Uniform bubbling

Prom the onset of creaming to the end of the rise during the expansion process, the gas must be retained completely in the form of bubbles, which ultimately result in the closed-ceU stmcture. Addition of surfactants faciUtates the production of very small uniform bubbles necessary for a fine-cell stmcture. 

Column Operation To assure intimate contact between the counterflowing interstitial streams, the volume fraction of liquid in the foam should be kept below about 10 percent—and the lower the better. Also, rather uniform bubble sizes are desirable. The foam bubbles will thus pack together as blunted polyhedra rather than as spheres, and the suction in the capillaries (Plateau borders) so formed vidll promote good liqiiid distribution and contact. To allow for this desirable deviation from sphericity, S = 6.3/d in the equations for enriching, stripping, and combined column operation [Lemhch, Chem. E/ig., 75(27), 95 (1968) 76(6), 5 (1969)]. Diameter d still refers to the sphere. 

The simplest model of a bubbling fluidized bed, with uniform bubbles exchanging matter with a dense phase of catalytic particles which promote a continuum of parallel first order reactions is considered. It is shown that the system behaves like a stirred tank with two feeds the one, direct at the inlet the other, distributed from the bubble train. The basic results can be extended to cases of catalyst replacement for a single reactant and to Astarita s uniform kinetics for the continuous mixture. 

The horizontal tube can be moved through the counter shield. To avoid complications associated with non-uniform flow and also to provide a direct means of measuring the flow velocity, air is allowed to leak into the system to form uniform bubbles spaced at regular intervals. The time taken for the bubbles to travel the distance between two markers 100 cm apart on the tube is measured with a stop-watch during each of the counting intervals. The experimental results are shown in Fig. 11, in which all of the data have been corrected for background. A half-life of 0.84 seconds for Pb2< m, with a probable error of 2, was obtained from these results. 

Bubble size distribution is one the most diffieult parameters to measure and analyze. However, its evolution with time is of great importance, because it is closely linked to the product quahty as perceived by a consumer (Hailing 1981, Campbell and Mougeot 1999). In some systems, a uniform bubble size may be desirable (e.g., bread dough and eake batter) which improve baking characteristics, while in others, a wide spread in the distribution may be advantageous to achieve specific mouth-feel responses. 

The transition from the uniform bubbling regime to the churn-turbulent regime depends upon the physical properties of the gas and liquid, the particle size and the fluid flow rates. Very little is known about this transition regime. 

It has been shown by Chavarie and Grace (15) that the decomposition of ozone in a fluidized-bed is best described by Kunii and Levenspiel s model (16) but that the Orcutt and Davidson models (17) gave the next best approximation for the overall behaviour and are easier to use and were chosen for the simulation. They suppose a uniform bubble size distribution with mass transfer accomplished by percolation and diffusion. The difference between the two models is the presumption of the type of gas flow in the emulsion phase piston flow, PF, for one model and a perfectly mixed, PM, emulsion phase for the other model. The two models give the following expressions at the surface of the fluidized bed for first-order reaction mechanism ... 

Uniform bubbly flow is unstable to void fraction disturbances above some critical value of the void fraction (>45%). 

Biesheuvel and Gorrisen (1990) have derived one-dimensional conservation equations for void fraction disturbances in a uniform bubbly fluid. They have studied the features of the propagation of void fraction disturbances and investigated the stability of uniform bubbly flows. They observed that the voidage fraction waves are unstable for void fractions above some critical value (>45%). Their method of approach was similar to that of Batchelor (1988) in many respects. 

The initial steady state of flow is the homogeneous flow regime (i.e., the uniform bubbly flow regime) and is represented by the following equations At time 1 = 0 ... 

For recirculation flow the Taylor dispersion mechanism was introduced by Shyu and Miyauchi (S13). Equation (4-12) is a revised result for it. For this flow regime, Ohki and Inoue (02) developed an expansion model with parameters adjusted to the data available, and also introduced the Taylor dispersion mechanism for the low-gas-velocity region of uniform bubble flow. 

