, system system
where Dvm = volume median droplet diameter C = surface tension, mN/m (dyn/cm) | = liquid viscosity, mPa- s (cP)
The exponential dependencies in Eq. (14-195) represent averages of values reported by a number of studies with particular weight given to Lefebvre ([Atomization and Sprays, Hemisphere, New York (1989)]. Since viscosity can vary over a much broader range than surface tension, it has much more leverage on drop size. For example, it is common to find an oil with 1000 times the viscosity of water, while most liquids fall within a factor of 3 of its surface tension. Liquid density is generally even closer to that of water, and since the data are not clear that a liquid density correction is needed, none is shown in Eq. (14-195). Vapor density also has an impact on dropsize but the impact is complex, involving conflicts of a number of effects, and vapor density is commonly omitted in atomizer dropsize correlations.
Effect of Pressure Drop and Nozzle Size For a nozzle with a developed pattern, the average drop size can be estimated to fall with rising AP (pressure drop) by Eq. (14-196):
AP2 AAP 1
For similar nozzles and constant AP,the drop size will increase with nozzle size as indicated by Eq. (14-197):
D1 Da orifice diameter! orifice diameter2
Once again, these relationships are averages of a number of reported values and are intended as rough guides.
The normal operating regime is well below turbulent breakup velocity. However the data of Kennedy [/. of Engineering for Gas Turbines and Power, 108,191 (1986)] at very high pressure drop in large nozzles shows a shift to a higher dependence on pressure drop. This data suggests that turbulent droplet breakup can also be governing with hydraulic spray nozzles, although this is unusual.
Spray Angle A shift to a smaller-angle nozzle gives slightly larger drops for a given type of nozzle because of the reduced tendency of the sheet to thin. Dietrich [Proc. 1st Int. Conf. Liq. Atomization Spray Systems, Tokyo (1978)] shows the following:
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