Chin. J. zyxwvutsrqponm Chen. zyxwvutsrq Eng., zyxwvutsrqpon 15(4) 611-615 (2007) zyxwvutsr RESEARCH NOTES Bubble Formation in Non-Newtonian Fluids Using Laser Image Measurement System* JIANG Shaokun( zyxwvuts % @ Z )", FAN Wenyuan( % zyxwvu (3 /3i and LI Huaizhi( &)b zyxwvutsrq a State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin Univer- sity, Tianjin 300072, China Laboratoire des Sciences du GCnie Chimique, CNRS-ENSIC-INPL, 1, rue Grandville, BP 451, 54001 Nancy cedex, France %)", ZHU Chunying(3k %%)", MA Youguang Abstract A self-developed laser image measurement system was established to study the behavior of bubble for- mation at a single orifice in non-Newtonian polyacrylamide (PAAm) solutions. Images of bubbles were captured by a CCD camera and volumes of bubbles were digitally analyzed online. The effects of rheological property of PAAm solution, orifice, reservoir, and gas flowrate on bubble formation were studied experimentally. It is found that the volume of bubble increases with the concentration of PAAm solution, the diameter of the orifice, and the gas flowrate, respectively, whereas little effect of reservoir is observed in experiments. Keywords bubble formation, laser, non-Newtonian, polyacrylamide 1 INTRODUCTION Bubble behavior in non-Newtonian fluids is of great academic interest and plays a key role in such diverse domains as waste water treatment, petroleum processing, fermentation, polymer devolatilization, composites processing, boiling, plastic foam process- ing, and environment remediation. Bubble behavior can be divided into three processes: bubble formation from submerged orifices, interaction, and coalescence during the ascent. For bubble formation in both shear dependent viscosity and viscoelasticity liquids and gas flowrate above 0.5X 10-6m'-s-', Acharya et al.[l] presented a prediction equation for the volumes of bubbles based on the assumption that shear and nor- mal stresses have very little influence upon the mode of bubble formation as compared with stresses due to inertia. Costes and Alran[2,3] further developed dif- ferent models for bubble formation at constant pres- sure and flowrate, respectively, in terms of the balance of different forces including buoyancy, force of sur- face tension, liquid resistance, and gas momentum rate during the bubble formation process in non-Newtonian fluids. Rabiger and Vogelpohl[4] studied the behavior of bubble formation in carboxymethylcellulose (CMC) and polyacrylamide (PAAm) water solutions and dis- cussed change in the shape of the bubble and the rela- tion between bubble diameter and gas flowrate. For determining bubble profile and the final bubble size, a high-speed photographic technique was used by Ghosh and Ulbrecht[S] combining dimensional analy- sis, a force balance, and a rheological equation. Con- sidering the pseudoplasticity of the continuous phase on the size and shape of bubble, they proposed a theoretical approach to calculate the bubble growth rate and the final bubble size. Assuming that PAAm aqueous solutions follow Maxwell's viscoelastic model, Terasaka and Tsuge[6] proposed a non-spherical bubble formation model to theoretically estimate the volume, shape, and growth curve of bubbles and dis- cussed the effects of operating conditions and concen- tration of PAAm aqueous solutions. Furthermore, tak- ing into account the influence of in-line interactions between bubbles due to fluid memory effects of the viscoelastic characteristics, Li et al.[7,8] developed a theoretical model which included the radial expansion and vertical ascension of the bubble interface com- bining the thermodynamic equations for the gas in the bubble and the reservoir below the orifice, as well as the fluid rheological equation for the non-spherical bubble formation at an orifice submerged in non-Newtonian fluids under constant flowrate condi- tions. Despite numerous theoretical and experimental investigations[9-131, the mechanism of bubble for- mation and detachment remains still far from fully understood up to now. In this work, a comprehensive experimental study of bubble formation at a single orifce in non-Newtonian PAAm solutions was performed employing a self-developed laser image measurement system and a CCD camera image capturing system for digitally analyzing the volumes of bubbles online. 2 EXPERIMENTAL The main features of the experimental setup con- sist of a plexiglass square tank surrounded by a square duct that was used to control the liquid temperature inside the square tank, as shown in Fig.1. The length of the side of the tank was 0.15m and its height was 0.3m. Nitrogen bubble was formed through three differ- ent orifices $wing diameters of 1 X 10-3m, 1.5X 10-3m and 2X 10- m, respectively, submerged in the liquid in the center of a Plexiglas plate in the middle section of the tank. Three reservoirs with the volumes of Received 2006-06- 14, accepted 2007-03- 13. Discipline to Universities (Grant No.BO6OO6). * Supported by the National Natural Science Foundation of China (No.20476073) and the Programs of Introducing Talents of ** To whom correspondence should be addressed. E-mail: ygma@tju.edu.cn