428 Science and Engineering of Droplets

positions of the droplets in the light beam. However, the Malvern particle sizer is capable of measuring droplet sizes only within a certain range, and inaccuracies may be incurred when the droplet sizes in the spray being measured are beyond the size range corresponding to the receiver lens used. Other potential problems, that may influence the data accuracy and repeatability of the Malvern particle sizer, may include variations in detector sensitivity/ responsivity,[698] beam steering,[699] multiple scattering,[700] and vignetting,[698] as reviewed by Lefebvre.[1]

Ridder et al.[676] used a laser diffraction instrument, operating on the principle of the Fraunhofer diffraction, to measure the size distribution of molten metal droplets in gas atomization. The instrument provided adequate time resolution for high gas flow rates (~0.18 kg/s or 230 scfm), particle concentrations (~106 particles/cm3), and short run times (~2–5 min) typical of the supersonic inert gas metal atomization process. The instrument has a dataacquisition repetition rate of 30 scans/s and a resolution of 10 ms/ reading. Mathematical modeling of the Fraunhofer diffraction was conducted, addressing the light-scattering phenomena and relating the particle size distribution vector to the light energy distribution vector due to the diffraction from the particles. An inverse transformation method capable of converting the measured light intensity distribution into the particle size distribution was developed on the basis of pattern recognition using the approach of feature map and content-addressable memories. Experiments were conducted to evaluate this novel pattern recognition scheme, and its performance was found to be fairly high for real-time particle size analysis.

6.1.4Other Methods

In addition to the techniques outlined above, many other methods have been developed for droplet sizing. These include, for example, intensity ratio method, phase optical-microwave method, and dual-cylindrical wave laser technique,[701] etc.

Measurement Techniques for Droplet Properties 429

Intensity ratio method for particle sizing is based on the ratio of measured light intensities at different angles and the dependence of the ratio on particle size.[702][703] Droplets in a spray scatter light in all directions when a light beam is passed through the spray. The relative intensity of the light scattered by droplets in the forward direction is a function of angle and may be calculated on the basis of the diffraction theory. The method is fairly insensitive to refractive index. However, the measurement size range is limited. An intensity ratio system for particle sizing has been developed by Spectron.

Zemlianskii and Yanovsky[704] suggested a strategy for sizing aerosols and water droplets in air using multiwavelength and multisensor technique. They conducted analyses on the basis of the Mie and Rayleigh theories. The phase optical-microwave system with fixed differential frequency shift between two beams of different wavelengths consists of two microwave detectors and two detector receiving optics. The proposed optical-microwave measurement method, theory and numerical treatment are based upon the assumptions that the particles to be sized are spherical and traverse a uniform field of plane waves.

An acoustical particle counter for counting and sizing fog droplets has been evaluated by Singh and Reist.[6] Fog droplets, mostly in the size range of 5–30 µm, were measured by the acoustical particle counter as well as an optical and an electron microscope for comparison. The mean droplet diameters estimated from the acoustical particle counter were in agreement with the microscope values. A Rich 100 condensation nuclei monitor was also operated simultaneously during the fog droplet counting to monitor condensation nuclei counts.

Korobochka and Pavlenko[705] proposed a simple model and nozzle design for the determination of exact droplet size distribution generated by an air-assist nozzle. The approach enables the direct measurements of droplet size and allows generation of a very narrow range of droplet size distribution.