(150a) Experimental Observation of the Initial Stages of Localized Fluidization

Localized fluidization is the process where fluidization is restricted to a section of the bed while the surrounding areas remain static. Localized fluidization is of importance for the operation of conical and spouted beds, as well as the understanding of soil erosion and ground water movement. Although fluidization has been extensively studied in the literature, there are fewer studies dealing with localized fluidization. In particular, there is very little experimental information on the initial stages of the formation of localized fluidization. Therefore, the objective of this paper was to conduct an experimental investigation of the early stages of localized fluidization.

The experiments were carried out in a 30 cm x 20 cm x 10 cm transparent rectangular box where different particle diameters, injection ports and bed heights were tested. To observe the formation of the fluidization, a combination of Planar Laser Induced Fluorescence (PLIF) and Refractive Index Matching (RIM) was used. RIM renders the medium transparent and PLIF allowed for the observation of a particular plane inside the bed away from wall interactions. This setup permitted the observation of the growth of the fluidized zone from the initial localized particle movement within a cavity to the fully fluidized state.

The results include the identification of two very distinct regimes for the expansion of the fluidized cavity: a regular regime that has been previously observed, and a newly found ultra-slow regime that requires much longer time to achieve full fluidization. It was also found that the duration of the expansion from cavity to fluidized chimney depends strongly on the injection port size in the ultra-slow regime, while it only depends weakly in the regular regime. In addition, the diameter of the injection port has a significant influence on the evolution of the fluidization in the area close to the injection, while having almost no effect in the region closer to the top of the bed. Furthermore, a parametric study of the particle sizes, diameter of the injection port, and bed height was performed for the regular regime. An empirical expression for this regime was developed that allows the collapse of the expansion time for all the different variables studied.