Breakup—High Turbulence

Droplet Breakup—High Turbulence This is the dominant breakup mechanism for many process applications. Breakup results from local variations in turbulent pressure that distort the droplet... 

Droplet Breakup—High Turbulence This is the dominant breakup mechanism for many process applications. Breakup results from local variations in turbulent pressure that distort the droplet shape. Hinze [Am. Inst. Chem. Eng.., 1, 289-295 (1953)] applied turbulence theory to obtain the form of Eq. (14-190) and took liquid-liquid data to define the coefficient ... 

Breakup in a highly turbulent field (L/velocity) ". This appears to be the dominant breakup process in distillation trays in the spray regime, pneumatic atomizers, and high-velocity pipehne contactors. 

Breakup will occur due to high turbulence and high shear rate. This will occur in the impeller region and close to the walls. In a stirred tank, almost all breakup of the bubbles occurs in the impeller region. According to Eq. (15.3), the energy required to break up a 5-mm bubble is on the order of 1W m 3, while 35 W m-3 is required to break up a 1-mm air bubble in water. A high rate of bubble breakup... 

There is little evidence showing the mode of breakup in turbulent flow fields. Hinze (HI7) speaks of a bulgy mode of breakup. Published photographs (C7, T12) show highly deformed bubbles and necking drops, protuberances and cell-like surface structures (see Fig. 12.8). Experimental evidence regarding single bubbles and drops in well-characterized turbulent fields would be most welcome. 

Interfacial tension (y) and buoyancy are of prime importance. Under conditions of highly turbulent agitation, the droplet size is determined by breakup with fluid eddies, and the relationship... 

A bubble owes its stabihty to the surface tension forces. It can break when the hydrodynamic stresses are sufficiently large to overcome the forces due to surface tension (Hinze 1955). Therefore, a relative estimate of these two opposing forces is necessary to determine the operating conditions that can cause breakup of bubbles. The discussion is limited to highly turbulent flow fields since industrial operations are invariably in the turbulent region. When the two opposing forces are equal, there is a quasi equilibrium. This situation is quantified as follows ... 

Two-Fluid (Pneumatic) Atomizers This general category includes such diverse apphcations as venturi atomizers and reac tor-effluent quench systems in addition to two-fluid spray nozzles. Depending on the manner in which the two fluids meet, several of the breakup mechanisms may be apphcable, but the final one is high-level turbulent rupture. 

As shown by Table 14-12, empirical correlations for two-fluid atomization show dependence on high gas velocity to supply atomizing energy, usually to a power dependence close to that for turbulent breakup. In addition, the correlations show a dependence on the ratio of gas to liquid and system dimension. 

In some practical processes, a high relative velocity may not exist and effects of turbulence on droplet breakup may become dominant. In such situations Kolmogorov, 280 and Hinze[27°l hypothesized that the turbulent fluctuations are responsible for droplet breakup, and the dynamic pressure forces of the turbulent motion determine the maximum stable droplet size. Using Clay s data, 2811 and assuming isotropic turbulence, an expression was derived for the critical Weber number 270 ... 

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