Air-blast atomizers

Figure 2.9. Schematic of prefilming type of air-blast atomizer.

Various correlations for mean droplet sizes generated by air-assist atomizers are given in Table 4.6. In these correlations, mA is the mass flow rate of air, h is the height of air annulus, tf0 is the initial film thickness defined as tj ) = dQw/dan, d0 is the outer diameter of pressure nozzle, dan is the diameter of annular gas nozzle, w is the slot width of pressure nozzle, C is a constant related to nozzle design, UA is the velocity of air, and MMDC is the modified mean droplet diameter for the conditions of droplet coalescence. Distinguishing air-assist and air-blast atomizers is often difficult. Moreover, many... 

Various correlations for mean droplet size generated by plain-jet, prefilming, and miscellaneous air-blast atomizers using air as atomization gas are listed in Tables 4.7, 4.8, 4.9, and 4.10, respectively. In these correlations, ALR is the mass flow rate ratio of air to liquid, ALR = mAlmL, Dp is the prefilmer diameter, Dh is the hydraulic mean diameter of air exit duct, vr is the kinematic viscosity ratio relative to water, a is the radial distance from cup lip, DL is the diameter of cup at lip, Up is the cup peripheral velocity, Ur is the air to liquid velocity ratio defined as U=UAIUp, Lw is the diameter of wetted periphery between air and liquid streams, Aa is the flow area of atomizing air stream, m is a power index, PA is the pressure of air, and B is a composite numerical factor. The important parameters influencing the mean droplet size include relative velocity between atomization air/gas and liquid, mass flow rate ratio of air to liquid, physical properties of liquid (viscosity, density, surface tension) and air (density), and atomizer geometry as described by nozzle diameter, prefilmer diameter, etc. 

Table 4.7. Correlations for Mean Droplet Sizes Generated by Plain-Jet Air-Blast Atomizers...

Table 4.9. Correlations for Mean Droplet Size Generated by Miscellaneous Air-Blast Atomizers with Prefilming Effect...

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. 

Turbulent jet diflFusion flames. In this case, the oil is atomized by high-pressure air or steam (air blast atomizer), and the momentum of the fuel spray is so high that the entrained air is suflBcient to complete the combustion. 

Twin-Fluid Air-Blast Atomizer. Twin-fluid atomizers can be divided into internal and external mixing systems. Atomization occurs by passing a high-velocity gas stream over a liquid sheet or by mixing in the form of a Y jet. The gas stream is usually air although steam has been used to improve the injection characteristics of heavy viscous fuels. The air stream is usually derived from the main air flow to the combustor, thus utihzing a portion of the combustor pressure drop. 

An air-blast atomizer, designed on the basis of industrial-type twin-fluid atomizers, was supplied with kerosene under low pressure from a nitrogen cylinder and under high pressure air from a compressor. The atomizer nozzle diameter was 1.5 mm, a stabilizer disk with 89-mm diameter was fitted into an annular air duct of 316-mm diameter, kerosene flow rate was 2.3 kg/hr, and atomizing air flow rate was 0.69 kg/hr. 

The concept of fractals has been applied to describe aqueous droplet size distributions generated from an air blast atomizer. A critical fractal dimension, Dc (value of 3), was used as an indicator to show whether liquid breakup or aggregation of droplets dominates during atomization. If the fractal dimension D = Dc, then the number of droplets in the spray stays constant D > Dq implies that aggregation governs the process, whereas D < Dq implies that the breakup process dominates and the number of droplets increases. The fractal dimension, D, is obtained from ... 

Rizkalla, A.A. Lefebvre, A.H. Influence of liquid properties on air-blast atomizer spray characteristics. ASME Gas Turbine Conference 1974, 1-5, Paper No. 74-GT-l. 

Nebulizers and atomizers used in aerosol research produce a polydisperse aerosol consisting of particles under 10 pm in diameter. Most nebulizers use compressed air for atomization, whereas some use ultrasonics. Many models of compressed-air nebulizers have been developed, but they basically use the principle of air blast atomization of liquids issuing through a small orifice. Impaction plates or baffles are used to remove the larger droplets. Mass median diameters normally range from 2 to 5 pm, with a compressed-air pressure of 20-30psig. A detailed discussion of nebulizers can be found in Raabe [5]. Most nebulizers or atomizers tend to have a small liquid reservoir and cannot be used for long duration unless the reservoir is refilled continuously. 

D. Schmidt, L. Chiappetta, G. Goldin, R. Madabhushi, Transient multidimensional modeling of air-blast atomizers. Atomization and Sprays 13 (4) (2003) 373-394. 

---