Falling film flow patterns

Falling film flow. The extent of basic modeling of two-phase annular flow is still very limited, because annular flow is the pattern that is least well understood... 

The velocity distribution equation (27) indicates that in the absence of surface tension effects the maximum velocity in a film flowing in a flat channel of finite width should occur at the free surface of the film at the center of the channel. The surface velocity should then fall off to zero at the side walls. However, experimental observations have shown (BIO, H18, H19, F7) that the surface velocity does not follow this pattern but shows a marked increase as the wall is approached, falling to zero only within a very narrow zone immediately adjacent to the walls. The explanation of this behavior is simple because of surface tension forces, the liquid forms a meniscus near the side walls. Equation (12) shows that the surface velocity increases with the square of the local liquid depth, so the surface velocity increases sharply in the meniscus region until the side wall is approached so closely that the opposing viscous edge effect becomes dominant. 

R. Armbruster and J. Mitrovic, Patterns of Falling Film Flow over Horizontal Smooth Tubes, Proc. 10th Int. Heat Transfer Conf, Brighton, 3, pp. 275-280,1994. 

Figure 9.5 Flow pattern of a falling film in falling film microreactor (microchannel cross-section 1000pm X 300pm liquid 110ppm SLS solution), (a) Corner rivulet flow (Ql = 2 ml/min) (b) falling film flow with dry...

Figure 13.4 View through the glass roll of the coating bead as the flow rate is reduced. The roll speed was fixed capillary number (Ca) = 0.12 and no vacuum was applied to the upstream free surface. The cylinder was moving from bottom to top in each photograph. As the film thickness falls, the upstream free surface moves towards the feed slot and becomes three-dimensional. As the flow rate is decreased even further, the V pattern grows until the coating bead breaks. Reproduced from ref. 9 with the permission of Elsevier, 2014.

Grimley (G10, Gil) used an ultramicroscope technique to determine the velocities of colloidal particles suspended in a falling film of tap water. It was assumed that the particles moved with the local liquid velocity, so that, by observing the velocities of particles at different distances from the wall, a complete velocity profile could be obtained. These results indicated that the velocity did not follow the semiparabolic pattern predicted by Eq. (11) instead, the maximum velocity occurred a short distance below the free surface, while nearer the wall the experimental results were lower than those given by Eq. (11). It was found, however, that the velocity profile approached the theoretical shape when surface-active material was added and the waves were damped out, and, in the light of later results, it seems probable that the discrepancies in the presence of wavy flow are due to the inclusion of the fluctuating wavy velocities near the free surface. 

Annular flow The liquid phase flows along the pipe or channel walls, as a more or less continuous stream, with the gas phase acting as a core. The gas phase may carry droplets of liquid that may be generated by the breakup of waves on the surface of the liquid film. Some liquid drops may fall back into the liquid phase, so that there may be a continuous liquid interchange between the continuous liquid phase and the gas phase. Furthermore, the liquid may contain entrained gas bubbles. The pattern detail will depend very strongly on the flow conditions in the system. Hewitt and Hall Taylor describe a subpattern of annu-... 

Armbruster and Mitrovic [62] observed that liquid falls from tube to tube in three patterns discrete droplets, jets or columns, and sheets, depending on the flow rate (i.e., film Reynolds number) and fluid properties. In addition, depending on the tube arrangement and spacing, the condensate may cause ripples, waves, and turbulence to occur in the film splashing may occur, as well as nonuniform rivulet runoff of condensate because of tube inclination or local vapor velocity effects. As a result, it is impossible to arrive at an analytical expression to describe these complex bundle phenomena. In general, the effect of inundation may be accounted for using... 

FIGURE 15.142 Flow patterns in falling film waves for wave height to substrate height ratios of 2, 4, and 6 (from Jayanti and Hewitt [359], with permission of Elsevier Science). Note axial distance foreshortened by a factor of about 400 for presentational purposes. 

In order to overcome the coupling of power dissipation and mass transfer, we need to consider a different mechanism for gas-liquid contacting. If we turn to laminar flow, an external structure should be used to create or maintain the surface area. For example, in a falling-film reactor the gas /liquid interfacial area is roughly equal to the wall area. In capillaries at moderate velocities, the predominant flow pattern is called Taylor [29] flow, see Fig. 6.3. In Taylor flow, the gas bubbles are too large to retain their spherical shape and are stretched to fit inside the channel. Surface tension pushes the bubble towards the channel wall, and only a thin film remains between the bubble and the wall. 

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