Fluid dynamics of bubble swarms rising in Newtonian and non-Newtonian liquids in flat sheet membrane systems

• Fluid dynamic investigation of an aerated flat sheet geometry.
• First application of the electrodiffusion method in a non-Newtonian two-phase system.
• Gas hold-up results for 144 and shear stress results for 72 parameter combinations.
• Gas velocity and rheology have a significant influence on both properties.
• Higher shear stresses found in non-Newtonian liquid but fluctuations are damped.

The bubble swarm behavior in a system comparable to a flat sheet membrane module was investigated. The parameters channel depth, superficial gas velocity, superimposed liquid velocity and viscosity of the continuous phase were varied. Besides water as continuous phase, a non-Newtonian liquid was used with respect to the rheology of real waste water sludge. The gas hold-up was determined for 144 parameter combinations with high speed imaging and the shear stress – important for the fouling mitigation – was determined for 72 parameter combinations with the electrodiffusion method. To the authors׳ knowledge, this is the first application of the electrodiffusion method in a bubbly flow with a non-Newtonian liquid. For the two properties gas hold-up and the shear stress describing correlations were found taking all parameters into account. In both cases, the strongest influences were found for the gas velocity and the rheology of the continuous phase. Regarding the shear stresses, the median values were up to 3.2 Pa while the maximum values were up to 9.1 Pa. Depending on the parameter combination, a significant influence of the rheology was found disagreeing with the statement of the transferability of the results gained in water to real waste water systems mentioned in some papers.