Droplet solidification

Solidification. The liquid forming the droplet can be melted and a reduction of temperature will result in droplet solidification. In practice, solidification is usually combined with prilling and spraying (but is easily combined with emulsification)... 

Process conditions should be adjusted in order to avoid the particle disintegration and to prevent its adherence in the chamber. The time required to promoted the droplet solidification depends on the speed assumed by the droplet, the particle size and particle size distribution,specific heat of the material, temperature and speed of the cooling air (Cordeiro et al., 2013). 

Poliak, S., Petermann, M., Kareth, S., Kilzer, A. (2011). Thermal analysis of the droplet solidification in the PGSS-process. Journal cf Supercritical Fluids, 56, 299—303. 

Fig. 18.29 Droplet cooling and solidification in the PSGA process (a) droplet cooling rate in liquid state and (b) droplet solidification as a function of atomization gas pressure...

Along the droplet trajectory, the gas-droplet relative velocity, the particle number concentration, the Stokes number, and the droplet solidification degree at the droplet location are extracted as shown in Fig. 18.55a-d, respectively. Based on the information above and given a time interval Af= 0.0001 s, the accumulative particle-droplet collision number Q can be derived along the droplet path by use of (18.37). 

Fig. 18.64 Schematic of particle sticking on matrix particle surface and incorporation into matrix particle during matrix droplet solidification...

The other group of collisions will result in particle partial penetration according to the Re—We regime map. Since most of colhsions in this regime occur in the downstream spray flow, it is considered reasonable that most of the solid particles are likely to be captured by the droplet surface because of droplet solidification. The sticking efficiency of particulate reinforcements on the surface of MMC particles can be roughly described by the volume fraction of these particles to the droplet. 

During the formation of a spray, its properties vary with time and location. Depending on the atomizing system and operating conditions, variations can result from droplet dispersion, acceleration, deceleration, coUision, coalescence, secondary breakup, evaporation, entrainment, oxidation, and solidification. Therefore, it may be extremely difficult to identify the dominant physical processes that control the spray dynamics and configuration. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Waldvogel, I Diversiev, G. Poulikakos, D. Megaridis, C. Attinger, D. Xiong, B. Wallace, D. 1998. Impact and solidification of molten-metal droplets on electronic substrates. J. Heat Transfer 120 539. 

This process, originally designated as RSR (rapid solidification rate), was developed by Pratt and Whitney Aircraft Group and first operated in the late 1975 for the production of rapidly solidified nickel-base superalloy powders.[185][186] The major objective of the process is to achieve extremely high cooling rates in the atomized droplets via convective cooling in helium gas jets (dynamic helium quenching effects). Over the past decade, this technique has also been applied to the production of specialty aluminum alloy, steel, copper alloy, beryllium alloy, molybdenum, titanium alloy and sili-cide powders. The reactive metals (molybdenum and titanium) and... 

To ensure that a liquid metal is completely disintegrated into droplets before the initiation of any solidification in the gap, the heat transfer between the liquid metal and the rolls must be minimized. Materials with low thermal conductivity and poor wetting properties have been found to have the most significant effect on reducing the heat transfer. Epoxy and phenolic resin coatings have been applied to... 

In both atomization modes, as thin unstable ligaments, and/ or sheets disintegrate into round droplets, atomization gas may plausibly be trapped into the droplets under certain conditions. For alloys with alloying elements which readily react with atomization gas, for example, oxidize to form refractory oxides, solidification may be delayed and spheroidization is prevented so that rough flakes may form. For such alloys, the atmosphere in the spray chamber must be inert and protective to avoid the formation of any refractory and to foster spheroidal shape of droplets. 

Droplet Formation in Water Atomization. In water atomization of melts, liquid metal stream may be shattered by impact of water droplets, rather than by shear mechanism. When water droplets at high velocities strike the liquid metal stream, some liquid metal fragments are knocked out by the exploding steam packets originated from the water droplets and subsequently contract into spheroidal droplets under the effect of surface tension if spheroidization time is less than solidification time. It is assumed that each water droplet may be able to knock out one or more metal droplet. However, the actual number of metal droplets produced by each water droplet may vary, depending on operation conditions, material properties, and atomizer designs. 

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