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.