Nozzle-surface distance, influence

In the absence of surface tension influences, the drop formation at vertical orifices is expressed by the equation given for bubble formation. The force due to kinetic energy of the liquid is neglected as its component is zero in the vertical direction. The drop ascends right from the beginning according to the equation of motion and detaches when it has covered a distance equal to the diameter of the nozzle. 

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. 

Martin [87] provides design correlations for multiple-slot and round jets besides recommendations for the spatial arranganent of jet nozzles on the basis of maximizing heat transfer per unit fan energy. The optimal ratio of the pitch of the nozzles compared with the distance above the surface (0.7), which is recommended by Martin, is close to the reported critical value at which jet-to-jet interactions start influencing the heat transfer at the stagnation point under the jet axis [84]. 

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