Mean droplet size influencing parameters

Both effects can produce coarser atomization. However, the influence of Hquid viscosity on atomization appears to diminish for high Reynolds or Weber numbers. Liquid surface tension appears to be the only parameter independent of the mode of atomization. Mean droplet size increases with increasing surface tension in twin-fluid atomizers (34). is proportional to CJ, where the exponent n varies between 0.25 and 0.5. At high values of Weber number, however, drop size is nearly proportional to surface tension. 

Various correlations for mean droplet size generated by plain-jet, prefilming, and miscellaneous air-blast atomizers using air as atomization gas are listed in Tables 4.7, 4.8, 4.9, and 4.10, respectively. In these correlations, ALR is the mass flow rate ratio of air to liquid, ALR = mAlmL, Dp is the prefilmer diameter, Dh is the hydraulic mean diameter of air exit duct, vr is the kinematic viscosity ratio relative to water, a is the radial distance from cup lip, DL is the diameter of cup at lip, Up is the cup peripheral velocity, Ur is the air to liquid velocity ratio defined as U=UAIUp, Lw is the diameter of wetted periphery between air and liquid streams, Aa is the flow area of atomizing air stream, m is a power index, PA is the pressure of air, and B is a composite numerical factor. The important parameters influencing the mean droplet size include relative velocity between atomization air/gas and liquid, mass flow rate ratio of air to liquid, physical properties of liquid (viscosity, density, surface tension) and air (density), and atomizer geometry as described by nozzle diameter, prefilmer diameter, etc. 

Generally, the mean droplet size is proportional to liquid surface tension, and inversely proportional to liquid density and vibration frequency. The proportional power index is —1/3 for the surface tension, about -1/3 for the liquid density, and -2/3 for the vibration frequency. The mean droplet size may be influenced by two additional parameters, i.e., liquid viscosity and flow rate. As expected, increasing liquid viscosity, and/or flow rate leads to an increase in the mean droplet size,[13°h482] while the spray becomes more polydisperse at high flow rates.[482] The spray angle is also affected by the liquid flow rate, vibration frequency and amplitude. Moreover, the spray shape is greatly influenced by the direction of liquid flow (upwards, downwards, or horizontally).[482]... 

Xiong et al. [44] developed a three-dimensional model of droplet-gas two-phase flow and smdied the evolution of spray downstream along the exit orifice in an effervescent atomizer. The model was used to calculate the mean size and statistical distributions of atomized droplets under various operating conditions. Their key results show that the gas to Uquid mass ratio is one of the most important control parameters and increasing this parameter will decrease the droplet size gradually and finally tend to a certain limitation. They also foimd that a decreasing nozzle exit favors primary breakup, while high injection pressure has more influence on the secondary atomizatiOTi. 

A few distributions of VCM suspensions in water viewed by light microscopy into specially designed pressure cells appear in the literature (23,24), but no analyses of droplet size distribution under different conditions of reactor agitation or polymeric additive addition have been reported. A technique for fixing VCM emulsions by osmium tetroxide (25) may prove useful to study the VCM/water system in greater detail. Mersmann and Grossmann (26) have studied the dispersion of liquids in non-miscible two-phase systems, which include chlorinated liquids such as carbon tetrachloride in water. The influence of stirrer type and speed on the development of an equilibrium droplet size distribution is discussed. Different empirical relationships to calculate the Sauter mean diameter of droplet distributions from reactor operating parameters are also reviewed. 

Influence of Emulsifier Concentration Emulsions of small droplet sizes and narrow droplet size distributions can be obtained at high emulsifier concentration. Figure 13.12 shows the volume density distributions of emulsions of a disperse phase fraction of cp = 72% and two different emulsifier (Tween 80) concentrations, 2.4% and 4.6%. The production parameters were trans-membrane pressure difference of 12 bar, three passes and membrane mean pore size of 0.8 pm. In both cases the emulsifier concentration is above the critical micelle concentration (CMC). 

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