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. 

This paper summarizes the results of an experimental study to visualize and compare spray formation processes... 

Physical properties of the three test fuels are presented in Table I. Except for the surface tension of No. 6 fuel oil, which was a typical value, all properties were measured for the specific samples tested. The primary differences between the SRC-II middle distillate and the No. 2 fuel were the higher specific gravity, surface tension, and viscosity of the SRC-II. The No. 6 grade fuel, a residual fuel oil, had a much higher viscosity than either of the distillate fuels. Both the SRC-II and No. 2 fuel oil were sprayed at a nominal temperature of 80°F to simulate usage in a non-preheat combustion system. The No. 6 fuel oil was sprayed at temperatures ranging from 150° to 240°F in order to assess spray formation processes and spray quality over a broad range of viscosities. 

Particle size data were obtained for selected test conditions directly from holograms and photographs. Spray measurements were made at an axial distance of approximately 2.5 cm (1 in.) from the injector where spray formation processes had been completed and spherical droplets formed. 

A study to visualize and compare spray formation processes for both a coal-derived synthetic fuel and comparable petroleum fuels has been completed. Although the results were primarily qualitative, a limited amount of particle size data... 

All three fuels exhibited the same basic spray formation processes. No new or exotic modes of droplet formation were observed with the SRC-II, although distinct differ-ences in ligament length and breakup time were seen. 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

Fig. 4.11. Unexpected formation of a metastable phase of acetaminophen as a result of exposure to the molten wax formulation during spray-congeal processing. Note The three curves have been manually offset on the F-axis from the normal zero milliwatt baselines in order to display the relative X-axis (temperature) differences between the three samples.

For example, amorphous clarithromycin was prepared by grind and spray-drying processes, and XRPD was used to follow changes in crystallinity upon exposure to elevated temperature and relative humidity [59]. Exposure of either substance to a 40°C/82% RH environment for seven days led to the formation of the crystalline form, but the spray-dried material yielded more crystalline product than did the ground material. This finding, when supported with thermal analysis studies, led to the conclusion that the amorphous substances produced by the different processing methods were not equivalent. 

Figure 1.1. Schematic of spray combustion process (a) annular combustion chamber in a single spool turbojet with an axial flow compressor (b) fuel injection and droplet formation in combustion chamber.

Figure 1.8. Schematic showing (a) plasma spray deposition process and (b) droplet formation, acceleration, surface impact, and coating formation.

Figure 7.23 Schematic diagram of spray drying process for the formation of calcined magnesium zinc ferrite. From J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John Wiley and Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Hagerty (8C) presents an excellent summary of the work on continuous fuel spray conducted at the University of Michigan. Five major problems closely related in the field of spray research include the stability of the liquid phase, drop-size factors, spray distribution, metering characteristics, and the effect of all these on the resulting combustion. Hagerty discusses these factors in relation to theoretical and experimental studies of the process of spray formation. 

A recent application of particle formation by solvent evaporation and spray-drying techniques is based on the concept of the aerodynamic diameter. According to Eq. (8.5), the aerodynamic diameter dAer is correlated with the true particle diameter dP and the particle density pp° 5. It is evident that particles formed in a particle-formation process can be much bigger, provided that their density is very small. Increased bioavailability of such large porous insulin particles (Fig. 8.14) has been demonstrated on inhalation by rats and has been correlated with a... 

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. 

Of the approximately 12 motion pictures we made of the impact initiation process, all show that the structure of the air bubble is broken down and replaced by a turbulence area. Ignition occurs at the former site of the bubble after an induction period. The compression ratio of the air bubble appears to be the major factor determining probability of initiation by impact. The mechanism for impact initiation of nitroglycerin therefore appears to be a quasi-adiabatic compression of the gas, with heat transfer accelerated by spray formation. Hot spots formed at the former site of the bubble undergo an accelerating exothermic reaction which proceeds to a deflagration. The possibility that liquid explosives under reduced pressure may be sensitized to weak impacts must be considered. 

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