Modellierung von Tropfen/Partikel-Kollisionen in Sprühvorgängen von Schmelzen
Final Report Abstract
Powders often are produced by atomization of melts, where a spray of molten droplets is created and the droplets typically solidify in flight to form the powder with requested properties. An important powder feature is its flowability that may be impaired due to nonspherical or aggregated or agglomerated particles. Droplet collisions in the spray may e.g. lead to coalescence or sticking or aggregation of the particles. The most important parameter for distinct melt droplet collision outcome is the solidification state of both droplets during binary collision. Drop collisions in an inter-spray arrangement: The collision process of solidifying melt droplets in sprays has been studied. Therfore, the outcome of melt droplets collisions in an intersecting spray (inter-spray) configuration has been investigated. An apparatus that forms two intersecting melt sprays with atomized wax has been developed. Spray experiments with Carnauba wax that is separately dyed in yellow and red for the two sprays are performed. The sprayed particle product is charaterized. The results illustrate a characterization of collision outcomes of the atomized Carnauba wax in an inter-spray. Particle size distribution with histogram and cumulative fraction representation, with location, standard deviation and peakedness descriptors have been determined. The atomization experiments confirm the strong influence of the spray intersection angle (a=0°~45°) on mass and particle size distribution. It is shown that increasing the spray inclination angle leads to an increase of the total collision frequency in the spray and the percentage and width of the particle size distribution measured by its standard deviation. With increasing the spray inclination angle (from 0° to 45°), the overall number and percentage of in flight collided particles of the two sprays (yellow and red) will increase from 4% to 16%. Binary melt droplet collisions: Detailed analysis of binary melt droplets collisions has been investigated in the second part of the project. The material for the melt droplet collision investigation is Carnauba wax with a melting point of app. 85°C. Numerical models of heat transfer and solidification of wax droplets in flight were established. The motion and thermal history including solid fraction of droplets in the spray process are calculated. For the present processing conditions, the Carnauba wax droplets of 380pm are completely solidified at a flight distance of 1.6m. Due to the large size of the droplets, binary droplet collision processes can be performed only in the liquid-liquid state. With increasing impact parameter, the collision process changes from head-on to off-center collision. After the collision, there is a rotation of droplets caused by the angular momentum of droplets which plays an important role in the process of stretching separation and formation of secondary particles after the collision.
Publications
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Droplet Collisions in Overlapping Melt Sprays. Proc. Spray Deposition and Melt Atomization Conf. SDMA 2009, 7.-9. Sept. 2009, Bremen, Germany
Gao, S.; Fritsching, U.
