Spatially-resolved characterization of oil-in-water emulsion sprays. (December 2021)

• Record Type: Journal Article
• Authors: Li, Cheng; He, Ruichen; He, Zilong; Kumar, S. Santosh; Fredericks, Steven A.; Hogan, Christopher J.; Hong, Jiarong
• Is Part Of: International journal of multiphase flow. Volume 145(2021)
• Publication Date: 2021-12
• Subjects: Spray -- Oil-in-water emulsion -- Droplet size distribution -- Joint velocity-size distribution -- Joint size-eccentricity distribution -- Digital in-line holography; Multiphase flow -- Periodicals; Écoulement polyphasique -- Périodiques; Multiphase flow; Periodicals; 620.1064
• DOI: 10.1016/j.ijmultiphaseflow.2021.103813
• Languages: English
• ISSNs: 0301-9322

Abstract

Highlights: Systematic study of influence of oil-in-water emulsion on spray droplet generation. Imaging of spray breakup and 3D droplet distribution via digital inline holography. Oil emulsion suppresses the formation of small ligaments and thus small droplets. Oil emulsion decreases drop eccentricity by changing lamella sheet characteristics. Oil emulsion induces earlier spray breakup and leads to higher droplet velocities. Abstract: When an oil-in-water emulsion is utilized for application in a flat-fan spray, micrometer-sized oil droplets are thought to facilitate hole formation on the spray lamella, leading to an earlier breakup of the spray sheet and an increase in resulting droplet sizes. However, prior work has largely focused on changes to the mean or median droplet size, and has not examined the influence of oil-in-water emulsions on the complete droplet size distribution, as well as other droplet properties such as droplet shape and droplet velocity distributions in flat fan sprays. This study concerns the effects of pressure, spatial location, and application of oil emulsions on the resulting droplet size, eccentricity, as well as velocity distributions, all of which are crucial information in determining the dispersion dynamics of the droplets during the spray applications. Experiments were conducted with a TP6515 nozzle, with the abovementioned droplets information measured using digital inline holography (DIH). Results show that the DIH-inferred volumetric droplet size distributions (VDSD) span widely from sub-100 μm to over 2 mm in size. The application of an oil-in-water emulsion results largely in suppression of smaller droplets, while the VDSD is relatively insensitive to increasing the oil volume fraction beyond a critical level. DIH results are consistent with the observation that smaller ligaments are observed only in the breakup regions for the single-phase water sprays, which break up more violently and earlier than larger ligaments. It is these smaller ligaments that are suppressed in the oil-in-water emulsion, which yield sub-millimeter droplets, some with sizes approaching sub-100 μm dimensions, corresponding to driftable sizes in agricultural settings. Interestingly, the application of oil-in-water emulsion generally decreases the eccentricity more significantly at the center than at the edge of the spray fan. We attribute this decrease to the increase in lamella sheet thickness and thus decrease in characteristic shrinkage rates, consistent with observations of high speed shadowgraphs. In all instances, oil-in-water emulsion droplets have higher velocities than equivalent sized water droplets. We attribute this to the earlier action of the spray breakup process in the oil-in-water emulsion, reduced surface energy generation during breakup (larger droplets), and reduced energy dissipation during breakup with oil-in-water emulsions, leading to increased translational energy after the breakup process. Therefore, it appears that oil-in-water emulsion application simultaneously suppresses small droplet formation and increases droplet velocity, and hence spray penetration in agricultural application.