and processing 2019-09, Vol.143, p.107588, Article 107588
• Main Authors: McDonough, J.R., Ahmed, S.M.R., Phan, A.N., Harvey, A.P.
• Format: Article
• Language: English
• Subjects: Coil reactor; Computational fluid dynamics; Oscillatory baffled reactor; Particle image velocimetry; Swirling; Vortex
• ISSN: 0255-2701; 1873-3204
• DOI: 10.1016/j.cep.2019.107588

[Display omitted] •Flow around helical coils with a central rod resembles that in a coiled tube•Dual vortices that resemble Dean vortices are subsequently observed•These are predictable using the Dean number if the swirling velocity is known•The primary vortex forms due to flow separation around the helical coil•The second vortex is induced through a torque between inward and outward acting acceleration When a liquid is oscillated in a tube containing a helical coil, there are two key flow phenomena: vortex-shedding and “swirl”. Together, they cause plug flow to be achievable over a wide range of conditions in this design. When an additional small cylindrical rod is placed in the centre of the helical coil, a new dual vortex regime is realised. This new flow regime was studied via CFD and PIV. It was demonstrated that a significant swirling velocity was generated by the helical coils, inducing an outward centrifugal force and inward pressure gradient, creating an instability resulting in the formation of a pair of counter-rotating vortices. The oscillatory amplitudes and frequencies necessary for the formation of these dual vortices at this scale (5 mm diameter) were centre-to-peak amplitude of 2–4 mm and frequency ≥ 3 Hz. The vortex pairs were also visualised in 3D, and exhibited a double-helix shape. Additionally, the transition of this flow to a more turbulent-like state was investigated. It was found to occur in the range Reo = 600–630. Through analysis of the swirl and radial numbers, it is clear that swirling dominates the flow structures at amplitudes of xo ≥ 4 mm.