Hydrodynamic drag and rise velocity of microbubbles in papermaking process waters

Diversity of dissolved, colloidal and solid materials present in papermaking process waters influences the rise of microbubbles by increasing their drag. This effect is known to reduce the gas separation efficiency but its importance thus far has not been quantified by experimental studies. In this paper the terminal velocities and drag coefficients of bubbles as a function of the bubble Reynolds number are studied experimentally in papermaking process waters with a high-speed CMOS camera and a submersed back-light illumination in a pressurised bubble column. Bubbles are tracked in time to provide time series data for every bubble that passes the focal plane of the imaging system. Image sequences are analysed with automatic image processing algorithms that measure the velocity and size of bubbles, and also the velocity of the fluid surrounding the bubbles revealing the instantaneous slip velocity of each bubble. Results show how suspension viscosity, surface tension and solids content affect the kinetics of microbubbles. Changes in microbubble formation during a pressure drop and differences of bubble size distributions in a variety of process waters and model solutions are also shown. Finally, an empirical correlation between the bubble drag coefficient and the bubble Reynolds number is generated for the investigated process waters.