On the hydrodynamic stability of a viscous liquid jet enclosed in a narrow tube
Gijs Ooms (TU Delft - Fluid Mechanics)
Mathieu Pourquie (TU Delft - Fluid Mechanics)
Wim Paul Breugem (TU Delft - Multi Phase Systems)
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Abstract
We studied the hydrodynamic stability of a viscous liquid jet enclosed by a much less viscous fluid in a narrow vertical tube. In literature, this flow pattern is also known as perfect core-annular flow. The main objective is to unravel the competition between capillary and shear-driven instability mechanisms acting on the flow. The temporal stability of the flow was tested under laminar conditions for a small axisymmetric sinusoidal perturbation of the interface. To this purpose, numerical simulations were conducted using a finite-volume two-phase flow solver combined with a geometric Volume-of-Fluid method to capture the interface between the immiscible fluids. The simulation results are interpreted using linear stability theory for thin liquid jets in free space. The main conclusion is that perfect core-annular flow is hydrodynamically unstable, either through a capillary or a shear-driven instability. The competition between the two instability mechanisms is characterized by the Weber number based on the annular layer thickness, Wea. For Wea≪1, the flow is prone to a capillary instability, while for Wea≫1, the liquid jet may undergo atomization. Evidence is also found for a reduced growth rate of capillary instabilities in the presence of strong shear at high Wea.
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