Dispersed bubble flow

Dispersed bubbly flow (DB) is usually characterized by the presence of discrete gas bubbles in the continuous liquid phase. As indicated in Fig. 5.2, for the channel of db = 2.886 mm, dispersed bubbles appeared at a low gas superficial velocity but a very high liquid superficial velocity. It is known that for large circular mbes dispersed bubbles usually take a sphere-like shape. For the triangular channel of dh = 2.886 mm, however, it is observed from Fig. 5.2 that the discrete bubbles in the liquid phase were of irregular shapes. The deformation of the gas bubbles was caused by rather high liquid velocities in the channel. 

Dispersed bubbles are observed (Fig. 5.6a) when the gas flow rate is very small such as [/gs = 0.0083 m/s. Two kinds of bubbles are observed one type is finely dispersed with a size smaller than the tube diameter, and the other type has a length of near to or a little larger than the mbe diameter with spherical cap and tail. The distance between two consecutive bubbles may be longer than ten times the tube diameter. This flow pattern is also considered as a dispersed bubbly flow. Often in air-water flow two kinds of bubbles appear together as pairs of bubbles in which the small-sized bubbles follow the larger ones. 

Figure 5.16c indicates that as the channel size was reduced to Jh = 0.866 mm, the dispersed bubbly flow pattern vanished from the flow regime map. Figure 5.16a-c indicates that the slug-churn flow transition line shifted to the right, as the channel size was reduced. Similar trends were also found in small circular tubes by the... 

Second, when a liquid is flowing in a long tube, a liquid velocity profile will develop, and bubbles near the center of the tube will rise more rapidly than those near the wall. In slug flow, all slugs rise with nearly the maximum velocity, but in dispersed bubble flow, the variations in speed tend to cancel and give a component to the bubble velocity due to the average liquid velocity. As before, from continuity, this mean liquid velocity at any cross section must be (Qo + Qt)A, so that the bubble rise velocity is... 

Fig. 5. Types of two-phase How in a horizontal pipeline (a) Stratified smooth flow where gas velocity is low. Liquid flows along bottom portion of pipelines with essentially a smooth surface, (b) Stratified flow with a wavy. surface, the waviness caused by increased gas flow velocity, (c) Liquid bridges the pipeline cross section, thus causing slugs or plugs of liquid, which move at a velocity approximately that of ihe flowing gas, (d) Annular flow, in which the liquid essentially flows as an annular film on the pipe wall while gas flows as in a centra) core of the pipe, (e) Dispersed bubble flow usually results when liquid flow rates are high and gas rates are low. Because of comparative density differences, most bubbles are found above the pipe center line. Conditions vary somewhat when the pipeline is in a vertical orientation. After Cindric, Gandhi, and Williams)...

The hydrodynamics control the mass transfer rate from gas to liquid and the same from liquid to the solid, often catalytic, particles. In concurrently operated columns not only the gas-continuous flow regime is used for operation as with countercurrent flow, but also the pulsing flow regime and the dispersed bubble flow regime (2). Many chemical reactors perform at the border be-... 

Ultimately at high frequencies the pulses overlap and we arrive in the dispersed bubble flow regime. Thus we consider the pulses to be zones of the bed already in the dispersed bubble flow, spaced by moving compartments that are still in the gas-continuous flow regime. This concept is very helpful in calculating mass transfer and mixing phenomena, as well as in pressure drop relations (9) where it appears that above the transition point the pressure drop can be correlated linearly with the pulse frequency. Pulses are to be considered as porous to the gas flow as is shown when we plot the pulse velocity versus the real gas flow rate, figure 5. 

The value of k a, a being the gas-liquid contact area per unit volume, k the corresponding liquid side mass transfer coefficient, is considerably higher in the pulsing than in the gas-continuous flow regime. It has been tried in the past, and partially success-full, to correlate the mass transfer data to the energy dissipation rate in the bed. We made the premise, that pulses are parts of the bed already in the dispersed bubble flow regime and therefore must accredit for an increase in the transfer rate proportional to their presence in the bed. 

