Jet atomization

FIG, 27-26 Swirl pressure-jet atomizer. (From Lefehvre, Atomization and Sprays, Hemisphere, New Yo7 k, 1989. Rep7oduced with pe7missio7x. Adi 7ights 7 ese7 oed.)... 

Atomization of melts has, in principle, some similarity to the atomization of normal liquids. The atomization processes originally developed for normal liquids, such as swirl jet atomization, two-fluid atomization, centrifugal atomization, effervescent atomization, ultrasonic piezoelectric vibratory atomization, and Hartmann-whistle acoustic atomization, have been deployed, modified, and/or further developed for the atomization of melts. However, water atomization used for melts is not a viable technique for normal liquids. Nevertheless, useful information and insights derived from the atomization of normal liquids, such as the fundamental knowledge of design and performance of atomizers, can be applied to the atomization of melts. 

In internal mixing atomization (for example centrifugal-pneumatic atomization), 159] the liquid metal and gas enter the swirl jet atomizer tangentially under pressure (Fig. 2.13)J159] The two fluids rotate, form a mixture, and accelerate in the confuser. Due to the strong centrifugal force, the liquid metal forms a film at the nozzle exit even without the presence of the gas. With the applied gas, the liquid film is atomized into a fine dispersion of droplets outside the nozzle. 

The liquid properties of primary importance are density, viscosity and surface tension. Unfortunately, there is no incontrovertible evidence for the effects of liquid viscosity and surface tension on droplet sizes, and in some cases the effects are conflicting. Gas density is generally considered to be the only thermophysical property of importance for the atomization of liquids in a gaseous medium. Gas density shows different influences in different atomization processes. For example, in a fan spray, or a swirl jet atomization process, an increase in the gas density can generally improve... 

Table 4.3. Correlations for Mean, Minimum and Maximum Droplet Sizes Generated in Pressure Jet Atomization by Plain-Orifice Atomizers... 

The influence of liquid density on the mean droplet size is relatively small but complex. An increase in liquid density may reduce the mean droplet size due to a decrease in sheet thickness at the atomizing lip of a prefilming atomizer, or due to an increase in the relative velocity between liquid and air for a plain-jet atomizer. However, increasing liquid density may also increase the mean droplet size because a liquid sheet may extend further downstream of the atomizing lip of a prefilming atomizer so that the sheet breakup may take place at lower relative velocity between liquid and air. 

In the empirical correlation proposed by Kato et al.,[503] the mean droplet size is inversely proportional to the water pressure, with a power index of 0.5 for conical shaped annular-jet atomizers, and 0.7-1.0 for V-shaped flat-jet atomizers. This suggests a lower efficiency of the annular-jet atomizers in terms of spray fineness at high water pressures. The data of Kato et al.15031 were obtained for water pressures lower than 10 MPa. Seki et al.15021 observed the similar trend in the water atomization of nickel and various steels at higher water pressures (>10 MPa). Since k is dependent on both... 

In the empirical correlation proposed by Kishidaka15041 for two-jet atomizers, melt nozzle diameter and physical properties, water velocity, and water to melt ratio are included. The constant A is again a function of atomizer geometry. The water velocity may be estimated with the following equation assuming loss-free water flow in the water nozzle(s) ... 

For a stationary spray without scanning, a Gaussian shaped mass distribution typically develops with an annular-jet or discrete-jet atomizer. The radial mass distribution in the spray can be formulated in terms of mass flux)632]... 

FIG. 24-24 Swirl pressure-jet atomizer. (Fn mL fcor, Atomization and Spra) , Hemisphere, New York, 1989. Reproduced with permission. AU rights reserved.)... 

Presser, C., A.K. Gupta, and H. G. Semerjian. 1993. Aerodynamic characteristics of swirling spray-pressure jet atomizer. Combustion Flame 92 25-44. 

Actual values of SMD (or some other mean droplet size) for various types of fuel injectors are given for orifice injectors (23, 25, J+5, 47, 61, 95, 107, 113) swirl injectors (8, 18, 20, 26. 34, 46, 63, 76-78, 81, 87, 93, 99, 100, 104, 112, US, U8, 123) air-blast atomizers (5, 13, 33, 50, 64, 74, 94, 101) impinging-jet atomizers (30) rotating-element atomizers (7, 28, 53) and droplet formation caused by collapse of gas bubbles rising through a liquid surface (32, 91). Maximum values of x are reported for orifice injection (88) and for air-blast atomization (72). [The air-blast atomization mechanism may be controlling when the air blast is in reality only a high relative velocity between the air and the injected liquid (61).]... 

B) have found excellent correlation between the measured sizes of drops atomized by high-velocity gas streams with the equations developed by Nukiyama and Tanasawa (6L), so long as conditions are held within certain limits. The behavior of sprays of 7i-heptane, benzene, toluene, and other fuels has been studied by Garner and Henny (SB) by use of a small air-blast atomizer under reduced pressures. A marked increase in the Sauter mean diameter was obtained for benzene and toluene as compared with n-heptane, which parallels their poor performance in gas turbines. Duffie and Marshall (2B) give a theoretical analysis of the breakup characteristics of a viscous-jet atomizer and show high-speed photographs of the process. 

Ibrahim E, Przekwas A. (1991) Impinging jets atomization. Phys Fluids A 3(12) 2981-2987. 

Jet Spray, The mechanism that controls the breakup of a Hquid jet has been analyzed by many researchers (22,23). These studies indicate that Hquid jet atomization can be attributed to various effects such as Hquid—gas aerodynamic interaction, gas- and Hquid-phase turbulence, capillary pinching, gas pressure fluctuation, and disturbances initiated inside the atomizer. In spite of different theories and experimental observations, there is agreement that capiUary pinching is the dominant mechanism for low velocity jets. As jet vdocity increases, there is some uncertainty as to which effect is most important in causing breakup. 

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