This paper presents numerical simulations of two-phase flow with high-density ratio, taking into account mass transport of a soluble component and its interfacial mass transfer. The mathematical model and the numerical method allow for different solubility of the species in the respective fluid phases, while volume changes due to mass transfer a reneglected. The discontinuous changes in species concentration sat the interface are modeled by means of Henry’s law. Simulations are carried out with a next ended version of the highly parallelized code FS3D, which employs an advanced Volume-Of-Fluid(VOF) method. For the examination of mass transfer, single bubbles are held in counter-flow to investigate the transient dissolution of a dilute species. In case of small bubbles with laminar wake a rotational symmetric concentration profile occurs, in agreement with theoretical considerations, while the local mass concentrations in the wake of larger bubbles show complex patterns of varying concentration as it also has been recently observed experimentally.

Direct numerical simulation of mass transfer between rising gas bubbles and water

https://hal.archives-ouvertes.fr/hal-01268232/document

Mass transfer in the wake of non-spherical air bubbles quantified by quenching of fluorescence

Direct numerical simulation of mass transfer from Taylor bubble flow through a circular capillary

The impact of local phenomena on mass transfer from single bubbles and bubbles rising under swarm conditions with the bubble diameter and the viscosity as variation parameters is investigated by using laser-induced fluorescence. It is reported that the wake shedding frequency of ellipsoid bubbles is decisively responsible for the convective mass transfer. A novel mass transfer correlation is developed considering the Strouhal number and swarm conditions.



On a etudie par la fluorescence induite par laser l'impact des phenomenes locaux sur le transfert de matiere d'une bulle unique ou de bulles en ascension dans des conditions d'essaims avec le diametre de bulle et la viscosite comme parametres de variation. On mentionne que la frequence des tourbillons de sillage pour des bulles ellipsoides est clairement responsable du transfert de matiere convectif. On a mis au point une nouvelle correlation de transfert de matiere qui prend en compte le nombre de Strouhal et les conditions d'essaims.

The Impact of Local Phenomena on Mass Transfer in Gas-Liquid Systems

Macroporous microcarriers are commonly applied to fixed and fluidized bed bioreactors for the cultivation of stringent adherent cells. Several investigations showed that these carriers are advantageous in respect to a large surface area (Griffiths, 1990; Looby, 1990a). When growing a rC-127 cell line on Cytoline 2 (Pharmacia Biotech), no satisfactory product yield could be achieved. A possible limitation in the supply of nutrient components was investigated to explain these poor results. No significant concentration gradients could be detected. Nevertheless, fluorescence staining revealed a decreasing viability, particularly inside the macroporous structure. Therefore, oxygen transfer to and into the carriers was examined by means of an oxygen microprobe during the entire process. Additional mathematical modeling supported these results. The maximum penetration depth of oxygen was determined to be 300 μm. A critical value influencing the oxygen uptake rate of the rC-127 cells occured at a dissolved oxygen concentration of 8% of air saturation. A significant mass transfer resistance within a laminar boundary film at the surface of the carrier could be detected. This boundary layer had a depth of 170 μm. The results showed that even a 40% air saturation in the bulk liquid could not provide an efficient oxygenation of the surface biofilm during the exponential growth phase. Fluorescent staining reveals a poor viability of cells growing inside the carrier volume. Thus, oxygen supply limits the growth of rC-127 cells on macroporous microcarriers. Poor process performance and low product yield could be explained this way.

Investigations on oxygen limitations of adherent cells growing on macroporous microcarriers.

For the investigation of mass transfer from gas bubbles into liquids the concentration gradient of oxygen migrating from air bubbles was measured by means of oxygen microelectrodes. For this purpose a single air bubble was fixed by a platinum wire spiral with the liquid flowing downward. Thus the ascent of the bubble in an aerated liquid was simulated. Liquid-side mass transfer coefficients determined from concentration gradients were higher than values calculated from theory. Sherwood numbers obtained from experimental results for bubbles of larger diameters were distinctly higher than those for smaller bubbles (diameter ≅1 mm); the difference corresponds approximately to that predicted theoretically between bubbles with mobile and those with rigid interfaces.

Determination of oxygen transfer from single air bubbles to liquids by oxygen microelectrodes

Oxygen transfer from a gas bubble into the surrounding liquid is examined by measuring oxygen pressure with Po2-needle electrodes at different distances from the bubble From there, the mass transfer coefficient can be calculated First measurements yielded results in the range to be expected

M. Riethues

Papers

Determination of oxygen transfer from single air bubbles to liquids by oxygen microelectrodes

Oxygen transfer from gas bubble into liquid.