Bubble rise velocities and drag coefficients in non-Newtonian polysaccharide solutions

Microbially produced polysaccharides have properties which are extremely us eful in different applications. Polysaccharide producing fermentations star t with liquid broths having Newtonian rheology and end as highly viscous no n-Newtonian solutions. Since aerobic microorganisms are used to produce the se polysaccharides, it is of great importance to know the mass transfer rat e of oxygen from a rising air bubble to the liquid phase, where the microor ganisms need the oxygen to grow. One of the most important parameters deter mining the oxygen transfer rate is the terminal rise velocity of air bubble . The dynamics of the rise of air bubbles in the aqueous solutions of diffe rent, mostly microbially produced polysaccharides was studied in this work. Solutions with a wide variety of polysaccharide concentrations and rheolog ical properties were studied. The bubble sizes varied between 0.01 mm(3) an d 10 cm(3). The terminal rise velocities as a function of air bubble volume were studied for 21 different polysaccharide solutions with different rheo logical properties. It was found that the terminal velocities reached a pla teau at higher bubble volumes, and the value of the plateau was nearly cons tant, between 23 and 27 cm/s, for all solutions studied. The data were anal yzed to produce the functional relationship between the drag coefficient an d Reynolds number (drag curves). It was found out that all the experimental data obtained from 21 polysaccharide solutions (431 experimental points), can be represented by a new single drag curve. At low values of Reynolds nu mbers, below 1.0, this curve could be described by the modofoed Hadamard-Ry bczynski model, while at Re > 60 the drag coefficient was a constant, equal to 0.95. the latter finding is similar to that observed for bubble rise in Newtonian liquids which was explained on the basis of the "solid bubble" a pproach. (C) 1999 John Wiley & Sons, Inc.