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 (DNS). 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.
|Number of pages||8|
|Publication status||Published - 2008|
|Event||6th International Conference on CFD in the Oil & Gas, Metallurgical and Process Industries 2008 - Trondheim, Norway|
Duration: 10 Jun 2008 → 12 Jun 2008
|Conference||6th International Conference on CFD in the Oil & Gas, Metallurgical and Process Industries 2008|
|Period||10/06/08 → 12/06/08|
|Other||10-12 June 2008|
Dijkhuizen, W., van Sint Annaland, M., & Kuipers, J. A. M. (2008). Direct Numerical Simulation of the Lift Force in Bubbly Flows. Paper presented at 6th International Conference on CFD in the Oil & Gas, Metallurgical and Process Industries 2008, Trondheim, Norway.