Droplet velocity

A spray comprises a cloud of liquid droplets randomly dispersed ia a gas phase. Depending on the appHcation, sprays may be produced ia many different ways. The purposes of most sprays are (/) creation of a spectmm of droplet sizes to iacrease the Hquid surface-to-volume ratio, (2) metering or control of the hquid throughput, (J) dispersion of the Hquid ia a certain pattern, or (4) generation of droplet velocity and momentum. 

Mumber Density and Volume Flux. The deterrnination of number density and volume dux requires accurate information on the sample volume cross-sectional area, droplet size and velocity, as well as the number of droplets passing through the sample volume at any given instant of time. Depending on the instmmentation, the sample volume may vary with the optical components and droplet sizes. The number density represents the number of droplets contained in a specified volume of space at a given instant. It can be expressed as follows, where u is the mean droplet velocity, t the sample time, andM the representative cross-sectional area at the sampling location. 

Increasing either the gas velocity or the liquid droplet velocity in a scrubber will increase the efficiency because of the greater number of collisions per unit time. The ultimate scrubber in this respect is the venturi scrubber, which operates at extremely high gas and liquid velocities with a very high pressure drop across the venturi throat. Figure 29-8 illustrates a commercial venturi scrubber unit... 

The hot wire technique appears to be the most useful one among the various electrical methods. However, the hot wire anemometer can operate only at relatively low droplet velocities. 

The Malvern particle sizer is one of the most widely used, most effective, simple, and reliable methods commercially available for rapid measurements of ensemble characteristics of a spray. It is able to handle high droplet concentrations. It is easy to use and does not require comprehensive knowledge of its basic principles for operation. The primary advantage of the system is the speed of data acquisition and analysis. In addition, measurements of droplet size distributions can be made at any droplet velocities due to the fact that the diffraction patterns generated by droplets are independent of the... 

Figure 9 Example of how the type of spray gun used, atomizing air pressure, and gun-to-bed distance can influence droplet velocity.

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