Atomizers spray formation processes

A visualization study of fuel atomization using a pulsed laser holography/photography technique indicates that basic spray formation processes are the same for both a coal-derived synthetic fuel (SRC-II) and comparable petroleum fuels (No. 2 and No. 6 grade). Measurements were made on both pressure swirl and air assisted atomizers in a cold spray facility having well controlled fuel temperature. Quality of the sprays formed with SRC-II was between that of the No. 2 and No. 6 fuel sprays and was consistent with measured fuel viscosity. Sauter mean droplet diameter (SMD) was found to correlate with fuel viscosity, atomization pressure, and fuel flow rate. For all three fuels, a smaller SMD could be obtained with the air assisted than with the pressure swirl atomizer. 

Droplet formation occurs primarily through the surface tension and viscosity dominated breakup of these liquid threads due to symmetric (or dilational) waves as described by Rayleigh (6) for inviscid liquids and by Weber (J) for viscous fluids. Figure 3 shows the double pulsed image of the droplet formation process for No. 2 and SRC-II fuel sprays under identical atomizer conditions. These two photographs illustrate typical differences seen between these two fuels. 

Within the scope of this work, the initial spray breakup process, providing information about the dense spray core, will be investigated. The formation of fuel drops will be simulated based on first-principles and will offer detailed insight into primary atomization. The three-dimensional, transient calculation will track the interface evolution through droplet formation and breakup. Because the results will be based on conservation laws, they will be extremely general. This will lead to better models that can be used with confidence in the engine design process. 

Primary atomization, the formation of ligaments and drops by an atomizer, has already been a subject of study for over a century. The difficulty in experiments is that the numerous droplets reflect light, obscuring clear views of the atomization process. In addition, the high speed and small size of practical fuel injection means that the experimental images are often not clear. Dense sprays and non-spherical drops also make quantitative data difficult to obtain with laser-based diagnostics. 

The spray-drying process first requires the formation of a slurry to be sprayed, which can be a concentrated solution of the agent to be dried or a dispersion of the agent into a suitable nondissolving medium. The dispersion is then atomized into droplets, which are exposed to a heated atmosphere to effect the drying process. Completion of the process yields a dry, freeflowing powder that ordinarily consists of spherical... 

Shell materials can be solvent-based, water-based, molten, reactive, or molecnlar. Variations of atomization, spray coating, and coextrusion are available to deposit shell or matrix materials from solvent, water, or as a molten material. For example, spray drying is snitable for encapsnlating with solvent-based or water-based matrix materials, while spray congealing nses molten fats or waxes. Fewer shell material selections are available with the emulsion-based processes. For example, complex coacervation is most often associated with the use of gelatin as the shell, and the generation of polyurea or polymelamine formaldehyde shells is associated with in situ polymerization. Further limited examples include the use of cyclodextrins for molecular complexation or phospholipids for the formation of liposomes. 

The DPF method [5-7] assumes spray formation is a combination of random and nonrandom processes. An instability analysis is used to describe primary breakup, which is uniquely determined for a given set of initial conditions (fluid physical properties and atomizer parameters) and a model of the breakup mechanism. The drop size distribution arises from fluctuations in the initial conditions due to such factors as gas and liquid turbulence, atomizer passage surface roughness, vortex shedding, liquid mixture composition, etc. 

Fig. 33.7. In this figure the atomizing gas enters fi om the top while the liquid enters firom a circumferential slot. As both fluids reach the core opening, the liquid is pushed toward the nozzle exit by the gas pressure. At an arbitrary time (tj), the liquid flow is redirected by the gas pressure and a thin film is formed at the nozzle wall. The hquid partially blocks the gas flow, building a pressure. As the pressure builds to a critical value, a hquid chunk is removed. This process causes an oscillatory spray formation. The frequency of this oscillation depends on the liquid and gas flow rates. The frequency increases with increasing the velocity of the liquid or the gas. Two separate variables are important for the pulsation (a) shear stresses at the liquid/gas interface, and (b) fluid momentum.

The concept or the basis of spray pyrolysis method assumes that one droplet forms one product particle. To date, submicrometer- to micrometer-sized particles are typically formed in a spray pyrolysis process. A variety of atomization techniques have been used ftn- solution aerosol formation, such as ultrasonic spray pyrolysis, electrospray pyrolysis, low pressure spray pyrolysis using a filter expansion aerosol generator (FEAG), salt-assisted spray pyrolysis, two-fluid pyrolysis method, etc. [15-18]. These atomization methods differ in droplet size, rate of atomization, and... 

Atomization is the process by which a liquid is disintegrated into many fine droplets. The formation of a spray with high surface/mass ratio is highly critical for optimum liquid evaporation conditions and, consequently, the desired properties of the resulting product. Although ideally the sizes of all droplets should be the same, in practical terms formation of droplets with a narrow size distribution would be satisfactory. 

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