Bubble rise phenomena in non-newtonian crystal suspensions

This chapter presents an experimental study of trajectory and shape of air bubbles and their effect on water, polymeric and massecuite equivalent non-Newtonian crystal suspensions when the bubbles rise. Massecuite is a fluid made from sugar crystals and sugar syrup (molasses) and is an important phase in raw sugar processing where vacuum pan (large cylindrical vessels with steam bubble rising through vertical heating surfaces) is used for sugar crystallisation. To study the bubble rise characteristics and their effect on flow in vacuum pan, various-suspensions were made by using different concentration of xanthan gum solutions with polystyrene crystal particles. The rheological properties for the solutions were measured using an Advanced Rheometric Expansion System (ARES) and the bubble size and bubble trajectory were measured using a combination of non-intrusive-high speed photographic method and digital image processing. A combination of different concentration of xanthan gum solutions with different concentrations of polystyrene crystal mixed with water were tested and characterized for the selection of the massecuite equivalent non-Newtonian fluid. The rheological results indicated that xanthan gum crystal suspensions exhibited shear thinning Power-Law flow behaviour and the solution with a concentration of 0.05% xanthan gum and 1% crystal showed the closest similarities to the rheological properties of massecuite at low shear rate. The viscous effects of the xanthan gum solutions were more pronounced than elastic effects and this phenomenon was observed for a high grade massecuite solution. The loss modulus (G’’) and storage modulus (G’) of crystal suspensions were investigated. The effects of the crystal suspensions on the bubble rise flow characteristics are discussed. In particular, how the bubble trajectory and the bubble shape are affected is reported. Bubble trajectory inside the stagnant liquids in water, polymeric solutions and crystal suspension are investigated corresponding to a 1.0 m height of liquid in the tube. The chapter also delineates the shapes observed in different liquids at three heights of liquid column when bubbles rise.The influences of the dimensionless numbers on the bubble trajectory and shape are reported and verified with the published literature. Particularly, the effect of Reynolds number, Weber Number, and Morton Number, on bubble trajectory are presented and the effect of, Eötvös number, and aspect ratio, E on bubbles’ shapes are described. The conditions for the bubbles’ path and shape oscillations are also identified and discussed.