Bubble-flow vertical tubes

FIGURE 17.51 Flow patterns in tube bundles (a) spray flow, (b) bubbly flow (vertical and horizontal), (c) chugging flow (vertical), (d) stratified spray flow, (e) horizontal stratified flow as defined by Grant and reported in Ref. 69. 

One of the first studies of two-phase flow was presented in 1935 by O Brien and Gosline in a paper entitled "Velocity of Large Bubbles in Vertical Tubes." The process was certainly susceptible to approximate analysis, but the style of investigation was similar to many others of this period, i.e., experiments and dimensional analysis. O Brien and Gosline expressed the latter as... 

The flow regime maps shown in Fig. 5.16a,b indicate that typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters ([Pg.216]

Mishima K, Hibiki T (1996) Some characteristics of air-water two-phase flow in small diameter vertical tubes. Int J Multiphase Flow 22 703-712 Moriyama K, Inoue A (1996) Thickness of the liquid film formed by growing bubble in a narrow gap between two horizontal plates. J Heat Transfer Trans ASME 118 132-139... 

Malnes, D., 1966, Slip Ratios and Friction Factors in the Bubble Flow Regime in Vertical Tubes, Norwegian Rep. KR-110, Inst, for Atomenergi, Oslo, Norway. (5)... 

Many of the considerations discussed in the previous section can be applied to vertical bubble flow. Two cases are of interest first, one in which there is no downward or upward liquid flow (Ql = 0), for example, on a distillation tray. With a shallow liquid layer there will be no liquid velocity profile, and hence bubble-rise velocities for uniformly sized bubbles will be the same at each cross-section. It can be shown (see Nicklin, N2) that if the rising velocity of a swarm of bubbles not continuously generated in a still liquid is Vo, then the rising velocity relative to the tube of a swarm of continuously generated bubbles will be, for... 

Lisseter and Fowler (1992) have derived a simple set of equations for bubbly flow through a vertical tube. They have shown that under steady flow conditions, the void fraction will relax from its inlet value to an asymptotic value within only a short distance from the inlet. They have obtained a relationship between the inlet void fraction and the imposed pressure drop and derived a simple expression for the equihbrium void fraction. They have also considered the wall friction in their analysis of bubbly flows. 

As depicted in Figure 8.34, the flow patterns formed in a vertical tube follow a trend with increasing gas flow of bubble flow, slug flow, chum flow, and annular flow. These regimes are described briefly below. 

Fig. 5 Boiling in a narrow vertical tube. (A) Boiling suppressed by head, natural convection is shown (B) bubble formation (C) slug formation due to bubble coagulation (D) fully developed slug flow (E) breakdown of slugs at high vapor rates (F) annular-flow-climbing film.

Fig. 2-12. Concentration distribution of uranium along a vertical tube. Concentration of fulvic acid Cfa=200mg/1, amplitude of mechanical vibration A=0.lmm, frequency 50 Hz. Duration of air bubble flow x, min 1- 0 2- 30 3- 120.

Fig. 4.44 Flow types in a vertical, unheated tube with upward flow, a bubble flow b plug flow c churn flow d wispy-annular flow e annular flow f spray or drop flow...

All these flow types appear more or less in a series one after the other during the evaporation of a liquid in a vertical tube, as Fig. 4.30 illustrates. The structure of a non-adiabatic vapour-liquid flow normally differs from that of an adiabatic two-phase flow, even when the local flow parameters, like the mass flux, quality, etc. agree with each other. The cause of this are the deviations from thermodynamic equilibrium created by the radial temperature differences, as well as the deviations from hydrodynamic equilibrium. Processes that lead to a change in the flow pattern, such as bubbles coalescing, the dragging of liquid drops in fast flowing vapour, the collapse of drops, and the like, all take time. Therefore, the quicker the evaporation takes place, the further the flow is away from hydrodynamic equilibrium. This means that certain flow patterns are more pronounced in heated than in unheated tubes, and in contrast to this some may possibly not appear at all. 

Thin-film evaporation. Here, a thin film of liquid flows over a heated surface (typically a vertical plate or the inside of a vertical tube) and evaporates. In many situations, this evaporation takes place directly at the surface of the liquid film, without the formation of bubbles at the solid surface. However, at higher heat fluxes, nucleate boiling occurs at the heated surface. 

Vertical Ducts. Typical flow patterns in upward vertical two-phase flow in a tube are presented in Fig. 17.48a. At low vapor qualities and low mass flow rates, the flow usually obeys the bubbly flow pattern. At higher vapor qualities and mass flow rates, slug or plug flow replaces the bubbly flow pattern. Further increase in vapor quality and/or mass flow rates leads to the appearance of the churn, annular, and wispy annular flow patterns. 

The simplest form of a bubble column is a vertical tube in which a gas distributor is placed at the bottom packed or plate bubble columns are also used. The gas bubbles rise through the liquid phase, which may flow through the column either cocurrent or countercurrent to the gas. As a result of the short residence time of the gas bubbles in the liquid phase, bubble column reactors are preferred for reactions which require a short gas and a long liquid reaction time. Therefore the residence time distribution of the liquid phase is a characteristic factor for the design of the reactor. The dependence of the residence time distribution upon the column diameter has to be known for any scale-up of bubble columns. 

The bubble-cap plate borrowed from the petroleum industry utilizes a smaller number of large openings through the plate in which a vertical tube or riser is inserted. This vertical tube is capped with an inverted cup that is serrated along its vertical walls to permit vapor-bubble flow through the liquid on the plate. The tube extends above the plate for a distance of 5( 75 mm and the inverted cup has a diameter of 75-100 mm. The center-to-center distance between bubble caps is on the order of 1.33-1.67 times the cap diameter. 

---