@phdthesis{Asegehegn2011, author = {Asegehegn, Teklay Weldeabzgi}, title = {Investigation of bubble hydrodynamics in gas-solid fluidized beds containing immersed horizontal tube banks for lignite drying application}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-23578}, school = {BTU Cottbus - Senftenberg}, year = {2011}, abstract = {In bubbling fluidized beds, bubble characteristics such as size, shape and velocity have a vital influence on the hydrodynamics of the bed and hence on its performance as a chemical reactor and/or a heat exchange unit. In many industrial applications such as lignite dryer heat transfer tubes are usually inserted to enhance the rate of heat and mass transfer and chemical conversion. However, their presence strongly influences the bubbling behavior of the beds. Therefore, reliable design and scale-up of these systems come only after fundamental understanding of the bubbling behavior is achieved. Therefore, in this research work the influences of tube bank geometries and particle size on bubble characteristics were thoroughly investigated. In this research work, both numerical and experimental studies were employed. For the experimental measurements, a new nonintrusive digital image analysis technique was developed. The technique allowed for the simultaneous measurements of bed expansion and various bubble properties. An in-house software was developed to fully automate the image acquisition and data processing procedure. For the numerical studies, the Eulerian-Eulerian two-fluid model based on the kinetic theory of granular flow was used. Though, this CFD model has been considered as a fundamental tool for modeling gas-solid fluidized beds, its quantitative validation remains insufficient for a wide range of reactor geometries and operating conditions. Therefore, in this work validation of the model using experimental measurements of bed expansion and bubble properties obtained from a pseudo-two-dimensional fluidized bed was performed. The influence of two-dimensional simulations and different modeling parameters such as the friction packing limit, drag model and solid-wall boundary conditions were investigated. The two-fluid model generally showed reasonable agreement with the experimental measurements of pressure drop, bed expansion and bubble properties in bubbling regime. However, as the gas superficial velocity is increased and the bed moved towards slugging and turbulent regimes a big deviation arose and the two-fluid model failed to predict reasonably the fluidized bed hydrodynamics for the freely bubbling bed. The mean bubble properties predicted by two-dimensional simulations were in reasonable agreement with experiments at lower superficial velocities. They deviated at higher bed height and this was more pronounced at higher gas superficial velocities. The results from three-dimensional simulations were in better agreement with the experimental measurements; however, the computational effort need was very high making them impractical for parametric studies and sensitivity analyses. It was also showed that the choice of friction packing limits, drag laws and specularity coefficients have little influence on the bubble properties. For a bubbling bed, both experimental measurements and numerical simulations showed that inserting horizontal tube banks had either no or marginal influence on the static bed pressure drop and bed expansion. On the other hand, bubble hydrodynamics were strongly influenced and controlled by the geometry of the immersed tubes. In freely bubbling fluidized beds bubble size as well as rise velocity increased with bed height and superficial velocity. In beds with immersed tubes, such general trends were completely disturbed. Tubes appeared to restrict rapidly growing bubbles. As a result the mean bubble diameter and rise velocity were lower in the vicinity of the tube banks than in the freely bubbling bed. Results from different particle sizes showed that in a freely bubbling bed increasing the mean particle size increased the bubble diameter and rise velocity as well as bed expansion. In fluidized beds with dense horizontal tubes on the other hand, the mean bubble properties were almost independent of the particle sizes.}, subject = {Wirbelschicht; Blase; Hydrodynamik; Zweiphasenstr{\"o}mung; Bildanalyse; Wirbelschicht; Blasenhydrodynamik; Eingetauchte horizontale Rohre; Two-Fluid Model; Automatisierte Bildanalyse; Fluidized Beds; Bubble Hydrodynamics; Immersed Horizontal Tubes; Two-Fluid Model; Digital Image Analysis Technique}, language = {en} }