Twin fluid atomization

FIG 27-25ii Common types of atomizers twin-fluid atomizers. (From Lefeh-V7 e, Atomization and Sprays, Hemisphere New Yof k, 1989. Reproduced with peimission. All rights reseroed. )... 

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. 

The size, shape, and length of the flames obtained from the two types of atomizers (twin-fluid air blast and Sonicore) show some fundamental differences between the two atomizers. A relatively large droplet size from the twin-fluid atomizer together with low concentrations of oxygen and temperature within the spray allow combustion to occur essentially at the spray boundary. 

This is a Nukiyama-Tanasawa type distribution. The parameter a > 1, guarantees that the number distribution vanishes at small D, as opposed to the expression derived by Cousin et al. [19] which corresponds to a = 1. Unfortunately this introduces a third parameter that needs to be determined. Lecompte and Dumouchel [21] suggest that there may be a tmique pair q and a that can represent all drop size distributions for a specific atomization process (ultrasonic atomization, twin-fluid atomization, etc.). This could transform the MEF into a pseudo-predictive method. 

Twin-fluid atomizer Twisted pair cable Twitchell splitting Twitchell s reagents Two-film theory... 

The principal parameters affecting the size of droplets produced by twin-fluid atomizers have also been discussed (34). These parameters include Hquid viscosity, surface tension, initial jet diameter (or film thickness), air density, relative velocity, and air—Hquid ratio. However, these parameters may have an insignificant effect on droplet size if atomization occurs very rapidly near the atomizer exit. 

Most studies indicate that air velocity has a profound influence on mean droplet size in twin-fluid atomizers. Generally, the droplet size is inversely proportional to the atomizing air velocity. However, the relative velocity between the Hquid and air stream is more important than the absolute air velocity. 

Both effects can produce coarser atomization. However, the influence of Hquid viscosity on atomization appears to diminish for high Reynolds or Weber numbers. Liquid surface tension appears to be the only parameter independent of the mode of atomization. Mean droplet size increases with increasing surface tension in twin-fluid atomizers (34). is proportional to CJ, where the exponent n varies between 0.25 and 0.5. At high values of Weber number, however, drop size is nearly proportional to surface tension. 

Fig. 9. (a) Pressure atomizers (b) rotary atomizer and (c) twin-fluid atomizers (89,90). 

Atomizers for large boiler burners are usually of the swid pressure jet or internally mixed twin-fluid types, producing hoUow conical sprays. Less common are the externally mixed twin-fluid types (89,90). 

FIG, 27-27 Y-jet twin-fluid atomizer. (From Lefehv70, Atomization and Sprays, Hemisphe7e, New Yo7 k, 1989. Rep7oduced with pe7mission. All 7 ights 7 ese7 oed.)... 

Two-fluid atomization, sometimes also termed twin-fluid atomization, two-phase atomization or aerodynamic atomization, is one of the commonly used techniques in many areas. Two-fluid... 

Recently, Knoll and Sojka[263] developed a semi-empirical correlation for the calculation of the Sauter mean diameter of the droplets after primary breakup of flat-sheets in twin-fluid atomization of high-viscosity liquids ... 

A number of techniques have been evolved to disperse liquids in gases in the form of fine droplets. The various atomizing techniques are jet injections, fan sprays, centrifugal nozzles, twin fluid atomizers, impinging jets, and rotary... 

Atomizers Atomization is the process of breaking up a continuous liquid phase into discrete droplets. Figure 24-22 shows the idealized process by which the surface area of a liquid sheet is increased until it forms droplets. Atomizers may be classified into two broad groups (see Fig. 24-23), pressure atomizers, in which fuel oil is injected at high pressure, and twin-fluid atomizers, in which fuel oil is injected at moderate pressure and a compressible fluid... 

FIG. 24-25 Y-jet twin-fluid atomizer. From Lefebvre, Atomization and Sprays, Hemisphere, New York, 1989. Reproduced with permission. All rights reserved.)... 

NAR is the ratio of air to liquid flow rates through the nozzle of a twin fluid atomizer expressed either in mass units or in volume units (air at STP). 

Aerosols (qv) are very finely divided sprays having droplet diameters of l ndash 30 Jim. They are used almost entirely as space sprays for application to enclosures, particularly against flying insects. Aerosols are most conveniendy applied by the familiar liquefied gas dispersion or bomb but can be generated on a larger scale by rotary atomizers or twin fluid atomizers. 

Bryce, W. B., Cox, N. W. and Joyce, W. I. (1978). On droplet production and size measurement from a twin fluid atomizer using real fluids. ICLAS 78, 258-263. 

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