Velocity entrainment

Entrainment velocity The velocity in a jet stream that effectively entrains the dust or gas particles that surround it. 

The critical section of the circuit (Figure 5.3) is where there is no liquid refrigerant left to help move the oil, i.e. the evaporator outlet and the suction pipe back to the compressor. Entrainment velocities of 5-7 m/ s are required to ensure that oil droplets will be carried back by the dry refrigerant gas to the compressor. The principle of continuous fluid velocity means that the evaporator will be in a continuous circuit. This does not imply that it has to be one pipe, since many pipes may be arranged in parallel to get the required heat transfer surface, providing the minimum velocity criteria are met. 

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. 

The model as formulated in this section cannot be used to predict a priori the solids entrainment rate into the jet because of the two empirical constants in Eq. (61). Lefroy and Davidson (1969) have developed a theoretical model based on a particle collision mechanism for entrainment of solid particles into a jet. The resulting equation for particle entrainment velocity is... 

The first term, (1 - ty) /(I - z), corrects for the voidage difference between that in thejet and that in the emulsion phase. The second term, ( - z) /(I - z), takes into account the fact that only a fraction of the particles having the entrainment velocity Ve will be entrained, the remainder rebounding back to thejet wall due to collisions with the particles already in thejet. Substituting Eq. (63) into Eq. (62), we have... 

The relative, normal entrainment velocity is uniform over the fireball and proportional to the rising velocity, dz/dt, i.e. ve = a0dz/df, with a an entrainment constant. 

In specifying settling chamber dimensions, gas flow velocities must be maintained below the re-entrainment velocity (pick-up velocity) of deposited particulate. As a general guideline, linear gas velocities are kept below 10 ft/sec (600 ft/min = 304.8 cm/sec). This is satisfactory for most materials however, some low-density particulates are re-entrained at lower velocities. 

Ayers, G. P. and Galbally, I. A preliminary investigation of a boundary layer - free troposphere entrainment velocity at Cape Grim, Baseline 92, Department of Arts, Sport, Environment, Tourism and Territories, Canberra A.C.T., Australia, 1994. 

In well-developed fires, the convective heat fraction is typically measured at more than about 65% of the total heat release rate (Heskestad, 2002). This heat is carried away by the plume above the flames. Prediction of plume velocity and temperatures above the flames serve as the basis for convective heat transfer calculations where overhead equipment exists. Widely used fire plume theory assumes a point source origin, and uniformity throughout the plume relative to air density, air entrainment, velocity profile, and buoyancy. 

Chapter 11). The simulations show enhanced vorticity in all three directions with a rich fine structure at later times in the evolution of the jet and an entrainment velocity field qualitatively different from that of around unheated turbulent jets. 

Figure 6.5 Radial entrainment velocity measured at a distance of 0.14Z) above the flame base plate for different flame forcing conditions 1 — proper fuel injection 2 — wrong fuel injection 3 — air forcing oifly and 4 — natural flame...

A major question concerning the development of jet flow subjected to heating is the effect on entrainment. Now because of the periodic boundary conditions imposed on the computational domain in the present simulation, the net entrainment over the domain has to vanish. Nevertheless, as shown below, considerable insight into the problem can be obtained by examining the entraining velocity field, which displays velocity vectors in the ambient fluid in the immediate neighbourhood surrounding the jet. 

Figure 11.5 shows the computed entraining velocity fields at the widest transverse cross-sections at t = 35 in the heated and unheated jets. (This corresponds to looking at the flow in the plane of the cross-section of the spatially developing jet.) The figure shows velocity vectors in the ambient fluid, and contours of streamwise vorticity within the jet. 

Figure 11.5 Comparison of computed entraining velocity fields at the widest transverse sections of the (a) unheated and (6) heated jet at time t = 35. The contonrs of streamwise vorticity, at intervals of 0.5, are shown using solid lines for positive values, and dotted lines for negative values. Contour for level 0 is not shown... 

A recent publication by Zhivaikin (Zl) has reviewed Soviet work on film thickness and entrainment from films, and has presented correlations both for the thickness and for critical entrainment velocities. The relationships, given in terms of dimensionless quantities for downward cocurrent flow, upward cocurrent flow, and gas-liquid counterflow, follow. For laminar flow of liquid alone vertically downward along a pipe wall, the Nusselt equation was found to apply. 

As the velocity of the gas is increased furtlier, the bed continues to expand and its height increases with only slight increase in the pressure drop. As the velocity of the gas is further increased, the bed continues to expand and its height increases, whereas the concentration of particles per unit volume of the bed decreases. At a certain velocity of the fluidizing medium, known as the entrainment velocity, particles are carried over by the gas. This phenomenon is called entrainment. When the volumetric concentration of solid particles is uniform throughout the bed at all times, the fluidization is termed particular. When the concentration of sohds is... 

Meroney and Neff 1984 To calculate the behavior of heavy and cold gas clouds subject to dilution by a water spray curtain, using a numerical model. Work included model validation. Variations in curtain position, spray entrainment velocity, wind speed and source conditions resulted in a wide range of spray-curtain performances. 

A simple concept can be used to illustrate important features of vapor barriers that alter cloud dispersion behavior in the near field (Meroney, 1991 Meroney and Neff, 1985). The concept is based on adding an entrainment velocity ( e) contribution, which is attributed to the vapor fence. 

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