Two-phase jet

Gas jets in fluidized beds were reviewed by Massimilla (1985). A more recent review is by Roach (1993) who also developed models to differentiate three jet flow regimes jetting, bubbling and the transition. However, most of the data were from jets smaller than 25 mm. The discussion here will emphasize primarily large jets, up to 0.4 m in diameter, and operation at high temperatures and high pressures. The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

Momentum Dissipation of a Gas-Solid Two-Phase Jet. Gas velocity profiles in a gas-solid two-phase jet inside a fluidized bed were determined at five different horizontal planes perpendicular to jet direction using a pitot tube (Yang and Keaims, 1980). The velocity profiles were integrated graphically, and gas entrainment into a jet was found to occur primarily at the base of the jet. 

For concentric jets and gas-solid two-phase jets, the jet momentum flux term pf(73 can be evaluated as follows, as suggested by Yang et al. (1984b) ... 

The solids particle velocity in the gas-solid two-phase jet can be calculated as shown in Eq. (27), assuming that the slip velocity between the gas and the solid particles equals the terminal velocity of a single particle. It should be noted that calculation of jet momentum flux by Eq. (26) for concentric jets and for gas-solid two-phase jets is only an approximation. It involves an implicit assumption that the momentum transfer between the concentric jets is very fast, essentially complete at the jet nozzle. This assumption seems to work out fine. No further refinement is necessary at this time. For a high velocity ratio between the concentric jets, some modification may be necessary. 

Solid Entrainment Rate into Gas and Gas-Solid Two-Phase Jets. 

A mathematical model for solid entrainment into a permanent flamelike jet in a fluidized bed was proposed by Yang and Keaims (1982). The model was supplemented by particle velocity data obtained by following movies frame by frame in a motion analyzer. The experiments were performed at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings ranging from 0 to 2.75. The particle entrainment velocity into the jet was found to increase with increases in distance from the jet nozzle, to increase with increases in jet velocity, and to decrease with increases in solid loading in the gas-solid, two-phase jet. 

Equation (64) predicts correctly the increase in solid entrainment into thejet with increases in jet velocity and the decrease with increases in solid loading in a two-phase jet. Since neither the voidage nor the particle velocity inside thejet were measured, direct verification of Eq. (64) was not performed. 

Yang, W. C., and Keaims, D. L., Solid Entrainment Rate into Gas and Gas-Solid, Two-Phase Jets in a Fluidized Bed, Powder Technol., 33 89 (1982)... 

Gas-Solid Two-Phase Jet in a Fluidized Bed. In Fluidization. Ed. Davidson and Keaims. Cambridge Cambridge University Press. 

Important design aspects of fixed water sprays include release orientation, release momentum, and release contact with the water sprays. If the release is a two-phase jet and has a momentum that is larger than that of the water spray, the jet will penetrate the water spray with little interaction which will lead to a poor removal efficiency. Water-spray efficiency can be maximized by ensuring that all of the water comes in contact with the vapor cloud, that is, by ensuring that the cloud will spread out across the entire area covered by the water spray such that all the water is used. 

The experimental evaporator (micro two-phase jets generator) is shown on Fig. 10 -11. The heater block of the evaporator includes a hole machined directly in the centre of the copper cylinder with thick walls and is used for the installation of a nickel sintered powder evaporator. Some thermocouples are disposed inside the copper block to control the heat flow to the evaporator from the electric heater disposed on its outer surface. To minimize heat losses, the heater block was insulated. Heat input to the evaporator was calculated by conduction analysis using thermocouples that were placed at a known distance apart in the copper heater block. 

The presence of the dispersed phase, for small holdup fraction, should not have a signiflcant effect on the turbulent characteristics of the dispersion. Investigators (Cll, C12, RIS, R16) found that for high holdup fraction their developed models for drop size distribution fltted experimental data better by taking into account a damping effect on turbulence by the dispersed phase. Doulah (Dll) developed a theory for the increase of drop size due to this damping of turbulence effect. Experiments (LI) on two-phase jet flows show that the damping of turbulence can be approximated by... 

Issa Rl, Oliveira PJ (1995) Numerical Prediction of Turbulent Dispersion in Two-Phase Jet Flows. In Celata GP, Shah RK (eds) Two-Phase Flow Modelling and Experimentation, pp. 421-428... 

Jiang, X., Siamas, G. A., Jagus, K. Karayiannis, T. G. 2010 Physical modelling and advanced simulations of gas-liquid two-phase jet flows in atomization and sprays. Progress in Energy and Combustion Science 36 (2), 131-167. 

Flow Regimes in the Main Holding Vessel The momentum of the two-phase jet issuing out of the diffuser is decided mainly by the momentum of the liquid phase. The latter in turn is decided by The power input per unit... 

Noz L [ Noz L cnt twO phase jet reaches the bottom and the entire holding vessel behaves as a multiphase contactor. The core of the holding vessel acts as a downflow bubble column, whereas the outer annulus is similar to the conventional bubble column. The [ Noz L crit i important for solid suspension in the main vessel as discussed later in some detail in Section 8.8. 

Effect of a Draft Tube Bhutada and Pangarkar (1987) and Panchal et al. (1991) have shown that the presence of a draft tube improves the gas holdup above a power input, which allows the two-phase jet to reach the bottom of the holding vessel (Fig. 8.15). There is no information on k a for the standard venturi loop system incorporating a draft tube. In this case also, the logic in Sections 8.7.2.8 and 10.10.2 indicates that a draft tube with proper configuration (Appj,/Ajj 0.25) may yield higher value of k a. 

On closer examination, it was found that the differences in the above correlations were mainly due to their different gas induction characteristics. These differences were eliminated when the solid suspension criterion was redefined in the form of the critical gas rate at which the two-phase jet reached the vessel bottom as shown in the last stage of Figure 8.16 denoted by the subscript s. . This approach gave a unified correlation independent of the nozzle diameter, diffuser type, etc. 

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