J. Non-Newtonian Fluid Mech. 135 (2006) 16–31 Flow around individual Taylor bubbles rising in stagnant polyacrylamide (PAA) solutions R.G. Sousa a , M.L. Riethmuller b , A.M.F.R. Pinto a , J.B.L.M. Campos a, ∗ a Centro de Estudos de Fen´ omenos de Transporte, Departamento de Engenharia Qu´ ımica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias 4200-465 Porto, Portugal b von Karman Institute for Fluid Dynamics, 72 Chauss´ ee de Waterloo, 1640 Rhode-Saint-Gen` ese, Belgium Received 7 July 2005; received in revised form 5 December 2005; accepted 10 December 2005 Abstract The flow around single Taylor bubbles rising in stagnant non-Newtonian solutions of polyacrylamide (PAA) polymer was studied using a technique employing simultaneous particle image velocimetry (PIV) and shadowgraphy. Solutions with different weight percentages of polymer, varying from 0.01 to 0.80wt.%, were used to cover a wide range of flow regimes. The rheological fluid properties and pipe dimension yielded Reynolds numbers between 2 and 1160, and Deborah numbers up to 115. The shape of the bubbles rising in the different solutions was compared and quantified by fitting correlations. The flow around the nose of the bubbles was found to be similar for all conditions studied. Velocity profiles were measured and analysed in the liquid film around the bubbles. A comparison of bubble wake flow patterns was made. For the 0.10 and 0.20 wt.% PAA solutions, long wakes with a recirculation region were observed. Below the wakes, a flow of stretched liquid was found. Negative wakes were also observed for the more concentrated solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: Taylor bubble; Multiphase flow; Non-Newtonian fluids; Viscoelasticity; Particle image velocimetry (PIV); Films 1. Introduction Slug flow is a two-phase flow regime, found when gas and liquid flow simultaneously in a pipe over a specific range of flow-rates. This regime is characterised by elongated gas bub- bles (Taylor bubbles or gas slugs) which almost fill the pipe cross-section, causing the liquid to flow around and between the bubbles. This type of flow is found in various geothermal, fermentation and polymer devolatilisation processes, pipeline transport in oil and gas wells, air-lift reactors, and many others. Slug flow is used in some chemical processes to increase the reaction rate by taking advantage of the mixing action of the wake of the Taylor bubbles. The flow of Taylor bubbles in Newtonian liquids has been deeply studied since the 1940s. Dumitrescu [1] and Davis and Taylor [2] were among the pioneers studying the shape and ve- locity of these elongated bubbles. Since then, several studies of Taylor bubbles have also dealt with the liquid film region sur- rounding the bubble [3–8], and the bubble wake region [8–13]. ∗ Corresponding author. Fax: +351 225081449. E-mail address: jmc@fe.up.pt (J.B.L.M. Campos). Recently the flow around Taylor bubbles rising in Newtonian liquids was studied using particle image velocimetry (PIV), and the most relevant studies are described in [14–17]. The flow of Taylor bubbles rising in non-Newtonian liquids has not been so deeply studied, despite being frequently found in industrial processes. Due to the complex liquid rheology, the gas–liquid flow patterns have several unusual characteristics. The effects of power law rheology and pipe inclination on slug bubble rise velocity were studied by Carew et al. [18]. Otten and Fayed [19] studied the pressure drop and friction drag re- duction in two-phase non-Newtonian slug flow. Rosehart et al. [20] measured the void fraction, slug velocity and frequency for co-current slug flow of air bubbles in highly viscous non- Newtonian fluids. Terasaka and Tsuge [21] made gas hold-up measurements for gas slugs rising in viscous liquids with a yield stress. Kamıs ¸lı[22] derived a one-dimensional flow equation for the motion of a long bubble rising steadily in vertical and in- clined tubes filled with a power-law fluid. Sousa et al. [23] applied PIV and shadowgraphy simul- taneously to describe the flow of Taylor bubbles rising in stagnant carboxymethylcellulose (CMC) solutions. The au- thors described the wake flow patterns found in solutions with 0377-0257/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnnfm.2005.12.007