Regime bubble coalescing

The bubbling fluidization regime, as shown in Fig. 9.3(b), is reached with an increase in the gas velocity beyond (7mb. Bubbles form and induce vigorous motion of the particles. In the bubbling fluidization regime, bubble coalescence and breakup take place. With increasing gas velocity, the tendency of bubble coalescence is enhanced. Two distinct phases, i.e., the bubble phase and the emulsion phase, are present in this regime. 

Classical bubbles do not exist in the vigorously bubbling, or turbulent fluidization regimes. Rather, bubbles coalesce constantly, and the bed can be treated as a pseudohomogenous reactor. Small bubble size improves heat transfer and conversion, as shown in Figure 5b. Increasing fines levels beyond 30—40% tends to lower heat transfer and conversion as the powder moves into Group C. 

Fig. 2.32 Diabatic flow pattern map for vaporizing flow in uniformly heated micro-channel, R-134a, d = 0.5 mm, L = 70 mm, Tg = 30 °C, = 50 kW/m without subcooling at inlet. Flow patterns isolated bubble regime (IB), coalescing bubble regime (CB), annular (completely coalesced) regime (A), post-dryout regime (PD). Reprinted from Thome et al. (2006) with permission...

Fluidization Regime. As for traditional fluidization applications, the fluidization regime—dispersed bubble, coalesced bubble, or slugging—in which a three-phase fluidized bioreactor operates depends strongly on the system parameters and operating conditions. Generally, desirable fluidization is considered to be characterized by stable operation with uniform phase holdups, typical of the dispersed bubble regime. It would be useful to be able to predict what conditions will produce such behavior. 

Additionally, macroscopic flow structure of 3-D bubble columns were studied [10]. The results reported can be resumed as follows (a) In disperse regime, the bubbles rise linearly and the liquid flow falls downward between the bubble stream, (b) If gas velocity increases, the gas-liquid flow presents a vortical-spiral flow regime. Then, cluster of bubbles (coalesced bubbles) forms the central bubble stream moving in a spiral manner and 4-flow region can be identified (descending, vortical-spiral, fast bubble and central flow region). Figure 10 shows an illustrative schemes of the results found in [10]. 

The important variables that affect the bubble dynamics and flow regime in a bubble column are gas velocity, fluid properties (e.g. viscosity, surface tension etc.), nature of the gas distributor, and column diameter. Generally, at low superficial gas velocities (approximately less than 5 cm/sec) bubbles will be small and uniform though their nature will depend on the properties of the liquid. The size and uniformity of bubbles also depends on the nature of the gas distributor and the column diameter. Bubble coalescence rate along the column is small, so that if the gas is distributed uniformly at the column inlet, a homogeneous bubble column will be obtained. 

The bubble coalescence criterion (Figure 3) using the propagation velocity was developed by Gidaspow (18). The void propagation equation in one dimension was derived from the continuity equations. The propagation velocity (C) in coalesced bubble regime becomes as follows ... 

Turbulent Fluidization. For Group A particles, as shown in Fig. 13.2, the onset velocity of the transition to the turbulent regime is commonly defined as the gas velocity corresponding to the peak Uc, whereas the leveling point Uk may be recognized as the onset of the turbulent regime proper [14]. However, based on direct observation of bed phenomena, appreciable variations in bubble behavior occur at gas velocities around Uc. Specifically, the bubble interaction is dominated by bubble coalescence at gas velocities less than Uc, while it is dominated by bubble breakup at gas velocities greater than Uc. 

The successful operation of effervescent atomizers has been dependent on the ability to maintain a stable uniform sized bubble flow. Bubbly flows depend on nozzle geometry, air and liquid flow rates, and they become influenced by pressure and velocity instabilities. The stability of bubbly flows depends upon two factors (1) bubble coalescence and (2) characteristics of the bubble formation, which may affect their coalescence. The bubble formation may occur at various regimes, which... 

Transition velocity tjetween Ug j, dispersed bubble regime and coalesced bubble regime... 

Fig. 4.27 The types of holes formed of detached hydrogen bubbles by different PC and RC regimes (a) coalesced hole obtained by the PC regime ...

Using the presented computational method we provide a systematic study of diverse shape regimes for a single buoyant bubble, recovering all main regimes in a full agreement with available experimental data (for detailed analysis see (Smolianski et al. )). Next, we present results on the bubble coalescence phenomena. 

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