Effects of the Injection Period on the Rise Velocity and Shape of a Bubble in a Non-Newtonian Fluid

The present work aims at studying the influence of the injection period on the bubble rise velocity and their corresponding shape. Three different fluids were employed: a Newtonian fluid (99.5% glycerol); an inelastic shear thinning fluid (2% carboxymethylcellulose—CMC in water) and an elastic shear thinning fluid (0.5% polyacrylamide— PAAm in water). Five different injection periods ranging from 0.3 to 60 s were applied to bubble volumes from 20 to 1000 × 10−9 m3. Except for the case of the injection period of 0.3 s, the rise velocity in the glycerol and CMC solutions does not depend on the injection period. However, the rise velocity decreases significantly with the injection period in the PAAm solution. In the glycerol solution, bubbles take an oblate spheroid shape, and are flattened vertically. On the other side, in the CMC and PAAm solutions, bubbles are vertically elongated and have a teardrop shape with a tail. However, the recent flow field results around a rising bubble obtained by PIV tend to show a disconnection between the bubble shape and surrounding flow field.