Lydia Achelis

Atomization Using a Pressure-Gas-Atomizer

An update and the latest results on molten metal atomization using a Pressure-Gas- Atomizer will be given. This atomizer combines a swirl-pressure atomizer, to generate a liquid hollow cone film and a gas atomizer to atomize the film and/or the fragments of the film. The paper is focused on powder production, but this atomization system is also applicable for deposition purposes. Different alloys (Sn, SnCu) were atomized to study the characteristics of the Pressure- Gas-Atomizer. The powders produced were analyzed by laser diffraction and image processing. Among other parameters, the molten metal mass flow (~140 – 200 kg/h), the gas mass flow and the atomizer design were varied. The results include the effects of these variances on particle size and particle shape.

Two Fluid Atomization Fundamentals

In this chapter we review the major types of atomization process configurations: free-fall gas atomization with an unconfined melt stream and close-coupled gas atomization with controlled melt introduction to an energetic gas flow. Studies will be reported of several types of devices, termed atomization nozzles, which are used to perform two-fluid atomization processes that involve the disintegration of a molten metal by interaction with a high velocity atomization gas. The resulting atomization process is a complex physical phenomena consisting of stages that start with melt stream pre-filming and distribution to the primary atomization zone, where melt sheets or ligaments form and initial droplet breakup (primary atomization) occurs by the interaction of a high density, hot melt with a high velocity (high kinetic energy, but low temperature) atomization gas, typically. Primary atomization is followed in the near-field region by secondary breakup, if a high enough gas velocity and sufficient mismatch velocity with the melt fragments are maintained to cause significant production of further droplets. Thus, the atomization processes described in this chapter essentially involve momentum and heat exchange between gas and melt, while other chapters will discuss the subsequent processes of droplet solidification, droplet-droplet or particle-droplet collisions and other spray phenomena that are important to spray deposition. Primarily, this chapter will deal with our state of understanding of melt breakup physics and the various types of gas atomization nozzles that can be used to generate an atomized molten metal spray.

Novel Atomization Process for Large Scale Nanoparticle Production by Flame Spray Pyrolysis

An advanced atomizer concept to obtain larger production rates of nano-particles by the Flame Spray Pyrolysis process (FSP) is investigated. In the conventional FSP process (external mixing gas/liquid nozzle) production rates may be varied by increasing the precursor feed rate and/or the precursor concentration. However, both measures typically result in the formation of larger nanoparticles. These effects may be avoided by the development and integration of advanced atomizer concepts. The aim is to address the spray structure in a way that keeps the flame height constant and modifies the flame width. Therefore, the time scales and the residence time-temperature histories of droplets and nanoparticles are expected to be similar while the production rate is increased. The atomizer concept for creation of a modified spray and flame combines a swirling liquid film generation that is atomized with an external swirling gas flow.In the first step a hollow cone of liquid ligaments and primary droplets is generated through a conventional pressure-swirl nozzle. The liquid phase is atomized in the second step, by the expanding gas of a circular ring nozzle. To study the main characteristics of the combined atomizer in model experiments, water and water/glycerol mixtures are used as the liquid phase and air as the gas phase. For investigation of the atomizer and spray properties, the relation between liquid outlet angle, inlet angle of the gas, the gas/liquid flow rates, the spray cone geometry and droplet size distribution are investigated. The spray structure and the breakup of the film are analyzed by high speed images. Laser diffraction is used to measure the droplet size distribution in the spray.A numerical model is developed and used to simulate the cold gas flow and spray distribution as in the adapted atomizer concept. The Eulerian-Lagranian approach is solved by means of a computational fluid dynamics (CFD) code. The process parameters such as liquid composition, liquid and gas flow rates are varied to meet the specific requirements of the nanoparticle production in the FSP process. The experimental and numerical investigation showed that an enlarged and steady spray resulted from an increased outlet angle of the liquid and gas swirl. Increased tangential velocities increase the entrainment of surrounding gas, widening and providing a more uniform velocity profile to the spray. Spray droplet mean diameters resulted in the desired range of ≤ 20 μm.Copyright

Influence of process parameters on particle characteristics using a combined pressure-swirl-gas atomizer

Abstract This paper summarizes the latest results in the development of a combined pressure-swirl-gas atomizer. The main goals of this investigation are to characterize the generated metal powders in terms of spray pattern and particle size distribution. With the combined pressure-swirl-gas atomizer it is possible to create 150kgh−1 of tin powder with mass medians between 30 and 110lm (tin alloys) and geometric standard deviations below 1.9 with a nitrogen consumption of 100kgh−1. Gas flow and melt mass flow can be varied independently of each other, while their characteristic ratio of mass flows can be held below 0.7, which opens new potential for low gas consumption in gas atomization. The experiments were conducted with water and tin melts as model fluids. The comparison of the dimensionless numbers shows that the model experiments are comparable and basically extendable to other melts as long as the material characteristics allow this.