Maximum droplet diameter

Droplet size, particularly at high velocities, is controlled primarily by the relative velocity between liquid and air and in part by fuel viscosity and density (7). Surface tension has a minor effect. Minimum droplet size is achieved when the nozzle is designed to provide maximum physical contact between air and fuel. Hence primary air is introduced within the nozzle to provide both swid and shearing forces. Vaporization time is characteristically related to the square of droplet diameter and is inversely proportional to pressure drop across the atomizer (7). 

Hsiang and Faeth[285] also presented measurement data for Dc max / Dini as a function of Werf in graphical form for Ohc/= l. 5 and 3.1. Thus, the maximum cross-stream droplet diameter over a range of Oh.d may be determined by interpolating these data. 

Analytical and empirical correlations for droplet sizes generated by ultrasonic atomization are listed in Table 4.14 for an overview. In these correlations, Dm is the median droplet diameter, X is the wavelength of capillary waves, co0 is the operating frequency, a is the amplitude, UL0 is the liquid velocity at the nozzle exit in USWA, /Jmax is the maximum sound pressure, and Us is the speed of sound in gas. Most of the analytical correlations are derived on the basis of the capillary wave theory. Experimental observations revealed that the mean droplet size generated from thin liquid films on... 

A two-color pyrometer has been used along with the phase-Doppler anemometer to simultaneously measure the local velocity and size of kerosene droplets and the temperature of burning soot mantle in a swirl burner.[648] The measurements were conducted within the flame brush that develops in the shear layer of a swirl-stabilized, gas-supported kerosene flame with a swirl number of about 0.19 and potential heat releases of 10.6 and 15.5 kW, respectively. The results showed that the maximum burning fraction of the droplets occurs adjacent to the region denoted as gas flame but the value ranges from 20 5 to 40 5% depending on the axial station, and decreases sharply across the shear layer. The flame mantle temperature was found to be independent of droplet diameter, which agrees with previous results in the literature. 

The design-base particle diameter to be separated in the vapor space should not exceed the pad-disengage-mt-nt droplet diameter. 1). To be on the safe side, a maximum droplet diameter should be limited to 400 pm. In horizontal drum design, the mesh pad should be regarded as a polisher for removing small droplets not separated in the vapor space—i.e., as a secondary mechanism in vapor-liquid separation... 

If there, were sufficient time for coalescence to occur in piping downstream of die dump valve, then maximum droplet diameter would be defined by tlie dispersion equation prior to water entering the first vessel in the water treating svsiern. 

An approach similar to that taken by Nomura and Harada was used by Samer to quantify the effects of droplet nucleation on emulsion polymerization kinetics in a CSTR. In their simplified analysis, it was assumed that radical capture by particles and droplets is proportional to the ratio of particle and droplet diameters. This assumption is reliable at low to moderate residence times, when polymer particles still closely resemble monomer droplets with respect to composition and surface characteristics. For predominant droplet nucleation, the maximum particle generation is limited by the concentration of monomer droplets in the feed. In Fig. 11 the steady state particle generation is given as a function of the residence time and temperature. Nucleation efficiency is defined as the number of particles divided by the number of droplets in the... 

Molag [375] actually found no droplets larger than d/dy2 = 2 in baffled tanks with 6-blade turbine stirrers. Thus the maximum stable droplet diameter is attained for dp 2d32, which corresponds to the critical Weber number. 

Expression (6.19) only apphes, if dp is much smaller that the Kolmogorov microscale and the viscosity of the dispersed phase is small (yr < 10 mPa s). With increasing /tj the resistance to deformation grows. Arai [5] determined, for > 100, the theoretical relationship for the maximum stable droplet diameter to be ... 

Since a droplet ruptures at values of the Weber number which exceed the critical value, Eq. (12-27) may be solved for the maximum value of the droplet diameter to give... 

All the curves in Figure 15.5 pass through a maximum. These maxima occur at the critical droplet diameter, Dpc,... 

Design methods are discussed elsewhere (417) and are based on limiting the maximum diameter of droplets entrained in the vapor stream. Maximum droplet diameter can be reduced by supplying a knockout facility upstream of the compressor. This can be either an enlargement of the vapor space above the top tray, an in-line liquid separator, or a separate knockout drum. Mist eliminators are effective and can be installed above the top tray or in the drum. If the vapor temperature is higher than the worst ambient conditions, the lines from the knockout facility to the compressor should either be kept short and insulated or be provided with liquid-removal facilities. 

The diameter of the droplets produced by a pneumatic nebulizer varies from < 5 ixm to about 25 xm. The spray chamber allows droplets to reach the burner which can be vaporized and atomized in the flame. If the spray chamber prevents small droplets (diameter of about 10 tm or less) from entering the flame, sensitivity will be decreased. On the other hand, if large droplets (>10/x-m) reach the flame, the flame noise will increase and the temperature will decrease. From the total mass of the sample nebulized, the maximum useful amount of droplets is about 10%, which gives the limit for the maximum attainable efficiency of the nebulizer. However, the nebuliza-tion efficiency can be improved in various ways by altering the droplet size distribution. A bead or bar placed close to the orifice of the nebulizer, or a counter flow nebulizer can be used for this purpose (Figure 37). 

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