Bubble induced shear stress in flat sheet membrane systems—Serial examination of single bubble experiments with the electrodiffusion method

With regard to the improvement of the cleaning process of flat sheet membrane modules by aeration, a statistical analysis of the shear stress generated by single bubbles is shown for 21 different parameter combinations of varied channel depths, bubble sizes and liquid velocities. A fully automated rig allowed the generation of a sufficient database of shear stress data gained with the electrodiffusion method (EDM). The maximum shear stresses and the global shear stress levels, where the latter represents the global shear stress fluctuations, were determined. To the authors' knowledge, for the first time in membrane research a transient correction of the EDM data was performed which is necessary in cases of highly transient processes. By showing different probability ranges gained from cumulative distribution functions in bar plots, a way of presenting the data was introduced that simplifies the comparison between the different geometrical and operating parameter combinations. Additionally, this type of diagram offers a very comprehensive overview of the measured data. Both the maximum shear stress and the global shear stress levels are larger when the bubble is larger than the channel depth and when an additional liquid velocity is superimposed. In all cases, the transient correction yielded higher values. Thus it is necessary to take this measure.In the investigated range, the highest maximum values were obtained for the case with superimposed liquid velocity, a channel depth of 5 mm and a bubble size of 9 mm with appr. 1.3 Pa (steady analysis) or 4.7 Pa (transient corrected analysis), respectively. The largest fluctuation ranges of the global shear stress levels were obtained for a channel depth of 5 mm and a bubble size of 9 mm with values from appr. 0.2 to 1.1 Pa (steady analysis) or 0 to 2.6 Pa (transient corrected analysis).

► Measurement of bubble induced shear stress for 21 parameter combinations.
► 40,000 bubble rises for building a sufficient database for the statistical analysis.
► Steady and transient corrected analysis of the electrodiffusion method data.
► Analysis of the maximum shear stress and the general shear stress level.