Jane P. Fife and Vladimir Kogan
Battelle Columbus Operations
SERG Project #2005/07
Year of Project: 2005
Report Received: 2006
Most liquid atomizers and nozzles used for forestry and agricultural pesticide application produce spatially segregated droplet spectra. Thus, an accurate characterization of the droplet size distribution generated by an atomizer or nozzle requires a detailed survey of the entire spray plume cross-section. An appropriate averaging methodology is necessary to determine the global, mass-weighted droplet size distribution. An adequate mass balance closure would then validate the accuracy of the spray plume’s average droplet size spectra.
A variety of different optical laser-based instruments can be and have been used to size and count spray droplets. Laser diffraction, such as that used in the Malvern and Sympatec instrument s, is the most widely used droplet-sizing technique for forestry and agricultural applications, and is the focus of this report.
Two independent methodologies have been developed for deriving a global, massweighted droplet size distribution from la ser diffraction measurements. The “Concentration- Based” method (Hewitt, 2002) exclusively uses the Malvern laser diffraction instrument’s obscuration readings to weight the droplet size data from each measurement location according to the droplet concentration. The “Flux-Based” method (Goguen et al., 1997) extends the spatially-derived Malvern measurements with flux-based calculations using droplet velocity so that the mass balance closure criteria can be evaluated.
Realization of differences in average droplet spectra between the two methods has raised questions within the forestry and agricultural spray communities regarding the accuracy of either approach, as well as the appropriateness of even using the laser diffraction technique for achieving mass closure. Therefore, the principal objectives of this study were: 1) to independently evaluate each methodology, and 2) to develop recommendations for future wind tunnel-based spray characterization based on results from the methods analysis and a review of literature.
To gain a better understanding of each method, the Concentration-Based and Flux-Based protocols were each executed using the same set of raw Malvern data provided in Goguen (1994). The resulting volume median diameters (VMD) derived from each approach varied by 5 um.
The primary difference between the methods (and presumably the reason for the two different VMD results) is that the Flux-Based approach attempts to account for the velocity of the droplets in the laser diffraction data. Droplet velocity influences spatially-derived measurements due to the residence time of in- flight droplets in the laser sampling volume. The Flux-Based method computed a smaller VMD compared to the Concentration-Based method, which was consistent with theory.
A thorough literature review was conducted to investigate the state-of-the-art in using optical instruments for determining the droplet size-resolved mass balance of sprays. Techniques used to sample the spray plume, averaging procedures used for data processing, and the capabilities of optical instruments currently on the market were evaluated. A separate literature review was conducted to identify wind tunnel techniques being used to measure droplet size distributions of sprayers under aerial application conditions.
Based on the synthesis of results from the methods analysis and literature review, it was assessed that no single experimental set-up can account for all spray testing scenarios; however, some general guidelines were developed and outlined in a flowchart diagram. The type of optical instrument and sampling technique employed will depend on the objectives of the test (mass balance closure requirement), spray characteristics (droplet size-velocity correlation; droplet size range; droplet density), and the experimental conditions (wind speed profile; type of atomizer or nozzle; spray liquid formulation).
In general, the most practical approach for conducting wind tunnel tests of spray atomizers is to use a laser diffraction instrument positioned sufficiently far downstream from the atomizer such that droplets have approached the wind velocity. In this way, the measurement bias due to droplet residence time within the laser sampling volume can be minimized. Depending on the spray density, then either a continuous, full-scan of the plume cross-section can be conducted, or in denser sprays, fixed, chordal measurements are necessary. In the latter case, an appropriate averaging procedure must be applied to the data.
Finally, it is recommended that the forestry and agricultural spray communities assess the importance of the mass balance closure criteria as part of a spray droplet size measurement standard. Mass balance closure effectively provides an indication of the overall accuracy of the droplet size measurements and averaging procedure by comparing the mass flux of droplets through the measurement plane with the supply mass flow rate. This requires that either a temporal-based instrument (such as phase-Doppler or optical array probe) be used for droplet
size measurement, or that a spatially-derived sample be converted to a flux-basis. The practical issues involved with using a phase-Doppler instrument (i.e., extent of time and formulated material required to conduct the necessary measurements; inaccuracies associated with measuring emulsion and air inclusion droplets) or an optical array probe (i.e., droplet size resolution of 34 mm) must be considered in developing a standard procedure. The Flux-Based method evaluated in this report is one method that could be employed under certain conditions to conduct mass balance analysis of laser diffraction measurements.