Droplet diameter, mean

D = mean droplet diameter (pm) s = surface tension (dyne- cm" ) d = density of liquid (g- cm" )... 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

For example, at a frequency of 1 MHz, the effect of ultrasound in water of surface tension 73 dyn-cm and density 1 g-cm- is to produce longitudinal waves of about 12 pm. The resulting mean droplet diameter (D) is given by Equation 19.2. 

Hollow Sprays. Most atomizers that impart swid to the Hquid tend to produce a cone-shaped hoUow spray. Although swid atomizers can produce varying degrees of hoUowness in the spray pattern, they aU seem to exhibit similar spray dynamic features. For example, detailed measurements made with simplex, duplex, dual-orifice, and pure airblast atomizers show similar dynamic stmctures in radial distributions of mean droplet diameter, velocity, and Hquid volume flux. Extensive studies have been made (30,31) on the spray dynamics associated with pressure swid atomizers. Based on these studies, some common features were observed. Test results obtained from a pressure swid atomizer spray could be used to iUustrate typical dynamic stmctures in hoUow sprays. The measurements were made using a phase Doppler spray analyzer. 

Droplet Size Corrections. The majority of correlations found in the Hterature deal with mean droplet diameters. A useflil equation for Sauter... 

Effect of Variables on Mean Droplet Size. Some of the principal variables affecting the mean droplet diameters for pressure swid atomizers may be expressed by equation 14. 

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 ... 

Plotting droplet size data on an arithmetic-probability graph paper will generate a straight line if the data follow normal distribution. Thus, the mean droplet diameter and standard deviation can be determined from such a plot. 

In many applications, a mean droplet size is a factor of foremost concern. Mean droplet size can be taken as a measure of the quality of an atomization process. It is also convenient to use only mean droplet size in calculations involving discrete droplets such as multiphase flow and mass transfer processes. Various definitions of mean droplet size have been employed in different applications, as summarized in Table 4.1. The concept and notation of mean droplet diameter have been generalized and standardized by Mugele and Evans.[423] The arithmetic, surface, and volume mean droplet diameter (D10, D2o, and D30) are some most common mean droplet diameters ... 

Table 4.1. Definitions of Mean Droplet Diameters and Their Applications 423 ...

Various correlations for mean droplet sizes generated by air-assist atomizers are given in Table 4.6. In these correlations, mA is the mass flow rate of air, h is the height of air annulus, tf0 is the initial film thickness defined as tj ) = dQw/dan, d0 is the outer diameter of pressure nozzle, dan is the diameter of annular gas nozzle, w is the slot width of pressure nozzle, C is a constant related to nozzle design, UA is the velocity of air, and MMDC is the modified mean droplet diameter for the conditions of droplet coalescence. Distinguishing air-assist and air-blast atomizers is often difficult. Moreover, many... 

An acoustical particle counter for counting and sizing fog droplets has been evaluated by Singh and Reist.161 Fog droplets, mostly in the size range of 5-30 pm, were measured by the acoustical particle counter as well as an optical and an electron microscope for comparison. The mean droplet diameters estimated from the acoustical particle counter were in agreement with the microscope values. A Rich 100 condensation nuclei monitor was also operated simultaneously during the fog droplet counting to monitor condensation nuclei counts. 

Minimum packing size. For reproducible results the minimum packing size should be such that the mean void height is not less than the mean droplet diameter. As reported by Gayler el alS21 this critical packing size is given by ... 

The absorption of ozone from the gas occurred simultaneously with the reaction of the PAH inside the oil droplets. In order to prove that the mass transfer rates of ozone were not limiting in this case, the mass transfer gas/water was optimized and the influence of the mass transfer water/oil was studied by ozonating various oil/water-emulsions with defined oil droplet size distributions. No influence of the mean droplet diameter (1.2 15 pm) on the reaction rate of PAH was observed, consequently the chemical reaction was not controlled by mass transfer at the water/oil interface or diffusion inside the oil droplets. Therefore, a microkinetic description was possible by a first order reaction with regard to the PAH concentration (Kornmuller et al., 1997 a). The effects of pH variation and addition of scavengers indicated a selective direct reaction mechanism of PAH inside the oil droplets... 

Whereas few actual values of n for sprays from various fuel injectors are reported, it is usually possible to obtain a fairly reliable estimate of x. This is so because x is uniquely related to various mean droplet diameters solely in terms of n, and data for Sauter mean diameter are rather frequently reported. Sauter mean diameter (SMD) is that diameter representative of the surface area per unit volume which is characteristic of the actual spray. 

Many of the averages and mean droplet diameters in current use may be expressed in terms of the moments of the various weighted size distributions. For example, the first moment of the surface-weighted size distribution is given by the expression... 

Figure D3.4.7 Change in cumulative particle size distribution of a 20% (w/v) oil-in-water emulsion stabilized by 2% (w/v) Tween 20 at the lower port (A) and upper port (B). (C) Change in mean droplet diameter and volume fraction of the emulsions as a function of time.

In (5) D00 is the median diameter and a is the standard deviation of the distribution. By fitting the experimental R-values, the parameters D0 0 and a can be determined and hence the size distribution of the droplets in the emulsion can be obtained. For microbiological safety aspects Dj 3 is more important. D3>3 is the volume weighted mean droplet diameter and a is the standard deviation of the logarithm of the droplet diameter. The parameter D3 3 is related to the parameter D00 according to ... 

Figure 6.21 The influence of emulsifier concentration on the relative viscosity of sorbitan mono-oleate stabilised W/O emulsions in paraffin. The emulsions had dispersed phase volume fractions in the range 0.37 to 0.68 and mean droplet diameters, am, as plotted along the x-axis. From data in Sherman [215].

From the theoretical point of view the key problem of the membrane emulsification is to explain and predict the dependence of the mean droplet diameter, Dmembrane emulsification parameters. Important quantities such as droplet-formation time can thus be successively predicted by the mean droplet diameter and disperse-phase flux. 

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. 

---