Slug-flow upward velocity

Chum flow or slug flow-annular (dispersed) flow transition. The flow becomes annular when the gas flow rates are enhanced to a certain point. The high gas flow rate causes a wavy interface of the liquid film. As a consequence, parts of the waves will enter the gas core as entrained drops. This results in an upward flow direction of the liquid, due both to interfacial shear and to drag on the waves and drag on the droplets. Annular flow can therefore exist only when the gas velocity is sufficient to lift the droplets in the gas core. The minimum gas velocity required to suspend a drop is determined from the balance between the gravity and drag forces on the drop. 

Two-phase flow patterns in vertical tubes. The liquid flow rate is upward at a small, constant velocity. The gas flow rate upward increases steadily from left to right. The annular pattern shown is often referred to as climbing film flow. [From D. J. Nicklin and J. F. Davidson, The onset of instability in two-phase slug flow, in Symposium in Two-Phase Flow, Institution of Mechanical Engineers, London, 1962. Reproduced by permission j of the publisher.]... 

Kinetics can be screened in a screw impeller-stirred reactor (SISR) [7] (Figure 9.8). The reactor system comprises a screw impeller that pumps the liquid upwards, and the high exit velocity of the liquid results in an effective foam formation in the top section of the reactor. A slug flow (Chapter 6) is thus established in the monolith channels. Consequently, the liquid and gas are pumped from the lower section to the upper section of the reactor, over and over again. In fact, the concept resembles that of a loop reactor. Cylindrical monoliths are placed in the stator of an SISR (Figure 9.8), and a foam of gas and liquid is forced through the monolith channels by a screw. 

Detailed consideration of the interaction between particles and fluids is given in Volume 2 to which reference should be made. Briefly, however, if a particle is introduced into a fluid stream flowing vertically upwards it will be transported by the fluid provided that the fluid velocity exceeds the terminal falling velocity m0 of the particle the relative or slip velocity will be approximately o- As the concentration of particles increases this slip velocity will become progressively less and, for a slug of fairly close packed particles, will approximate to the minimum fluidising velocity of the particles. (See Volume 2, Chapter 6.)... 

For turbulent upward liquid flow, it was found experimentally that the slug velocity is given for superficial liquid Reynolds numbers over 8000 by... 

However, when considering monoliths having comparable fractional catalyst volumes and SA/V ratios as typical catalyst particles in fixed beds, countercurrent flow of gas and liquid is still very problematic. At the small channel diameter of about 1 mm (see Table 2) and at realistic velocities of gas and liquid, the liquid, which should flow downward as a film along the wall, will easily bridge the channel and form a slug, which will be transported upward by the gas. Thus, instead of the desired annular countercurrent flow, a segmented flow, or Taylor flow, in the upward direction will be obtained. This phenomenon is akin to the flooding in packed beds. 

The third instability, classical choking, occurs on reducing the superficial gas velocity in the riser when slugs start to form right after a dense bed is created in the bottom section of a riser. Stable operation of gas-solids upward slug/plug flow may still be possible in... 

---