Spraying Atomization

In conventional spraying, atomization is the result of external mechanical forces, i.e., the exchange of momentum between two free jets (air and paint). Atomization may be classified as compressed air atomization (air 0.02-0.7 MPa, paint 0.02-0.3 MPa), airless atomization (paint 8 40 MPa), air-assisted airless atomization, also termed airmix process (air 0.02 0.25 MPa, paint 2-8 MPa), and special technologies (Table 8.3). 

Universally employable spraying experience required large-scale series coating 

Simple to use paint mists constitute a health applications (automobile 

Applicable to complicated ventilation required in enclosed repair and touch-up finishes 

Small amounts can be compressed air supply required domestic appliances, etc.) 

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Fan spray atomizers Fansteel process Faradaic current... 

The flow characteristics inside Hquid atomizers have been studied by numerous investigators (4—8). Of special interest to designers is the work reported on swid atomizers (4), fan spray atomizers (6,7), and plain jets (8). The foUowing discussion focuses on the flow characteristics of a swid atomizer. 

Drying conditions, because of turbulence and gas mixing, are uniform throughout the chamber i.e., the entire chamber is at the gas exit temperature—this fact has been well estabhshed in many chambers except in the immediate zone of gas inlet and spray atomization. 

Under better conditions, mechanical lubrication may be used. Force-feed lubricators can be installed that will provide a continuous and measured supply of oil to the meshing teeth or by spraying atomized lubricant onto the gears. 

Fanning friction factor, 73 260 Fan spray atomizers, 23 179 Fan sprays, 23 182 Fansteel process, 24 319 FAO Flax Group, 77 592 Faraday, Michael, 77 398 Faraday constant (F), 3 410 Faraday cup construction, 74 444 Faraday s law of electrolysis, 24 748 Faraday s laws, 9 593, 772 77 669 Far-Go/Triallate, 2 549t Farina, 26 284... 

Solid-cone spray atomizers usually generate relatively coarse droplets. In addition, the droplets in the center of the spray cone are larger than those in the periphery. In contrast, hollow-cone spray atomizers produce finer droplets, and the radial liquid distribution is also preferred for many industrial applications, particularly for combustion applications. However, in a simplex atomizer, the liquid flow rate varies as the square root of the injection pressure. To double the flow rate, a fourfold increase in the injection pressure is... 

Fan spray atomizers have been widely used in the spray coating industry (Fig. 2.5), in some small annular gas turbine combustors, and in other special applications that require a narrow elliptical spray pattern rather than the normal circular pattern. In particular, fan spray atomizers are ideal for small annular combustors because they can produce a good lateral spread of fuel, allowing to minimize the number of injection ports. 

Figure 2.5. Schematic of a flat spray atomizer used for paint spray.

In practical fan sheet breakup processes, sheet thickness diminishes as the sheet expands away from the atomizer orifice, and liquid viscosity affects the breakup and the resultant droplet size. Dombrowski and Johns[238] considered these realistic factors and derived an analytical correlation for the mean droplet diameter on the basis of an analysis of the aerodynamic instability and disintegration of viscous sheets with particular reference to those generated by fan spray atomizers ... 

Various correlations for mean droplet size generated using pressure-swirl and fan spray atomizers are summarized in Tables 4.4 and 4.5, respectively. In the correlations for pressure-swirl data, FN is the Flow number defined as FN = ml/APlpl) )5, l0 and d0 are the length and diameter of final orifice, respectively, ls and ds are the length and diameter of swirl chamber, respectively, Ap is the total inlet ports area, /yds the film thickness in final orifice, 6 is the half of spray cone angle, and Weyis the Weber number estimated with film... 

Table 4.5. Correlations for Mean Droplet Sizes Generated by Fan Spray Atomizers...

In fan spray atomization, the effects of process parameters on the mean droplet size are similar to those in pressure-swirl atomization. In general, the mean droplet size increases with an increase in liquid viscosity, surface tension, and/or liquid sheet thickness and length. It decreases with increasing liquid velocity, liquid density, gas density, spray angle, and/or relative velocity between liquid and surrounding air. 

As for normal liquids, modeling of droplet processes of melts provides tremendous opportunities to improve the understanding of the fundamental phenomena and underlying physics in the processes. It also provides basic guidelines for optimization and on-line control of the processes. This section is devoted to a comprehensive review of process models, computational methods, and numerical modeling results of the droplet processes of melts. The emphasis of this section will be placed on the droplet processes in spray atomization for metal powder production, and spray forming for near-net shape materials synthesis and manufacturing. Details of these processes have been described in Ref. 3. 

The problem of spraying, atomization, and bubble formation in agitated systems still need considerable study, though quite a bit of work has already been reported on them. The formation in the above cases has mostly been studied in the absence of heat and mass transfer and chemical reaction, the presence of which can greatly influence the bubble volume. This has to receive considerable attention if the performance of pertinent industrial equipment is to be adequately explained. 

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