Direct Numerical Simulation of the Lift Force in Bubbly Flows


Dijkhuizen, Wouter and Sint Annaland, Martin van and Kuipers, Hans (2008) Direct Numerical Simulation of the Lift Force in Bubbly Flows. In: 6th International Conference on Computational Fluid Dynamics in the Oil & Gas, Metallurgical and Process Industries, CFD 2008, 10-12 June 2008, Trondheim, Norway.

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Abstract:It is well-known that the lift force is responsible for the
segregation of small and large bubbles encountered in
bubbly flows through pipes and bubble columns: in the
case of up flow small spherical bubbles move to the
wall, while larger deformed bubbles move to the core
region. Depending on the fluid properties there is a
transition at a certain bubble diameter, which is
extremely critical if one wants to predict the correct
circulation pattern and gas-holdup. However, until now
quantitative knowledge about this force is limited to
spherical bubbles (Legendre & Magnaudet, 1998) and
deformed bubbles in moderately viscous liquids
(Tomiyama, 1998). Therefore, this work focuses on
extending the knowledge on the lift force, bridging the
gap towards a wide range of bubble diameters as well as
less viscous liquids, such as the industrially important
air-water system, using direct numerical simulations
To enable numerical simulation of small bubbles at high
density ratios, the surface tension treatment of a 3D
Front Tracking model has been significantly improved.
Also its numerical implementation has been carefully
optimized to reduce computation time, to be able to
efficiently run the large number of cases required in this
study. The numerical simulations have been carried out
using a cubic computational domain consisting of one
million grid cells, which yields good resolution at
reasonable calculation time (typically two weeks on a
single CPU). The initially spherical bubble is placed in
the centre of the computational domain and a window
shifting technique assures that it keeps this position.
The top, left and right boundaries are used to enforce
the linear shear field, using inflow and no-slip boundary
conditions respectively. They are supplemented by a
prescribed pressure outflow boundary at the bottom,
where the liquid is free to exit the domain, and free-slip
boundaries at the front and rear.
First of all, the results confirm that small spherical
bubbles move to the high negative velocity side (wall
region), while large deformed bubbles move in the
opposite direction. The transition in the lift force is
accompanied by a slanted wake structure behind the
larger bubbles. Secondly, the numerical values of the
lift coefficient show a good agreement with Tomiyama
et al. (2002) for moderate to low viscosity liquids.
Surprisingly, at higher viscosities there is a very
significant discrepancy. Finally, it was found that both
the drag and lift force coefficients are not a function of
the shear rate.
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