Liquid sheet disintegration

Fan Sprays, It was demonstrated around the 1950s that instability theory can be used to analyze the wave growth on a thin liquid sheet (18). This analysis predicted the existence of an optimum wavelength at which a wave would grow rapidly. This optimum wavelength, X, corresponds to a condition that leads to liquid sheet disintegration. It can be expressed as in equation 2 ... 

Roller atomization is a mechanical atomization process. It was invented in the mid 1970 sJ188] In this process, as schematically depicted in Fig. 2.22, a stream of molten metal is fed into the gap between two counter-rotating rolls and forms a thin liquid sheet that subsequently disintegrates into droplets by the mechanical forces. In the original design, a pair of rollers of 100 mm in diameter are mounted in the same horizontal plane and rotate at speeds up to 1250 radians/s. The roll gap is about 50-100 pm, and the metal flow rate is up to 6 kg/min. 

Taylod205 also conducted mathematical analysis of the generation of ripples by wind blowing over a viscous fluid. Using a relationship between the growth of the amplitude of disturbance waves and the surface stress, Taylor derived a criterion for the instability of waves. In Taylor s instability theory, the disintegration of a liquid sheet/film is visualized as a process in which droplets are detached from the liquid surface with a wave of optimum amplitude. The diameter of the most frequent droplets is then formulated as a function of air velocity over the liquid surface, liquid density, surface tension and viscosity, as well as air density. 

A) Flat Liquid Sheets into Quiescent Air Rim-Jet Disintegration. In quiescent air (without an air flow), a liquid sheet issuing from the 2-D nozzle will converge toward the axis of the sheet to form a round j et under the action of surface tension forces. The liquid jet subsequently breaks up into droplets. 

Although a liquid sheet may leave the nozzle with some perturbations, the principal cause of the instabilities is the interaction of the sheet with the high-velocity air streams whereby rapidly growing waves are imposed on the sheet. Disintegration may occur when the amplitude of these waves reaches a critical value. Each full sinusoidal wave is initially distorted to yield two half-waves of very similar forms. The constant stretching of the half-waves increases... 

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. 

Flat Sheets. Generally, the interface between a liquid sheet and air can be perturbed by aerodynamic, turbulent, inertial, surface tension, viscous, acoustic, or electrical forces. The stability of the sheet and the growth rate of unstable disturbances are determined by the relative magnitude of these forces. Theoretical and experimental studies 255112561 on disintegration mechanisms of flat sheets showed that the instability and wave formation at the interface between the continuous and discontinuous phases are the maj or factors leading to... 

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 ... 

Droplet Formation in Centrifugal Atomization. The mechanisms of centrifugal atomization of liquid metals are quite similar to those for normal liquids. Three atomization modes have been identified in rotating electrode atomization process, i.e., (I) Direct Droplet Formation, (2) Ligament Disintegration, and (3) Film/Sheet Disintegration.1[189][32°] are aiso applicable to the centrifugal atomiza-... 

As ambient air pressure is increased, the mean droplet size increases 455 " 458] up to a maximum and then turns to decline with further increase in ambient air pressure. ] The initial rise in the mean droplet size with ambient pressure is attributed to the reduction of sheet breakup length and spray cone angle. The former leads to droplet formation from a thicker liquid sheet, and the latter results in an increase in the opportunity for droplet coalescence and a decrease in the relative velocity between droplets and ambient air due to rapid acceleration. At low pressures, these effects prevail. Since the mean droplet size is proportional to the square root of liquid sheet thickness and inversely proportional to the relative velocity, the initial rise in the mean droplet size can be readily explained. With increasing ambient pressure, its effect on spray cone angle diminishes, allowing disintegration forces become dominant. Consequently, the mean droplet size turns to decline. Since ambient air pressure is directly related to air density, most correlations include air density as a variable to facilitate applications. Some experiments 452] revealed that ambient air temperature has essentially no effect on the mean droplet size. 

Dombrowski, N. and Fraser, R. P. Phil. Trans. 247A (1954) 101. A photographic investigation into the disintegration of liquid sheets. 

Liquid sheet formation by an appropriate nozzle is followed by rim disintegration, aerodynamic wave disintegration, and turbulent breakup. 

At low-discharge velocities and low film thicknesses, the sheet disintegration is due to the oscillations caused by air motion. In this case, the film thickness has a large impact on the droplet size. In contrast, it is insignificant whether a pure liquid or a lime-water suspension (mass portion cj) = 16-64%) is treated (21). 

The fuel emerged from the orifice as a conical liquid sheet and subsequently disintegrated into a conical spray. Delavan atomizers in three different sizes (nominal flow rates of 1, 2, and 5 gph), three different spray angles (45, 60, and 90 degrees) and with hollow cone spray patterns were investigated. Nominal fuel pressures considered were 50, 100, and 150 psig. 

---