Bubble behaviour investigation in a wet fluidized bed using digital image analysis

Abstract

Due to the presence of liquid bridge forces, wet particles reveal totally different fluidization behaviours than dry fluidized beds. This paper studies the bubble dynamics of wet Geldart‐D particles in a wet 2D fluidized bed. A digital image analysis method is adopted to identify bubbles and extract bubble properties based on MATLAB software. During fluidization, the bubble feature analysis of optimized binary images captures the bubble size variation, bubble fraction, and bubble shape. The results show that the increasing liquid saturation promotes the gas holding capacity of the emulsion phase, leading to the decreasing bubble fraction. When the particle size grows, the stability of the emulsion phase is promoted and hence the average bubble diameter increases. As the liquid saturation increases, the enhancing liquid bridge forces limit the bubble growth and promote the bubble breakage, which contributes to the small bubbles. With respect to the wet particles, of which the diameter is 1 mm and the liquid saturation is over 15%, slugging is likely to be observed, resulting in the increasing equivalent bubble diameter. The growing liquid saturation increases the wide aspect ratio, decreases the shape factor, and disperses the dependence between the two parameters. This indicates that small and irregular bubbles are likely to be seen in wet fluidized beds.

Investigation of Bubble Properties in a Bubbling Fluidized-Bed Gasification Reactor Using a Computational Particle Fluid Dynamic Model

“…The time step, 0.001 s, specified in Table 3 is the maximum allowable in the simulation. The actual time step is limited by (11) where Δx cell is the cell dimension, u is the fluid velocity, and Δt is the time step in the flow direction. In order to maintain stability, accuracy, and speed of the calculation, the VR automatically adjusts the time step to keep the CFL number between 0.8 and 1.5 as specified in this study.…”

“…Several studies have been carried out on the measurement of bubble properties in fluidized beds using both experimental and numerical techniques. For instance, nonintrusive experimental techniques such as electrical capacitance tomography (ECT) and digital image sensing are widely used to measure the bubble properties in fluidized beds without interfering with the fluid flow dynamic behavior. − However, most of these works are devoted to measuring bubble dynamics in fluidized bed columns containing one type of particle. By contrast, gasification reactors usually come with a bed of solids containing mixtures of inert particles and active biomass particles of different size distributions.…”

Characterizing bubble behavior in a pseudo‐2D fluidized bed of wet particles

“…By monitoring the incipient fluidization, it was found that liquid addition changes the fluidization of Geldart Group B particles, shifting the fluidization properties of particles to Group A and then to Group C. 9,[22][23][24] Thus, it can be speculated that the effect of liquid addition on bubble characteristic in wet fluidization would be similar to that of reducing particle size in dry-particle fluidization. Previous studies on the bubbling characteristics of wet fluidization reveals that as liquid content increases, the liquid-bridge force contributes to smaller, irregular bubbles with smaller aspect ratio 25 and lower bubble fraction and bubble flow rate. 14 These findings cannot support the promotion of the evolution of bubble characteristics shifting from Geldart Group B particles to Group A and then to Group C when liquid is added.…”