Taking the active pulse height as 0.05 m and the pulse velocity as 1 m/s, we derive for the mass transfer coefficient in the gas-continuous zone, 11, a value of 10 m/s and in the pulse proper, k, a value of 6 10 m/s. These values compare very well with those given in literature (5, 6) for both gas-continuous and dispersed bubble flow regimes. An estimate of k can also be made by means of the penetration theory, taking the respective liquid in and outside the pulse as the basic for the calculation of the con-... 

It is shown, that the performance of a pulsing packed column can be split up into its two component parts, the pulses and the zones in between pulses. The pulses can be described as parts of the bed already in the dispersed bubble flow regime the zones-in between the pulses as parts of the bed still in the gas-continuous regime. The pulse frequency is linearly dependent upon the real liquid velocity. The properties of the pulse, like holdup, velocity and height are quite independent upon all the parameters except gas flow rate. 

Combination of the empirically found correlations for these pulse properties in a model in which the parts of the bed in the gas-continuous resp. dispersed bubble flow are weighted, leads to a correlation of the mass transfer rate with predictive value. 

Hydraulic design aims at the realization of an intensive heat and mass transfer. For two-phase gas-liquid or gas-solid systems, the choice is between different regimes, such as dispersed bubbly flow, slug flow, churn-turbulent flow, dense-phase transport, dilute-phase transport, etc. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

Actually Sato et al. expressed their particle mass-transfer coefficients in terms of an enhancement factor representing the ratio of with two-phase flow to ks at the same liquid flow rate in single-phase flow. For pulsing and dispersed bubble flow this enhancement factor was found to be inversely proportional to liquid holdup j3, which in turn is a function of the two-phase parameter A or A (see Section IV,A,3,a). For comparison, the data for single-phase liquid flow are best represented by an equation of the same form as Eq. (115) but with a constant of 0.8. 

Prank T, Zwart PJ, Shi J-M, Krepper E, Lucas D, Rohde U (2005) Inhomogeneous MUSIG Model - a Population Balance Approach for Poly dispersed Bubbly Flows. Int Conf Nuclear Energy for New Europe 2005, Bled, Slovenia, September 5-8... 

D. Barnea, Transition from Annular Flow and from Dispersed Bubble Flow—Unified Models for the Whole Range of Pipe Inclinations, Int. J. Multiphase Flow, Vol. 12, pp. 733-744,1986. 

This type of flow, sometimes referred to as dispersed bubble flow, is characterised by a train of discrete gas bubbles moving mainly close to the upper wall of the pipe, at almost the same velocity as the liquid. As the liquid flowrate is increased, the bubbles become more evenly distributed over the cross-section of the pipe. 

Dispersed bubble flow In this flow regime many small gas bubbles are distributed uniformly across the entire tube cross section when the gas and liquid velocities are high. 

The following current trends emanate from the analysis of the radial heat transfer two-phase downflow and upflow fixed-bed literature [98] (i) radial heat transfer is strongly influenced by the flow regime [96,99,100] (ii) the bed radial effective thermal conductivity always increases with liquid flow rate for both two-phase downflow and upflow [96, 100] (iii) Ar is very little dependent on gas flow rate in trickle flow, and it decreases with gas flow rate in pulsing flow regime and increases in dispersed bubble flow regime [99,100] (iv) Ar decreases with the increase of the liquid viscosity [101] (v) the inhibition of coalescence induces higher Ar values [101] (vi) Ar always increases with... 

In spite of the widespread use of fixed-bed reactors, much remains to be done to define the dynamics of the reactor [30], Most of these reactors are operated in the concurrent mode at which the gas and the liquid both flow from the top to the bottom. A number of flow regimes have been distinguished for packed columns in downflow operation [31, 32], Based on the Reynolds number for liquid and gas flows, the flow regimes include (i) trickle flow, (ii) pulsed flow, (iii) dispersed bubble flow, (iv) wavy flow, and (v) spray flow (Figure 12.7) [17]. In general, countercurrent flows lead to much larger pressure drops across the bed, and this would be the case for FTS. Thus, the countercurrent flow mode is not used in today s plants. 

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