chemical engineering research and design 87 (2009) 573–586 Contents lists available at ScienceDirect Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd Application of fluorescent PIV and digital image analysis to measure turbulence properties of solid–liquid stirred suspensions H. Unadkat, C.D. Rielly * , G.K. Hargrave, Z.K. Nagy Department of Chemical Engineering, Loughborough University, Leicestershire LE11 3TU, UK abstract This study describes an experimental technique which combines Fluorescent Particle Image Velocimetry (FPIV) and digital image analysis, to quantify the hydrodynamics of a solid–liquid suspension stirred by a 45 ◦ pitched-blade turbine impeller. Soda-lime glass spheres of 1000 m diameter were employed for the dispersed phase, with up to volumetric concentrations of 0.5vol% in water. The magnitude of the continuous phase mean velocity did not change significantly in the impeller jet or bulk flow, with the addition of up to 0.5vol% dispersed phase. Turbulence levels of the continuous phase, in terms of rms velocities, turbulent kinetic energy and dissipation rate decreased above particle concentrations of 0.2 vol%, and the level of turbulence suppression remained constant up to 0.5 vol%. Continuous phase integral length scales remained unchanged in the presence of solids. The locally averaged particle concentration field showed high concentrations above and below the impeller and at the corner of the vessel base, extending up to the vessel wall. Particle turbulence levels measured at 0.5vol% dispersed phase were lower than the corresponding continuous phase. © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Mixing; Multiphase flows; Turbulence; Particle image velocimetry; Phase separation 1. Introduction The study of turbulent flows in multiphase systems has pre- sented a significant challenge to fluid dynamicists, being recognised as one of the most interesting fields of research. Investigating the dynamics of a continuous phase turbu- lent flow is coupled with the complexity of the response of the dispersed phase. The dispersed phase may modu- late the structure of the turbulent environment, and equally the continuous phase will have a compounding impact by transferring momentum to the dispersed phase, referred to as ‘two way coupling’ (Bachalo, 1994). Previous studies of multiphase flows have been conducted mainly in pipe and jet configurations, which have reported up to 50% turbu- lence damping by small particles, and up to 360% increase by larger particles (Gore and Crowe, 1991). It has been sug- gested that this transition occurs when the particle diameter to characteristic fluid length scale ratio is 0.1. Other the- ories have also been postulated to relate these effects to ∗ Corresponding author. Tel.: +44 1509 222504. E-mail address: C.D.Rielly@lboro.ac.uk (C.D. Rielly). Received 10 October 2008; Received in revised form 11 November 2008; Accepted 17 November 2008 the particle Reynolds number and wake shedding (Hetsroni, 1989). The focus of this paper is the study of solid-liquid stirred suspensions, which are a common unit operation in the chemical, pharmaceutical and food industries (Guiraud et al., 1997), and include steps such as solid-catalysed reactions, dissolution and crystal growth. These processes may involve micromixing or mass transfer, which strongly depend on the system turbulence. Although they are widespread in industry, there is little information regarding the velocity of either or both phases in stirred vessels, mainly due to the limitations of previously available measurement techniques. However, recent advances in laser diagnostics and digital imagery have improved the prospects of studying two-phase flows, and some experiments have even attempted to characterise turbulence modulation in stirred vessel configurations. Nouri and Whitelaw (1992) applied laser-Doppler anemometry (LDA) to quantify the mean flow and rms velocities of the dispersed phase up to 2.5 vol%. Guiraud et al. (1997) employed 0263-8762/$ – see front matter © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2008.11.011