In bubble columns, gas is dispersed in a continuous liquid phase. Uniform bubble size and bubble concentration characterize the homogeneous regime, particularly in the traverse direction indicating the absence of bulk liquid circulation. In contrast, the heterogeneous regime is characterized by a nonuniform bubble concentration, especially in the traverse direction, because of liquid circulation. 

Lance M, Bataille J (1991) Turbulence in the liquid phase of a uniform bubbly air-water flow. J Fluid Mech 222 95-118... 

Tomiyama [148] and Tomiyama and Shimada [150] adopted a N + 1)-fluid model for the prediction of 3D unsteady turbulent bubbly flows with non-uniform bubble sizes. Among the N + l)-fluids, one fluid corresponds to the liquid phase and the N fluids to gas bubbles. To demonstrate the potential of the proposed method, unsteady bubble plumes in a water filled vessel were simulated using both (3 + l)-fluid and two-fluid models. The gas bubbles were classified and fixed in three groups only, thus a (3 + 1)- or four-fluid model was used. The dispersions investigated were very dilute thus the bubble coalescence and breakage phenomena were neglected, whereas the inertia terms were retained in the 3 bubble phase momentum equations. No population balance model was then needed, and the phase continuity equations were solved for all phases. It was confirmed that the (3 + l)-fluid model gave better predictions than the two-fluid model for bubble plumes with non-uniform bubble... 

Traditional methods used for the study of foams employ one of three methods of generating a foam, namely single capillaries, sintered glass spargers and diffuser stones. Single capillaries give uniform bubbles but are relatively slow. The... 

Other methods generate a rapid foam with considerable turbulence, which adversely affects the reproducibility of the results. A turbulent-free foam can be created by using a uniform mesh to generate a foam of uniform bubble size. Direct measurements of single film thickness using optical methods can yield useful information on the critical film thickness. 

To keep all the catalyst particles suspended and to achieve a uniform dispersion of bubbles, a minimum amount of agitation is required. The minimum agitation speed for complete catalyst suspension depends, among other conditions, on catalyst loading and particle size [1,9,10]. Minimum agitation speed for uniform bubble dispersion depends mainly on the gas flow rate [11]. 

Foams are fluids that depend on shear history. The texture of a foam will reach an equilibrium state at a particular shear rate. Finer textured, more dynamically stable foams are produced at high shear rates, higher pressure, and with higher quantities of surfactant (36). Reidenbach et al. (11) observed that at higher shear rates, finer more uniform bubbles were created. This information indicates that at downhole conditions during fracture stimulation when conditions of high pressure and shear are present, foams are finely textured with parallel-piped uniform bubbles. 

Air that is acddentally entrapped in concrete does not improve frost resistance of concrete, since it is distributed in voids which are relatively large, few in number, and unevenly distributed. Conversely, by introducing air-entraining admixtures to the concrete, it is possible to incorporate a system of very tiny and uniform bubbles inside the cement paste. 

The gas-liquid interfacial area (a) is a fundamental parameter in designing bioreactors because the knowledge of this parameter is required to calculate individual gas-liquid mass transfer rates (Vasquez et al., 2000). The interfacial area is a challenge to quantify because it is influenced by the bioreactor geometry and operating conditions, as well as the physical and chemical properties of the gas-liquid system. In some cases, the interfacial area is estimated by assuming a uniform bubble diameter and measuring the overall gas holdup e. In this case, the gas-liquid interfacial area is estimated from Chisti (1989) ... 

Bubble formation and orifice activity are two important factors determining stability. Synchronous bubble formation, where almost all holes are active instantaneously, tends to produce a uniform bubble and gas holdup distribution. The uniform bubble distribution leads to a more stable homogeneous flow regime, less liquid recirculation, and higher gas holdup and gas-liquid mass transfer. Asynchronous orifice operation is often accompanied by alternating or oscillating orifice activity, which leads to flow instability. The instability creates more bubble-bubble interaction and leads to lower gas holdup and gas-liquid mass transfer. Hence, the gas distributor affects the critical superficial gas velocity at which the transition regime is detected. 

Biesheuvel, A., and Gorissen, W.C.M. (1990), Void fraction disturbances in a uniform bubbly fluid, International Journal of Multiphase Flow, 16(2) 211-231. 

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