Weber number, critical

Droplet delivery from an airblast nebulizer is governed by the surface tension, density and viscosity of the fluid, and the applied pressure, which can be passive or forced. Droplet breakup is illustrated in Fig. 6. Droplets form during this breakup at a critical Weber number (We) ... 

The critical resulting velocity of the liquid relative to the bubble, v el, can be evaluated from the critical Weber number, We ... 

For liquids of higher viscosities, the influence of liquid viscosity on droplet breakup needs to be considered. According to Hinze,[270] the critical Weber number may be modified to the following expression to account for the effect of liquid viscosity ... 

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 ... 

Generally, the occurrence of a specific mode is determined by droplet impact properties (size, velocity, temperature), surface properties (temperature, roughness, wetting), and their thermophysical properties (thermal conductivity, thermal capacity, density, surface tension, droplet viscosity). It appeared that the surface temperature and the impact Weber number are the most critical factors governing both the droplet breakup behavior and ensuing heat transfer. I335 412 415]... 

For a homogeneous and isotropic turbulent field, Hinze (H6) has shown that k in Eq. (159) equals [(l/2)(AVe )]3/5> where NWecr is the critical Weber number of maximum bubble size capable of survival. 

NFr Froude number Npr Substitution given in the text NFr Modified Froude group NRe Reynolds number Nrc Substitution given in the text Nr, Modified Reynolds number NwlCr Critical Weber number of maximum bubble size capable of survival... 

An interesting study of entrainment from horizontal liquid films has also been carried out by Van Rossum (VI). On the basis of a liquid film in laminar flow with a flat and smooth surface, he found that the mean actual film thickness was about 0.6 times the theoretical one. The critical Weber number corresponding to the onset of atomization was determined for five different liquids, with Weber number defined as... 

By exceeding a certain discharge velocity, turbulence forces increase to such an extent that film disruption takes place immediately at the orifice. Now the droplet size is independent of the film thickness. This state of atomization is described by the critical Weber number. Measuring data obtained with hollow cone nozzles of different geometry and pure liquids as well as lime-water suspensions are represented in Figure 19. Wep,crit... 

The experimental results show that c/n decreases up to a certain flow rate at which it was found that Arwe = 0.8, after which a different behavior is exhibited, with an increase in c/V, as ArRe increases. At NRe = 280, a further break was observed this is due to the onset of turbulence in the film, which alters the manner in which u varies with ArRe. Hence, as in the case of the wavelengths, there appears to be an important change in the wave characteristics near the critical Weber number of unity. 

We crit is the modified critical Weber number, which is close to 1. 

The exact result is influenced by the viscosities of the two liquid phases, the amount of mechanical shear produced, and the interfacial tension, among others. One representation is given by the critical Weber number [130-132]. The Weber number, We, is given by ... 

Figure 3.5 The critical Weber number for disruption of droplets in simple shear flow (solid curve), and forthe resulting average droplet size in a colloid mill (hatched area) as a function ofthe viscosity ratio for disperse to continuous phases. Redrawn from data in Walstra [131].

Figure 3.6 Droplet breakup as a function of viscosity ratio. The solid line represents the critical Weber number value above which droplet breakup will occur. Data from Isaacs and Chow [130].

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