(165f) Experimental and Computational Studies of the Fluid Dynamic Behaviour of Liquid-Solid Mixtures in Agitated Vessels Conference: AIChE Annual MeetingYear: 2018Proceeding: 2018 AIChE Annual MeetingGroup: North American Mixing ForumSession: Mixing in Rheologically Complex Fluids Time: Monday, October 29, 2018 - 2:35pm-3:00pm Authors: Meridiano, G., University College London Mazzei, L., University College London Angeli, P., University College London Weheliye, W. H., University College London Experimental and computational studies of the fluid dynamic behaviour of liquid-solid mixtures in agitated vessels Giovanni Meridiano, Weheliye Hashi Weheliye , Luca Mazzei, Panagiota Angeli Department of Chemical Engineering, University College London, Torrington Pl, London WC1E 6BTA, UK giovanni.merigiano.17@ucl.ac.uk; weheliye.weheliye.10@ucl.ac.uk; l.mazzei@ucl.ac.uk p.angeli@ucl.ac.uk, , Introduction Mixing of solids in viscous liquids in stirred vessels is a crucial step in many manufacturing processes; it is regularly encountered in a wide range of industrial sectors like health-care, pharmaceuticals, cement manufacturing and food processing. In many of these applications the liquids have complex nonNewtonian behaviour (Paul, Atieno-Obeng and Kresta, 2004). In these industrial processes, obtaining a uniform mixture is a difficult task, because there are no turbulent eddies to help distribute the components. The objective of this work is to understand and characterize the fluid dynamics of uniform liquid-solid mixtures with both Newtonian and nonNewtonian liquid matrixes in stirred vessels. For this purpose, we measured the power consumption and the velocity profiles at different solid volume fractions and compared the results with those obtained via Computational Fluid Dynamics (CFD) simulations. Materials and methods The power required for stirring the suspensions as well as the internal velocity fields obtained via Particle Image Velocimetry (PIV) were measured experimentally at different solid volume fractions in a stirred vessel. The measurements were carried out in a stirred tank specifically designed to reproduce real industrial scale equipment. It consists of a transparent cylindrical tank of internal diameter DT= 50 mm equipped with two baffles and a dual-blade impeller of diameter D = 37 mm. The height of the fluid, H, in the tank was set equal to the tank diameter. The impeller was located at the centre of the tank, 10 mm from the bottom, and was driven by a variable speed motor that could operate in the range of 502000 rpm (IKA Eurostar 20) (Fig. 1). As viscous Newtonian fluid, we employed a solution of corn syrup and water, while, as nonNewtonian fluid, we opted for a solution of corn syrup, water and xanthan gum. The solid in both cases is PMMA microspheres (3237 µm). Figure 1: Experimental setup for the measurement of power consumption in the vessel . Power consumption measurements A precise way to measure power consumption is to use an air bearing system. A schematic of the air bearing system used in this work is shown in Fig. 1. From the force that is required to stop the rotation of the rotational table the power required to agitate the fluid can be calculated as follows: where MIdenotes the axial torque applied to the fluid by the impeller and N is the impeller angular speed in rotations per unit time. We recorded this force with a load cell (Omega LCM601-1) and a data acquisition system and software (OmegaIN-USBH). PIV measurements Velocity field measurements were carried out using Particle Image Velocimetry. For these experiments, we used the same transparent tank used for the torque measurements, but in this case, the tank was enclosed in a square transparent box. This box was made of acrylic and filled with a refractive index matching fluid to avoid optical distortions on the surface of the cylindrical vessel. The PIV set-up includes a dual cavity Nd:Yag green laser (532 nm) (Litron Laser®, 15 Hz, 1200 mJ) and a straddling CCD camera with 1280 x 1024 pixels (TSI PowerView Plus) that operates at a maximum frequency of 15 frames per second. The camera is equipped with an AF Nikkor 50mm f/1.8D prime lens (Nikon®). A hall switch sensor was used to capture images at the same phase angle. As tracers we used fluorescent polymer particles (melamine resin based) coated with rhodamine B (20μm), which absorb at 532 nm and emit at 610 nm. They are