Abstract Summary (Max 250 words)
Mechanically agitated tanks are widely used in the chemical and biochemical industries. Although working fluids often present complex rheology, little work has been done to understand the hydrodynamics of non-Newtonian fluids in a mixing tank. Studying such a configuration is challenging because (i) the local apparent viscosity μ_a is shear dependent and (ii) the flow developing in mixing tanks is inherently 3D inducing strong heterogeneities of local shear rate. The objective of this study is to deepen the understanding of the impact of non-Newtonian rheology on flow in a mixing tank through the acquisition of high-quality experimental data. In this context, 4D-LPT (3D time-resolved Lagrangian Particle Tracking) measurements are conducted. To the author knowledge this work is one of the first attempts where such a technique has been employed to study laminar flow od non-Newtonian fluid in a mixing tank. It offers a unique opportunity to measure local and time-resolved velocity fields in a volume. The fluid investigated is an aqueous solution of CMC modeled with Carreau-Yasuda model. The impeller is a classical Rushton turbine. Several mixing frequencies are analyzed to cover flow regimes ranging from laminar to transitional (Re=ρND^2/μ∈[50,554], where N is the mixing frequency, D the impeller diameter and μ a characteristic dynamic viscosity based on Metzner-Otto correlation2). Four cameras (Vision Research VEO 640) are used with 100mm Zeiss Milvius lenses with an aperture of F/16 and 540nm band-pass filters and Scheimpflug V3 mounts (LaVision). The investigated volume is homogeneously illuminated with a high frequency laser Photonics Industries DMX60-527-DH (2×60mJ at 1kHz). The measurement volume has the following dimensions: x=175.5mm, y=232.5mm, z=80mm, starting 40mm above the bottom of the mixing tank. Local and time-resolved data are post-processed to evaluate ensemble and phase-averaged velocity fields in order to investigate organized structures. In particular, the development of coherent structures in the wake of the impeller is characterized. Furthermore, the local shear rate is characterized without any truncation or isotropy assumptions and the contributions of the mean and organized motions to the total shear rate are assessed through POD. Finally, the spatial distribution of the local apparent viscosity is evaluated.
