Bubble-flow interactions

Studies of individual bubbles rising in a two-dimensional gas—Hquid—soHd reactor provide detailed representations of bubble-wake interactions and projections of their impact on performance (Fig. 9). The details of flow, in this case bubble shapes, associated wake stmctures, and resultant bubble rise velocities and wake dynamics are important in characteri2ing reactor performance (26). 

A set of experiments on gas-liquid motion in a vertical column has been carried out to study its d3mamical behavior. Fluctuations volume fraction of the fluid were indirectly measured as time series. Similar techniques that previous section were used to study the system. Time-delay coordinates were used to reconstruct the underl3ung attractor. The characterization of such attractor was carried out via Lyapunov exponents, Poincare map and spectral analysis. The d3mamical behavior of gas-liquid bubbling flow was interpreted in terms of the interactions between bubbles. An important difference between this study case and former is that gas-liquid column is controlled in open-loop by manipulating the superficial velocity. The gas-liquid has been traditionally studied in the chaos (turbulence) context [24]. 

To clarify the mutual interactions between the gas bubbles and its surrounding liquid flow (mostly turbulent) in a bubbly flow, information of bubble s shape and motion is one of the key issues as well as the surrounding liquid velocity distribution. Tokuhiro et al. (1998, 1999) enhanced the PIV/LIF combination technique proposed by Philip et al. (1994) with supplementation of SIT to simultaneously measure the turbulent flow velocity distribution in liquid phase around the gas bubble(s) and the bubble s shape and motion in a downward flow in a vertical square channel. The typical experimental setup of the combination of PIV, LIF, and SIT is shown in Figure 14. The hybrid measurement system consists of two CCD cameras one for PIV/LIF (rear camera) and the other for SIT (front). The fluorescent particles are Rhodamine-B impregnated, nominally 1-10 pm in diameter with specific density of 1.02, and illuminated in a light sheet of approximately 1 mm thickness (Tokuhiro et al., 1998,1999). The fluorescence is recorded through a color filter (to cut reflections) by the rear camera. A shadow of the gas bubble is produced from infrared LEDs located behind the gas bubble. A square "window" set within the array of LEDs provides optical access for... 

The velocity vectors of the liquid phase and bubbles are accurately detected after the abovementioned data-processing procedures, so that the characteristics of turbulence in a channel flow modified by microbubbles injection and the bubble-turbulence interactions are able to be explored statistically (Kitagawa et al., 2005). 

When nucleate boiling occurs in narrow channel the flow regime could be different from it s in conventional tube. In conventional tube the bubble departure size is typically less that the tube diameter, so bubble flow pattern and nucleate boiling are preferable in wide range of vapor content. In case of narrow channel the bubble departure size could be comparable with the channel size, it causes strong interaction between bubbles and change both the flow pattern and heat transfer rate. 

Complex multiphase flows (e.g., slurry bubble columns and stirred tank reactors i.e., bubbly flows in slurries three phase fluidized beds, i.e., liquid droplets and particles in continuous gas) where many phases interact simultaneously. 

Pauchon C, Banerjee S (1988) Interphase momentum interaction effects in the averaged multifield model, P art II Kinematic waves and interfacial drag in bubbly flows. Int J Multiphase Flow 14(3) 253-264... 

Kolev [46] discussed the validity of these relations for fluid particle collisions considering the obvious discrepancies resulting from the different nature of the fluid particle collisions compared with the random molecular collisions. The basic assumptions in kinetic theory that the molecules are hard spheres and that the collisions are perfectly elastic and obey the classical conservation laws do not hold for real fluid particles because these particles are deformable, elastic and may agglomerate or even coalescence after random collisions. The collision density is thus not really an independent function of the coalescence probability. For bubbly flow Colella et al [15] also found the basic kinetic theory assumption that the particles are interacting only during collision violated, as the bubbles influence each other by means of their wakes. 

The flow regimes discussed above are often used to describe systems with 0i 1. Other flow regimes occur for 0i 1 (e.g. bubbly flows) because additional force models (such as added mass) introduce new dimensionless parameters, and because the momentum of the continuous phase becomes dominant. For example, in bubbly flow when RCp > 1 the turbulent liquid wakes behind bubbles lead to pseudo-turbulence (Mudde et al, 2009 Riboux et al, 2010 Sato Sekoguchi, 1975) that changes the nature of bubble-bubble interactions through the continuous phase. 

Some foams that have a drop-size distribution that is heavily weighted toward the smaller sizes will represent the most stable foam. In such cases, changes in the size distribution curve with time yield a measure of the stability of the foams. The bubble size distribution also has an important influence on the viscosity. For bubbles that interact electrostatically or sterically, foam viscosity will be higher when bubbles are smaller (for a given foam quality). This condition results because the increased interfacial area and thinner films increase the resistance to flow. The viscosity will also be higher when the bubble sizes are relatively homogeneous, that is, when the bubble size distribution is narrow rather than wide (also for given foam quality). 

The Larkins and Sweeney equations were developed for downward bubble flow. In trickle bed operation, the liquid and gas flow rate are not as high as in packed absorbers, so that there is much less interaction. Single-phase flow pressure drop equations could be used as a first approximation, with the void fraction reduced to... 

Bubble size is required to calculate, for example, the drag force imparted on a bubble. Most Eulerian-Eulerian CFD codes assume a single (average) bubble size, which is justified if one is modeling systems in which the bubble number density is small (e.g., bubbly flow in bubble columns). In this case, the bubble-bubble interactions are weak and bubble size tends to be narrowly distributed. However, most industrially relevant flows have a very large bubble number density where bubble-bubble interactions are significant and result in a wide bubble size distribution that may be substantially different from the average bubble size assumption. In these cases, a bubble population balance equation (BPBE) model may be implemented to describe the bubble size distribution (Chen et al., 2fX)5). 

The discrete phase simulation method described in Secs. 4.1 through 4.4 is capable of predicting the flow behavior in gas-liquid-solid three-phase flows. In this section, several simulation examples are given to demonstrate the capability of the computational model. First, the behavior of a bubble rising in a liquid-solid suspension at ambient pressure is simulated and compared to experimental observations. Then the effect of pressure on the bubble rise behavior is discussed, along with the bubble-particle interaction. Finally, a more complicated case, that is, multibubble formation dynamics with bubble bubble interactions, is illustrated. 

---