Surface bubble eruption

The larger particles were thrown high in the splash zone higher than predicted by a ballistic trajectory using the bubble rise velocity as the initial velocity and neglecting any air drag. Later observations of this model showed that, when bubbles erupt at the surface, the accompanying... 

Particles are ejected into the freeboard via two basic modes (1) ejection of particles from the bubble roof and (2) ejection of particles from the bubble wake, as illustrated in Fig. 9.19. The roof ejection occurs when the bubble approaches the surface of the bed, and a dome forms on the surface. As the bubble further approaches the bed surface, particles between the bubble roof and surface of the dome thin out [Peters et al., 1983]. At a certain dome thickness, eruption of bubbles with pressure higher than the surface pressure takes place, ejecting the particles present on top of the bubble roof to the freeboard. In wake ejection, as the bubble erupts on the surface, the inertia effect of the wake particles traveling at the same velocity as the bubble promptly ejects these particles to the freeboard. The gas leaving the bed surface then entrains these ejected particles to the freeboard. 

A final question that needs to be answered is the number of particles drawn into the gas jets during bubble eruptions. Based on the literature and experimental observations undertaken during the course of this study, it is assumed that a layer with a thickness equal to the mean particle diameter in the bed is involved in the ejection process. From the surface exposed to a particular gas jet the total mass of involved particles is then calculated. Size classes for char and sand particles are allocated the same percentage of the ejected particle mass as in the bed as a whole. 

Consider the case of gas forming a bubble within a mass of particles. The only way in which the solids can be moved out of the way to form a bubble is that there must be a force being exerted by the gas flowing past the solids. Most of the gas then finds its way back into the bubble to complete its passage through the bed. The resulting flow into and out of the emulsion phase enhances the mass transfer. In the case where the bubble eruption and collapse occur at the bed surface, there is also intensely enhanced mixing due to... 

With this arrangement the signal fluctuations about their mean value are strongly influenced by bubble eruption at the bed surface, reflecting... 

Particularly high stress occurs when bubbles burst on the surface of the liquid, whereby droplets are eruptive torn out of the surface [32-36]. According to theoretical calculations, maximum energy densities occur in the region of the boundary surface shortly before the droplets separate [36]. The results calculated by Boulton-Stone and Blake [34] show that these are exponentially dependent on bubble diameter dg. Whereas these authors found values of e = lO mVs with dg = 0.5 mm, these are only e 1 m /s with dg = 5 mm. The situation may be different regarding the droplet volume separated from the surface by the gas throughput and thus the number of particles which are exposed to high stress. The maximum for this value occurs with a bubble diameter of dg = 4 mm (see [34]), and it is therefore feasible that there could be an optimal bubble size. 

The eruption of these bubbles at the bed surface is responsible for the ejection of particles of all size classes into the freeboard. Very fine particles may even be entrained without the assistance of a bubble, if their terminal falling velocity is below the superficial gas velocity in both the bed and the freeboard. 

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