Liquid breakup

General References For an overall discussion of gas-liquid breakup... 

Liquid Breakup into Droplets There are four basic mechanisms for breakup of liquid into droplets ... 

Breakup of a low-velocity liquidjet (Ih/elocity) . This governs in special applications like prilling towers and is often an intermediate step in liquid breakup processes. 

Wu, Ruff and Faeth12491 studied the breakup of liquid jets with holography. Their measurements showed that the liquid volume fraction on the spray centerline starts to decrease from unit atZ/<70=150 for non-turbulent flows, whereas the decrease starts at aboutZ/<70=10 for fully developed turbulent flows. Their measurements of the primary breakup also showed that the classical linear wave growth theories were not effective, plausibly due to the non-linear nature of liquid breakup processes. 

Al-Roub et all421 identified three basic modes of liquid breakup during droplet impingement onto a liquid film (1) rim breakup, (2) cluster breakup, and (3) column breakup. The rim breakup mode involves the breakup and ejection of one or a few small droplets at the outer edge of the film, while the cluster breakup mode involves the breakup of liquid into clusters of many small droplets at the outer edge of the film. In the column breakup mode, liquid breaks up into one or a few droplets from a column of liquid at the center of the spreading droplet as a result of the surface waves reflecting back to their source. The diameter and number of the... 

Neglecting the energy for overcoming viscous force during liquid breakup, a simple equation for the theoretical energy efficiency has been derived by Yule and Dunkleyl5 for a pressure-swirl atomizer ... 

Current breakup models need to be extended to encompass the effects of liquid distortion, ligament and membrane formation, and stretching on the atomization process. The effects of nozzle internal flows and shear stresses due to gas viscosity on liquid breakup processes need to be ascertained. Experimental measurements and theoretical analyses are required to explore the mechanisms of breakup of liquid jets and sheets in dense (thick) spray regime. 

As described previously, in the atomization sub-model, 232 droplet parcels are injected with a size equal to the nozzle exit diameter. The subsequent breakups of the parcels and the resultant droplets are calculated with a breakup model that assumes that droplet breakup times and sizes are proportional to wave growth rates and wavelengths obtained from the liquid jet stability analysis. Other breakup mechanisms considered in the sub-model include the Kelvin-Helmholtz instability, Rayleigh-Taylor instability, 206 and boundary layer stripping mechanisms. The TAB model 310 is also included for modeling liquid breakup. 

Liquid breakup High resistivity liquids ( > 1010 ohm cm) Ionic liquids ( o < 1010 ohm cm) Statistical distribution due to ion concentration fluctuations (most probable charge = 0) Double layer disruption Unequal ion mobility or migration rates Interface contaminants (incl. surfactants)... 

It should be noted that the above phenomena all involve relative motion or charge segregation but no static electrification or true charge separation. The latter can only occur when the liquid is separated from the solid or is broken up. The above phenomena consequently can only create conditions that will permit charge separation. Any net charging process itself will, therefore, be very similar to that involved in liquid breakup except that the initial charge segregation will be influenced by double layers at the solid surface as well as by those at a liquid-gas interface. 

The ratio of energy dissipation by heat and by the droplet-making process is difficult to measure. Masters (30) suggested that less than 0.5% of applied energy is utilized in liquid-breakup. Virtually the whole amount is imparted to the liquid and air as kinetic energy. 

The transformation of bulk liquid to sprays can be achieved in many different ways. Basic techniques include applying hydraulic pressure, electiical, acoustic, or mechanical energy to overcome the cohesive forces within the liquid. Atomizers can be classified according to the energy source used to achieve liquid breakup. Table 1 provides a summary of various atomizers. 

Spray characteristics are those fluid dynamic parameters that can be observed or measured during liquid breakup and dispersal. They are used to identify and quantify the features of sprays for the purpose of evaluating atomizer and system performance, for establishing practical correlations, and for verifying computer model predictions. Spray characteristics provide information that is of value in understanding the fundamental physical laws that govern liquid atomization. 

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