Liquid velocity, increase

The purpose of these experiments was to characterize different flow details under conditions when the superficial gas velocity is constant and the superficial liquid velocity increases. The upward flow regimes are presented in Fig. 5.33. Figure 5.33a shows the stratified flow pattern at [/gs = 20 m/s and J/ls = 0.005 m/s. In the region of pure stratified flow the liquid layer is drawn upward by the gas via the interfacial shear stress. No droplets could be observed at the interface. Such a regime was also observed by Taitel and Dukler (1976), and Spedding et al. (1998). 

Fig. 12 shows the radial profile of the liquid velocity at different axial positions. Due of the baffles, the liquid is redistributed in the radial direction and the turbulent intensity is increased. The radial profile of the liquid velocity is almost uniform after passing the internal. Liquid velocity is lower at the center and higher near the wall as compared with that below the internal. With increasing distance from the internal, the turbulence intensity diminishes and the wall effect becomes more apparent, that is, the liquid velocity increases at the center and decreases near the wall. The radial profile obtained at the position of 114 cm from the internal is similar to that obtained below the internal and is the same as that at the position of 144 cm. 

Fluidization of solid particles with these two widely different classes of fluids—liquids and gases—leads to vastly different phenomena of solids behavior, as shown in Fig. 3. For L/S fluidization, as liquid velocity increases beyond the incipient fluidization point, the solids bed continues to expand as if it were an elastic continuum stretching under the dynamic forces of augmented flow, until, near the terminal velocity of the particles, the solid particles can be noted to be suspended sparsely. Throughout this process of liquid-velocity increase, the solid particles are dispersed quite uniformly, fully exhibiting their discrete behavior, essentially independent of one another. Therefore, L/S fluidization was named particulate. ... 

Fig. 35. Identification of location and size of local pulses within the trickle bed. A high spatial resolution image (in-plane spatial resolution 175 pm x 175 pm slice thickness 1mm) is overlayed with a standard deviation map calculated from images acquired at a spatial resolution of in-plane spatial resolution 1.4 mm x 2.8 mm, and slice thickness 2 mm. The standard deviation maps have been linearly interpolated to the same in-plane spatial resolution as the high resolution data. Images are shown for a constant gas velocity of 112mm/s (a) increasing liquid velocity, and (b) decreasing liquid velocity. The liquid velocities increase left to right 2.8, 3.7, 6.1 and 7.6 mm/s. Reprinted from Lim et al. (2004), with permission from Elsevier. Copyright (2004).

Wall-to-Bed Heat Transfer. The wall-to-bed heat transfer coefficient increases with an increase in liquid flow rate, or equivalently, bed voidage. This behavior is due to the reduction in the limiting boundary layer thickness that controls the heat transport as the liquid velocity increases. Patel and Simpson [94] studied the dependence of heat transfer coefficient on particle size and bed voidage for particulate and aggregative fluidized beds. They found that the heat transfer increased with increasing particle size, confirming that particle convection was relatively unimportant and eddy convection was the principal mechanism of heat transfer. They observed characteristic maxima in heat transfer coefficients at voidages near 0.7 for both the systems. 

This suggests that any correction factor which will cause the holdup data for shear-thinning fluids to collapse onto the Newtonian curve, must become progressively smaller as the liquid velocity increases and the flow behaviour index, n, decreases. Based on such intuitive and heuristic considerations, Farooqi and Richardson [1982] proposed a correction factor, J, to be applied to the Lockhart-Martinelli parameter, x, so that a modified parameter Xmod is defined as ... 

Liquid-solid fluidised systems are generally characterised by the regular expansion of the bed which takes place as the liquid velocity increases from the minimum fluidisation velocity to a value approaching the terminal falling velocity of the particles. The general form of relation between velocity and bed voidage is found to be similar for both Newtonian and inelastic power-law liquids. For fluidisation of uniform spheres by Newtonian liquids, equation (5.21), introduced earlier to represent hindered settling data, is equally applicable ... 

Upward flow of sufficient velocity causes a bed of resin to expand and fluidize unless it is restrained at the upper surface. The resin panicles move freely relative to one another during fluidization since they are no longer in contact, and the upper level expands as the liquid velocity increases. Fine particles are carried upward out of the bed. The pressure drop over the fluidized resin is marginally larger than the static head of liquid since the density of resin is only slightly greater than water. 

An adsorbed layer exists even when the liquid flow is turbulent. In this case the adsorbed layer maintains a narrow boundary layer in which the liquid flow is laminar, even though the bulk flow is turbulent. These regions of laminar flow and completely turbulent flow are separated by a narrow buffer layer, in which the flow is neither truly laminar nor truly turbulent. In the laminar region the liquid velocity increases monotonically with distance from the wall but, in the turbulent region, velocity as defined by equation [4.1] is no longer appropriate. Rather, a mean flow velocity u is used, defined by ... 

Provided that p remains practically constant, this equation shows that, even for a liquid with a coefficient of viscosity equal to zero, a pressure gradient becomes less effective in producing acceleration in a liquid particle as the liquid velocity increases. This may be termed the Bernoulli effect. 

---