Gas-liquid hydrodynamics in Taylor Flows with complex liquids

Universitá di Pisa Facoltá di Ingegneria Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali Relazione di tirocinio in Ingegneria Chimica Gas-liquid hydrodynamics in Taylor Flows with complex liquids Il candidato: Federico Alberini Il relatore: Prof. Elisabetta Brunazzi Controrelatore: Prof. Ing. Roberto Mauri Anno Accademico 2009-201

Blending of non-Newtonian fluids in static mixers: assessment via optical methods

The performance of KM static mixers has been assessed for the blending of Newtonian and time-independent non-Newtonian fluids using planar laser induced fluorescence (PLIF). A stream of dye is injected at the mixer inlet and the distribution of dye at the mixer outlet is analyzed from images obtained across the pipe cross section. The effect of superficial velocity, scale of static mixer, flow ratio between a primary and a secondary injected flow and finally the injection position, are investigated to determine the consequences on mixing performance. Different methods are discussed to characterize mixing performance, comparing CoV and maximum striation thickness. Conflicting trends are revealed in some experiments results, leading to the development of an areal based distribution of mixing intensity and a distribution of striation with high mixing intensity. For two-fluids blending, the addition of a high viscosity stream into the lower viscosity main flow causes very poor mixing performance, with unmixed spots of more viscous component observable in the PLIF image. The final part of the work is focused on a preliminary understanding of advective mechanisms such as shearing of non-Newtonian fluid drops and stretching of a non- Newtonian fluid filaments

Mixing in Biogas Fermenters: Experimental Characterization of a Scale-down Geometry

In this work, the fluid dynamics features of a real industrial configuration of a biogas fermenter, which consists in a cylindrical tank stirred with three top-entering shafts with multiple impellers, are investigated. The analysis is based on the experimental characterization of a laboratory model digester of 0.49 m in tank diameter obtained from the scale-down based on the geometrical similarity criterion of a full-scale digester of diameter equal to 17 m. The aim of the work is to evaluate the appropriateness of the design for the requirements of the biogas production process and to suggest possible improvements to the overall mixing operation. The fluid dynamics investigation is carried out using either water or an aqueous solution of xanthan gum, in order to assess the impact of the variation of the rheological properties at different impeller speeds and direction of rotation of the impellers on the mixing features. To this end, Particle Image Velocimetry is adopted to obtain the velocity fields for the different liquid phases. The data analysis allows to identify possible critical fluid dynamics characteristics that may affect the fermentation, as for example the presence of stagnant zones, where sinking layers might be expected, thus explaining the failure of the biogas production often observed in the biogas production plant

A CFD study on the change of scale of non-Newtonian stirred digesters at low Reynolds numbers

Biogas from anaerobic digestion of agricultural waste is proving to be a convincing way to reduce greenhouse gas emissions. To optimize the process energy efficiency, the CFD simulation of the laminar non-Newtonian fluid mixing in the digester would be an effective method, but the adoption of appropriate spatial discretization at the production scale is currently impossible. For this reason, the identification of change of scale rules for an effective design and for preliminary laboratory scale experimental investigations is still of paramount importance. This work is aimed at the identification of a methodology for the scale down of an industrial stirred anaerobic digester with a volume of 1500 m3, for which CFD simulations have an unacceptable computational cost. The investigation is based on the simulation of three different scale down geometries. The different blade rotational speeds were determined from four different change of scale approaches, which enforced constant blade tip speed, constant shear rate close to the blades, constant Reynolds number and constant power per unit volume, across the different digester sizes. The volume distributions of velocity magnitude, shear rate and shear stress can be exploited to assess the presence of dead zones or localized region where biogas production may be inhibited. The effect of the different change of scale rules on the local instantaneous fluid dynamics were quantified and discussed, finding that both the non-dimensional velocity and non-dimensional shear rate fields are constant across the different scales, when the Reynolds number, based on the Metzner and Otto concept, is constant

Use of PLIF to assess the mixing performance of small volume USP 2 apparatus in shear thinning media

AbstractPlanar Laser Induced Fluorescence (PLIF) was used to assess mixing in small volume USP 2 dissolution apparatus for a range of viscous fluids which mimic gastrointestinal media, especially in the fed state. The release into the media from a specially prepared tablet containing Rhodamine 6G dye was tracked in time and the areal distribution method developed by Alberini et al. (2014a) was implemented to characterise the mixing performance. The distributions of the individual striations for selected mixing levels were also presented. These findings illustrate the poor mixing performance of the apparatus resulting in high variance of the dissolution data when working with viscous media. Analysis of data using CoV gives misleading results for the mixing performance of the small volume USP 2 dissolution apparatus. The results showed that the best mixing was mainly located above the blade and close to the wall, i.e. in the region where intensive motion takes place. This work presents important guidelines and precautions for choosing the proper sampling point for a wide range of liquid viscosities to minimize the variability of the dissolution data

Non-intrusive Experimental Monitoring of Gluten-free Dough Mixing in 1L Scale Mixer Using Electrical Resistance Tomography

In this study, for the first time Electro Resistance Tomography is employed for macroscale (bulk) characterization of gluten free dough during mixing in 1L scale vessel. Gluten free dough is a complex fluid, which presents a non-Newtonian behaviour. Its rheological behaviour is complex because it is not only shear rate dependent, but also time dependent. The investigated gluten-free dough consists of a mixture of gluten free flours (chickpeas, corn and potato starch), a thickening agent, instantaneous yeast and water. As thickening agent, Xanthan gum is used, which is a polysaccharide with many industrial uses, including food additive. The dough has been prepared in a 3D printed mixer equipped with double rotation shaft lid. Thus a “pizza” hook rotates off-centre (respect to the lid of the vessel) and on the axis of rotation of the hook itself. At the wall of the vessel, the electrodes for the tomography measurements are added in 4 circular planes. With the current work, a new approach to dynamically monitor the mixing developments is suggested, showing the potentiality of the technique to not intrusively identify inhomogeneity in the dough during the process. A standard operating procedure is used for the preparation of the dough, which consists in precise steps in time of material addition into the mixture

Influence of DC electric field upon the production of oil-in-water-in-oil double emulsions in upwards mm-scale channels at low electric field strength

AbstractA novel approach to create O/W/O is developed and described to achieve uniform oil drop size coated with thin layers of water. Drops were created using a test cell where the DC field is applied between different internal diameter (ID) needles (from which the O/W emulsion emits upwards into a continuous oil phase) and a grounded metal ring which was located at selected distances from the needle top. The advantages compared to the previous techniques consist of possibility of control on drop size and coating layer of the water using low electric field. A high speed imaging technique has been applied to determine drop size under different flow and electric field conditions. Without the electric field, several flow regimes were observed; stable formation of both the O/W/O emulsion and the O/W emulsion upstream of the cell was possible over a range of Reynolds numbers from 80 to 100. The effect of the electric field was found to be reverse below electric field strength of 60kVm−1, beyond this critical value there was significant impact upon the flow regime, drop size and emulsion structure. The impact of the electric field strength upon flow pattern and emulsion structure and a quantitative analysis of droplet size are presented. The work shows the results for the controlled creation of complex emulsion droplets combining electric field and mm scale channels. The differences with the other physical processes reported in the literature are discussed