Liquid sheet breakup

Liquid-Sheet Breakup The basic principle of most hydraulic atomizers is to form a thin sheet that breaks via a variety of mechanisms to form ligaments of liquid which in turn yield chains of droplets. See Fig. 14-86. 

Linear stability theories have also been applied to analyses of liquid sheet breakup processes. The capillary instability of thin liquid sheets was first studied by Squire[258] who showed that instability and breakup of a liquid sheet are caused by the growth of sinuous waves, i.e., sideways deflections of the sheet centerline. For a low viscosity liquid sheet, Fraser et al.[73] derived an expression for the wavelength of the dominant unstable wave. A similar formulation was derived by Li[539] who considered both sinuous and varicose instabilities. Clark and DombrowskF540 and Reitz and Diwakar13161 formulated equations for liquid sheet breakup length. 

It is observed that the breakup time is reduced as the Weber number is increased, which is expected. It is now evident that the liquid sheet breakup occurs at halfwavelength intervals, as observed earlier, and this parcel of liquid is expected to contract into a ligament under the force of surface tension. There does not exist any indication of satellite ligament formation from the liquid sheet breakup. 

Lozano A, GarcTa-Olivares A, Dopazo C, The instability growth leading to a liquid sheet breakup, Phys. Fluids 10(9), 2188-2197, 1998. 

Fig. 18.8 Liquid sheet breakup (left) characterization of sheet breakup length by positions of perforation- and droplet formation (right) liquid sheet breakup length vs. melt mass flow rate, experimental data from Achelis et al. [13]...

However, the model overpredicts the liquid sheet breakup lengths at very low melt mass flow rates. Figure 18.9 shows the propagation of liquid sheets from both 3D and 2D axisymmetric simulations at a melt mass flow rate 130 kg/h. The droplet formation has not yet started, in spite of a large penetration depth of liquid sheet, for example 30 mm in the 3D simulation and over 40 mm in the 2D axisymmetric simulation. 

Data extracted from numerical simulations and experiments on swirling liquid sheet breakup can be applied as inflow conditions for the subsequent modelling and simulation of secondary droplet breakup and spray development in a spray process based on the Eulerian-Lagrangian approach. 

FIG. 27-24 Idealized process of drop formation by breakup of a liquid sheet. After Domhrowski and Johns, Chem. Eng. Sci. 18 203, 1963. )... 

Table 3.2. Classification and Criteria of Breakup Regimes of Round Liquid Jets in Co-flowing Air as Compared to Those of Thin Liquid Sheets and Spherical Droplets in Air Stream 210 ...

Figure 3.8. Liquid sheet/film breakup modes Successive stages in the idealized breakup of (a) a sheet with a thick rim, (b) a wavy sheet, and (c) a perforated sheet.

B) Flat Liquid Sheets into Air Streams Mechanical and Aerodynamic Disintegration. In air streams (with an air flow), a liquid sheet issuing from the 2-D nozzle will form a quasi-2-D expanding spray. The breakup modes are divided into two groups (1) mechanical mode due to the action of liquid injection pressure, and (2) aerodynamic mode due to the action of air friction. 

The aerodynamic breakup mode occurs in the liquid sheet between the rims. In aerodynamic breakup, the perforation and wave... 

Farago and Chigier 2l() found that at similar aerodynamic Weber numbers, the disintegration modes of a thin liquid sheet in air streams are similar to those of a round liquid jet in a coaxial air stream (Table 3.2). At high aerodynamic Weber numbers, Membrane-Type or Fiber-Type breakup mode may set in. 

Arai and Hashimoto[2611 studied disintegration of a thin liquid sheet in a co-flowing air stream. For a constant sheet thickness, an empirical correlation was derived for the sheet breakup length as ... 

In practical fan sheet breakup processes, sheet thickness diminishes as the sheet expands away from the atomizer orifice, and liquid viscosity affects the breakup and the resultant droplet size. Dombrowski and Johns[238] considered these realistic factors and derived an analytical correlation for the mean droplet diameter on the basis of an analysis of the aerodynamic instability and disintegration of viscous sheets with particular reference to those generated by fan spray atomizers ... 

Droplet Formation in Gas Atomization. Experimental and modeling studiesl160 161 169] 318] 319] 321]- 325] have shown that gas atomization of liquid metals in spray forming and powder metallurgy processes may take place in two primary modes, i.e., liquid jet-ligament breakup and liquid film-sheet breakup. 

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