Atomization spray formation

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. 

The influence of design and operating variables on spray formation for pressure swirl atomizers has been well studied (e.g., Effects of fuel pressure and nominal flow rate... 

Effects of Atomizer Size and Fuel Pressure on Spray Formation. Atomizer is Delavan 60A. Fuel is SRC II Middle Distillate. 

Effect of Fuel Viscosity on Spray Formation. Atomizer is Sonicore 052H. Fuel is No. 6 Oil. 

Its formation is accompanied by the generation of a spray, resulting from the vibrations at the liquid surface and cavitation at the liquid-gas interface. The quantity of spray is a function of the intensity. Ultrasonic atomization is accomplished using an appropriate transducer made of PZT located at the bottom of the liquid container. A 500-1000 kHz transducer is generally adequate. The atomized spray which goes up in a column fixed to the liquid container is deposited onto a suitable solid substrate and then heat treated to obtain the film of the material concerned. The flow rate of the spray is controlled by the flow rate of air or any other gas. The liquid is heated to some extent, but its vaporization should be avoided. 

This chapter deals with US assistance to operations that are even closer to the detection step such as nebulization or spray formation immediately before the insertion of sample into atomic or mass deteotors, respeotively levitation to obtain a sample droplet (usually for in situ deteotion, with or without preconcentration or previous derivatization) and electroanalytioal determinations (either to facilitate preparation of the electrode or influence the electroanalytical step itself). In nebulization and levitation, US competes — most often advantageously — with alternative ohoioes having a similar effect in electroanalytical methods, US has a characteristio effeot. Both traditional and novel detection teohniques oan therefore benefit from US assistance in some way. 

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. 

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. 

O. Kurt, U. Fritsching, G. Schulte Secondary droplet formation during binary suspension droplet collision, Atomization Sprays 19, 457 72 (2009). 

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. 

A model for the atomization and spray formation by splash plate nozzles is developed by Sarchami et al. [30]. This model is based on the liquid sheet formation theory due to an oblique impingement of a liquid jet on a solid surface. The continuous liquid sheet formed by the jet impingement is replaced with a set of dispersed droplets. The initial droplet sizes and velocities are determined based on theoretically predicted liquid sheet thickness and velocity. A Lagrangian spray code is used to model the spray dynamics and droplet size distribution further downstream of the nozzle. 

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 basic approach to classify powder production methods is based on whether a method is top-down or bottom-up. In a top-down method, micro- and nano-particles are produced due to the stracture and size refinement through the breakdown of the larger particles in a bottom-up method, the mechanism of particle formation is usually by means of nucleatimi, growth and aggregation of atoms and molecules. In a more practical approach, one may divide the powder synthesis methods as follows (1) wet chemistry, such as the chemical precipitation, sol-gel, microemulsion, sonochemistry, and hydrothermal synthesis methods (2) mechanical attrition, grinding and milling (3) gas phase methods, such as the chemical and physical vapor deposition (4) liquid phase spray methods, such as the molten metal spray atomization, spray pyrolysis, and spray drying, and (5) liquid/gas phase methods. 

IPS. Inert plasma spraying (q.v.). Iridizing. The formation of a film of metal oxide on the surface of a ceramic, particularly of glass. The surface to be treated is heated and then exposed to the vapour, or to atomized spray, of a metal salt. Such surface films may be applied as decoration or to provide a surface that is electrically conducting, e.g. for anti-frosting windscreens. 

Formation of the atomized spray requires application of a force. The commercially available systems employ one of the following in order to create an atomized spray centrifugal energy, pressure energy, kinetic energy or sonic energy, and vibrations. 

Hollow-Cone Sprays. In swid atomizers, the Hquid emerges from the exit orifice ia the form of a cooical sheet. As the Hquid sheet spreads radially outward, aerodyaamic iastabiHty ioimediately takes place and leads to the formation of waves which subsequently disiategrate iato ligaments and droplets. Figure 3 illustrates the breakup process ia an annular Hquid sheet. 

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