Velocity superficial liquid

At relatively low liquid superficial velocities, increasing the mixture volumetric flux led to longer bubbles and shorter liquid slugs, eventually leading to the merging of elongated bubbles, and the development of the slug-annular flow pattern, repre-... 

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. 

As shown in Fig. 5.2, churn flow (C) appeared at moderate gas superficial velocities and the entire range of liquid superficial velocities. How was extremely chaotic and the gas-liquid interface was rather irregular. The gas phase and liquid phase had no distinct shapes. 

Annular flow (A) existed at high gas superficial velocities and at the entire range of liquid superficial velocities. In annular flow, liquid film formed at the side wall with part of the liquid remaining in the three corners of the channel, while the continuous gas core flowed concurrently with the liquid phase. 

In Fig. 5.25 the void fraction a is plotted versus a homogeneous void fraction jS with different symbols used for different ranges of liquid superficial velocity [/ls-The void fraction can be correlated with the homogeneous void fraction ... 

Experiments in annular and slug flow were carried out also by Ghajar et al. (2004). The test section was a 25.4 mm stainless steel pipe with a length-to-diameter ratio of 100. The authors showed that heat transfer coefficient increases with increase in liquid superficial velocity not only in annular, but also in slug flow regimes. 

On the other hand Bao et al. (2000) reported that the measured heat transfer coefficients for the air-water system are always higher than would be expected for the corresponding single-phase liquid flow, so that the addition of air can be considered to have an enhancing effect. This paper reports an experimental study of non-boiling air-water flows in a narrow horizontal tube (diameter 1.95 mm). Results are presented for pressure drop characteristics and for local heat transfer coefficients over a wide range of gas superficial velocity (0.1-50m/s), liquid superficial velocity (0.08-0.5 m/s) and wall heat flux (3-58 kW/m ). 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Fig.6 and Fig.7 illustrate the effect of draft-tube diameter on liquid superficial velocity, liquid circulating flowrate and gas hold-up. Results show that the liquid superficial velocity in the riser increases with increasing the draft-tube diameter while the liquid velocity in the... 

The height of the draft-tube also Influences the flow characteristics in the ALR. Fig.9 and Fig. 10 show that the liquid superficial velocity increases with increasing the hei t (H) of the draft-tube, while the liquid superficial velocity remains approximately unchanged when H exceeds 0.51m. With a hi er position of draft-tube, the flow area at the outlet of the draft-tube becomes larger, so the liquid velocity at the outlet decreases. 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

Reactions were carried out at room temperature in the annular-dry flow regime (gas superficial velocity, 1.4 m s liquid superficial velocity, 5.6 10 m s ) [13]. 

GL 26] [R 3] [P 28] Conversions from 2 to 42% were found for the oxidation of butyraldehyde [10], The highest conversions were obtained for large gas and liquid flows. On increasing the ratio of gas and liquid superficial velocities from 5 to 53, an increase in conversion from 10 to 41% resulted. 

CSS steady state at the time-average liquid superficial velocity... 

In Figure 3.49, the minimum liquid superficial velocity versus particle size in order to have a wetting efficiency higher than 90% for water as liquid phase at 25 °C is presented. 

This result illustrates the effect of Fogler s assumption of zero liquid superficial velocity, analyzed in Section 3.7.2 and eq. (3.374). Indeed, for a low liquid superficial velocity, the models result in almost the same values. Thus, it can be stated that Smiths approach is more accurate for high liquid superficial velocities. However, Fogler s approach is more useful and accurate in the case of considerable pressure drop and gas-phase expansion. 

First, we have to check the wetting efficiency of the bed because the simple trickle-bed model assumes complete wetting and thus it is not applicable otherwise. By using the El-Hisnawi et al. correlation, the wetting efficiency can be estimated (eq. (3.411)). To do that, we need the Galileo number and the Reynolds number. The liquid superficial velocity can be evaluated as follows ... 

The amount of level swell is correlated with the superficial velocity, jg, of gas or vapour at the surface of the liquid. Superficial velocity is the volumetric flow of gas or vapour, divided by the vessel cross-sectional area (i.e., with no attempt to account for the fraction of the cross-sectional area occupied by liquid). Within a particular flow regime, level swell increases with increasing superficial velocity. 

Fig. 31. (a) Dynamic liquid hold-up, and (b) wetting efficiency as a function of liquid superficial velocity for 1.5- and 3-mm cylinders. Gas fiow rate was constant at 31.3mms . The line shows the best fit of the data to the percolation model of Crine et al. (104). Reprinted from reference (103) with permission from Elsevier, Copyright (2003). 

Only the trickle-flow regime was investigated, where the mass-transfer resistances are probably the highest compared to other flow regimes. The results are reported mainly qualitatively [32], and only one correlation was given by Larachi et al. [55], In this, the reduced interfacial area a/ac grows by increasing both the gas-and liquid superficial velocity. 

TABLE 10.3. Mixing of Liquids Power and Impeller Speed (hp/rpm) for Two Viscosities, as a Function of the Liquid Superficial Velocity Pitched Blade Turbine Impeller... 

Ul = liquid superficial velocity, m/s ap = packing specific surface area, m2/m3 g = acceleration due to gravity, m/s Pi, = liquid viscosity, kg/(m-s)... 

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