High-velocity jets atomization

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. 

Figure 10 shows the nozzle in place in the liquid. As air under pressure is applied to the atomizer nozzle, a high velocity air stream emerges from the orifice. The bulk liquid is aspirated through the liquid feed line hole, and the liquid column near the region of the jet is atomized. 

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. 

There are a number of modes of jet breakup, but we consider only the breakup into spherical drops caused by capillary forces, which implies low jet velocities into an external medium whose density is low by comparison with the jet, say, air. At high velocities the dynamic effect of the surrounding medium on the jet surface will alter the surface pressure, and tangential stresses at the surface due to viscosity may also affect the breakup. It is a well-known observation that at sufficiently high velocities a jet will atomize into a large number of small droplets compared with a relatively smaller number of big drops at low velocities. 

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