Gravity induced mixing in a vertical tube

Marie Debacq (1) , Vincent Fanguet (1) , Jean-Pierre Hulin (1) , Dominique Salin (1) , Bernard Perrin (2)

1 FAST - Fluides, automatique, systèmes thermiques
2 LPA - Laboratoire Pierre Aigrain

The objective of this study is to analyse the macroscopically diffusive character of gravity induced mixing of two liquids in the confined geometry of a vertical tube. A 4m high vertical tube, 20mm in diameter, is separated in two by a slot valve : the upper and lower halves are respectively filled with an heavy transparent fluid and a lighter dyed one initially at rest. Mixing is induced by opening of the valve; a quantitative image analysis procedure allows to analyse spatiotemporal variations of the dye concentration inside the tube (each experiment lasts for about 20 minutes). At high density contrasts (from 3 to 20 %), mixing is diffusive on a macroscopic scale, and the corresponding diffusion coefficient increases linearly with the density contrast up to 5.6 10-4 m2 s-1 for a density contrast of 20 %. This diffusion coefficient increases also linearly with viscosity up to 1.25 10-3 m2 s-1 for a viscosity of 6 mPa s and a density contrast of 5 %. At lower density contrasts (down to 0.03 %) a diffusive regime is only reached at long times with a constant diffusion coefficient (2.5 10-4 m2 s-1). In this case, significant concentration fluctuations are observed and their motion is visible over large distances.

Domaines: Mécanique des fluides [physics.class-ph]; Génie des procédés

Marie Debacq, Vincent Fanguet, Jean-Pierre Hulin, Dominique Salin, Bernard Perrin. Gravity induced mixing in a vertical tube. International Workshop on Miscible Interfaces, Jul 2001, Paris, France. ⟨hal-01500256⟩