Measurement of Multiphase Flow Characteristics Via Image Analysis Techniques: The Fluidization Case Study

In this chapter, an overview on some imaging-based experimental techniques for the analysis of complex multiphase systems is reported. In particular, some techniques aimed at the study of fluidization dynamics will be analyzed and discussed, as developed by our research grou

Lagrangian Simulation of Bubbling Dynamics in a Lab-Scale 2D Fluidized Bed

The present work focuses on the development of a novel computational code able to predict with a reasonable level of accuracy the bubble behavior in gas fluidized beds with minimum computational demands. The code simulates the bubble chaotic rise motion and coalescence along bed height via simple lagrangian tracking of bubbles. An original empirical model for the assessment of bubble-bubble interactions is developed. The code is used to simulate a lab-scale unit in bubbling and slugging mode. On this basis, fast simulations are performed to successfully predict bubble population and fluxes within the bed. The main aim of this code is to be embedded within CAPE codes for the process simulation. The model adopted by the code is also well suited for multi-scale modeling approach since physical parameters can be obtained from both experimental data or CFD simulation. Preliminary results of the simulations, in terms of distributions for bubble size and number as well as local hold up values, are compared with relevant experimental data

Measurement techniques and modelling of multiphase systems.

Multiphase systems are often encountered in process industry. Widely diffused applications and operations involve the contact between different phases (gas-solid, gasliquid, solid liquid, etc.) for different purposes, such as chemical reactions of physical operations (heat transfer, mass transfer). Multiphase flows are therefore one of the most frequent applied fields in chemical engineering. The present thesis is aimed at the development of experimental techniques for the investigation of multiphase flow in general, with particular focus on the analysis of gas-solid and gas-liquid systems. Notably, the development of image analysis techniques is the central thread of the present contribution, and it will be shown that the knowledge so far accumulated allowed to obtain considerable results in both fields investigated. The approach to multiphase systems here adopted is based on the development of novel reliable experimental techniques for the assessment of multiphase system properties and subsequent collection of new experimental information through application of the original techniques here developed. The techniques are mainly based on image analysis, they are non-intrusive, capable of securing several properties simultaneously and cost effective. In particular, the expertise in digital image processing was applied to the investigation of two different classes of multiphase systems, i.e. gas-liquid dispersions and dense gas-solid systems (fluidized beds). With reference to gas liquid dispersions, an effective experimental technique for measuring local gas hold-up and interfacial area, as well as bubble size distribution, was developed and subsequently exploited for collecting experimental information. The technique, named Laser Induced Fluorescence with Shadow Analysis for Bubble Sizing (LIF-SABS) is based on laser sheet illumination of the gas-liquid dispersion and synchronized image acquisition, i.e. on equipment typically available within PIV set-ups. With reference to fluidized beds, a digital image analysis technique was developed vi to study the fluidization dynamics of a lab-scale two-dimensional bubbling bed. Several significant bubble properties were simultaneously measured, ranging from overall bed properties to bubble size and bubble velocity distributions. Moreover, since a lack of knowledge exists on the bubbling dynamics of mixed powders, a large experimental campaign was set up to investigate the fluidization behavior of such powder mixtures. In the field of fluidized bed modeling, a novel linear stability criterion for the state of homogeneous fluidization regime was developed, based on a new mathematical model for gas-fluidized beds. A fully predictive criterion for the stability of homogeneous fluidization state was proposed and validated with literature data

A NOVEL TECHNIQUE FOR MEASURING LOCAL BUBBLE SIZE DISTRIBUTION

A novel experimental technique for measuring the local gas hold-up and the statistical distribution of local bubble size, is proposed. The technique is based on laser sheet illumination of the gas-liquid dispersion and synchronized camera, i.e. on equipment typically available in PIV set-ups. The liquid phase is made fluorescent by a suitable dye, and a band-pass optical filter is placed in front of the camera optics, in order to allow only fluoresced light to reach the camera CCD. In this way bubbles intercepted by the laser sheet are clearly identified thanks to the neat shade resulting in the images. This allows excluding from subsequent analysis all bubbles visible in the images but not actually intercepted by the laser sheet, so resulting in better spatial resolution and data reliability. Preliminary data obtained in a stirred gas-liquid dispersion confirm the technique viability and reliability

Modeling of Magnetic-Field-Assisted Fluidization: Model Development and CFD Simulation of Magnetically Stabilized Fluidized Beds

Magnetic-field-assisted fluidization is starting to be considered as a viable alternative to standard fluidized beds for those operations (such as particle separations, filtration, adsorption) in which the solid phase can be made of magnetic particles or, alternatively, the fluidizing agent is a ferro-fluid; thus the fluid bed responds to the action of magnetic fields, and stabilized fluidization regimes can be generated. One of the major difficulties to be tackled is the development of a predictive model capable of estimating the stabilized-to-bubbling transition velocity for a given magnetic field or, on the other hand, the magnetic field intensity required to stabilize the bed to a quiescent condition. The fluid dynamics prediction of a stabilized bed is also a challenging task at the moment. On this basis, a very simple model for the description of MSFB was derived in this contribution starting from basic fluid dynamics and magnetodynamics equations. The model was implemented in a commercial CFD code in order to simulate the effect of the magnetic field onset on a freely bubbling fluidized bed

Free-surface shape in unbaffled stirred vessels: experimental study via digital image analysis

There is a growing interest in using unbaffled stirred tanks for addressing a number of processing needs such as low shear damage (sensitive biocultures), low attrition (solid\u2013liquid applications), deep-cleaning/ sterilization (pharmaceutical applications). The main feature of uncovered, unbaffled stirred tanks is highly swirling motion of the fluid that results in a deformation of the free liquid surface. At sufficiently high agitation speeds the resulting whirlpool reaches the impeller and gives rise to a gas\u2013liquid dispersion, so leading to the formation of a dispersion without the use of gas-sparger; the so-called self-inducing operation of the vessel. In this work, digital image analysis coupled with a suitable shadowgraphy-based technique is used to investigate the shape of the free-surface that forms in uncovered unbaffled stirred tanks, when different stirrer geometries are considered. The technique is based on back-lighting the vessel and suitably averaging over time the recorded free surface shape. For each investigated geometry, the deformed free-surface was analyzed at different impeller speeds. Different geometries of the vessel were analyzed, by varying impeller distance from vessel bottom as well as agitator type (Rushton turbine, Lightnin A310, Pitched Blade Turbine). It is shown that impeller design strongly affects the free surface profile, and in turn the impeller speed at which the free surface reaches the impeller. A model was developed to fully describe free-surface profile at all agitation speeds and for all investigated geometries, suitable for being adapted to experiments by means of physically consistent parameters adjustment

Experimental analysis of gas-liquid dispersion in mechanically stirred tank by means of quantitative Electrical Resistance Tomography

Electrical Resistance Tomography (ERT) is adopted extensively to measure fundamental dispersion properties in dense multiphase systems, where other techniques are either impossible to use (e.g. optical techniques), or very expensive (CT, PEPT, especially at large scales). Quite satisfying qualitative results can be obtained by ERT (sufficient for the accurate assessment of regime transitions or blending times), but the ability to reconstruct quantitative data has still to be fully addressed. In this paper, a practical method for obtaining quantitative data from ERT measurements is proposed and validated through experimental data. Several aspects are addressed: the effect of iterative vs non-iterative conductivity tomogram algorithms, determination of the optimal iteration number for iterative procedures and the choice of the most suitable conductivity-holdup relation. The general method proposed is applied to the dispersion of gas in a mechanically stirred tank agitated by a pitched blade turbine, finally resulting in affordable data for CFD models validation