neutrally buoyant in the fluids considered and, at the experimental conditions explored; their relaxation time is negligible compared to the convection time. The laser and the camera were synchronized by means of a Laser Pulse Synchroniser (Model 610035 TSI) and were controlled via the Insight 4G (TSI) software. The laser beam passed through a collimator (Model 610026 TSI) and two cylindrical lenses (25 mm, and 15 mm) to create a narrow plane of 1 mm thickness. The laser plane was then reflected on a 45º silver coated mirror and entered the stirred vessel from the bottom. A sketch of the setup is shown in Figure 2. Figure 2: Sketch of the main components of the experimental set-up for the PIV measurements Computational fluid dynamics simulations Computational Fluid Dynamics is a powerful tool that yields relevant process information that can be used to design and assess the performance of stirred vessels. A number of researchers have used CFD to study the behaviour of different types of liquid-solid systems in mechanically stirred tanks with different types of impellers (Blais et al., 2016) (Konz and Windhab, 2016) (Fradette et al., 2007). All the previous studies highlighted the importance of validating the CFD models against experimental data. The model employed in this study follows a EulerianEulerian approach. This means that the transport equations are written in term of volumeaveraged quantities. For the mixture stress closure an experimental constitutive equation is implemented that correlates the suspension viscosity to the solid volume fraction. Therefore, the first step of our work focussed on the rheological characterization of the two suspensions mentioned in Section 2 and on the development of suitable rheological constitutive equations for the CFD model. We compared the CFD findings against the power measurements in the stirred vessels and the velocity fields for different impeller speeds and solid loadings. Preliminary Results We characterized the rheology of the Newtonian fluidparticles system, at different solid volume fractions, using a HR-3 Discovery Hybrid Rheometer (TA Instruments®). Some results are shown in Fig.3. Figure 3: Evolution of viscosity with solid volume fraction F As expected, the viscosity increases with the solid volume fraction; moreover, the overall behaviour of the suspension remains Newtonian at solid volume fractions below 0.2 while at higher solid volume fraction the suspension shows a slightly shear-thinning behaviour. This phenomenon has been widely documented in the literature (Stickel and Powell, 2005). In addition, the change in viscosity with volume fraction is well predicted by the KriegerDougherty equation, whose parameters assume values similar to those found in the literature. The power consumption for the Newtonian fluid alone has been measured to calibrate the instruments and verify the reproducibility of the results. Furthermore, we ran some simulation for the same system. In Fig. 4, we report both the experimental and computational power numbers against the impeller Reynolds number. Figure 4: Power number against Reynolds number for the Newtonian fluidparticles system We were able to recover the well-established relation between the power number and the Reynolds number in the laminar regime (Paul, Atieno-Obeng and Kresta, 2004). Moreover, there is good agreement between the experimental and computational results. Bibliography Blais, B. et al. (2016) Development of an unresolved CFD-DEM model for the flow of viscous suspensions and its application to solid-liquid mixing, Journal of Computational Physics. Elsevier Inc., 318, pp. 201221. doi: 10.1016/j.jcp.2016.05.008. Fradette, L. et al. (2007) CFD phenomenological model of solid-liquid mixing in stirred vessels, Computers and Chemical Engineering, 31(4), pp. 334345. doi: 10.1016/j.compchemeng.2006.07.013. Konz, A. K. and Windhab, E. (2016) Experimental and computational study of a high speed pin mixer via PEPT, visualization and CFD, Chemical Engineering Science. Elsevier, 155, pp. 221232. doi: 10.1016/j.ces.2016.08.007. Paul, E. L., Atieno-Obeng, V. A. and Kresta, S. M. (2004) Handbook of Industrial Mixing, Wiley Online Library. doi: 10.1002/0471451452. Stickel, J. J. and Powell, R. L. (2005) Fluid Mechanics and Rheology of Dense Suspensions, Annual Review of Fluid Mechanics, 37(1), pp. 129149. doi: 10.1146/annurev.fluid.36.050802.122132. Topics: Mixing