| Gas atomization becomes the most important technology for producing powder materials. However, the theory of the atomization isn’t systematic and most of the researches on the atomization technology are at the level of experience yet. In this paper, the flow fields using eleven types of atomizers have been simulated through the model of computational fluid dynamics (CFD), respectively. Furthermore, the results of simulation have been proved by abundant experiments on preparing magnetic powders. A comparative study of the pros and cons of eleven kinds of atomizer has been carned out. The structure parameters about Laval nozzle of atomizer and process parameters of atomization have been improved. Finally, the quantitative relationship among atomization pressure P, cooling rate V and the cooling time t has been built up. Based on above main contents, several important conclusions can be summarized as follows:Various atomization nozzles are classified according to their structural characteristics, like pressure, velocity and temperature which can be calculated by discrete phase models (DPM). The results suggest that the mechanism of atomization can be divided four kinds in the eleven types of atomizers. The four categories are:up spray atomization and horizontal atomization; the other two categories are positive spray atomization and negative spray atomization according to the values of their contours curvature. Furthermore, to study the stability of atomization process various flow field are analyzed, and to study the ability of atomizers to crush objects Shock wave and kinetic energy are analyzed. The results show that the Laval nozzle has the strongest ability to atomization.The geometry structure parameters of Laval nozzle were optimized based on the DPM. When the influence on atomization from process stability, crushing effect of particle and Mach disc are considered, under the condition of the specific magnetic powder the results suggest that as spray material the optimal structure parameters of the atomizer should be chosen as following: the spay angle is 45, the diameter of delivery tube is 6mm, the height of the protrusion delivery tube is 6mm, the opening shape of nozzle is shown as arc concave, gas path is smooth and the best gap is 0.2 mm. The order of influence on the flow field during the atomization process is that:the height of the protrusion delivery tube> gap> the opening shape of nozzle> the spay angle> the smoothness of gas path> the diameter of delivery tube. Compared with the unoptimized nozzle, the size of powders is refinement obviously using optimized nozzle, the under the same conditions, and D50 reach to 14.5μm.The process parameters of atomization using the Laval nozzle were optimized through the DPM. The results suggest that the best atomization pressure is in the range of 35-70atm, and the ’atomization pressure should be chosen different value according to the atomization medium and the impact of flow field transition, like transition of open-closed wake. The numerical analysis shows that the superheat is bigger, the effect of atomization is better. However, the superheat is bigger, the melt oxidation is more obvious. Hence, the proposed value of superheat is controlled in the range of 150-250K, based on the compromise of melt oxidation and actual production, Besides, the effects on atomization the nitrogen and argon have not difference in many kinds of gas according to the results. In contrast, helium has a great advantage to atomization. To improve atomization, low melting point and low viscosity materials should be chosen. The significant order of effects from process parameters on flow fields is atomization pressure> superheat> the sort of gas> the nature of melting materials.For the above nozzle, the quantitative relationships of the atomization pressure, cooling rate and the cooling time are built up. The relationships are described as the following equations V=57.543P0.5194、V=1.28t-1 and t=0.0222P-0.5194, respectively. In this paper, the particle size can be directly predicted in theory using the above relationships combined other equations.In this paper, the atomization process is simulated by the advanced large eddy simulation (LES) model and volume of fluid (VOF) model. The results from the LES model and VOF model can offer at details, but the advantage of the DPM model exhibits on the universality of simulations and computing rate, versus the LES model and VOF model. In this paper, the simulation using the VOF model clearly reproduced the crushing process of atomization. The results show that the location of Mach disc apart from nozzle first increases then decreases. The simulation results using the three models are all show that the secondary breaking don’t present in the atomization process, since the reduction of velocity and temperature of melting material result in improving Weber value. |
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