金属熔体雾化喷嘴流场数值研究

发布时间：2018-05-25 16:21

本文选题：磁性粉末 + 喷嘴　；参考：《福州大学》2014年博士论文

【摘要】：雾化法已经发展成为生产粉末材料最重要的方法,但对该方法的研究大多仍停留在经验层次尚未形成一套完整的理论。本文采用计算流体力学方法,对具有代表性的十一种雾化器的雾化流场进行数值模拟研究,并通过磁性粉末的制备进行实验验证。本文比较了十一种雾化器的雾化能力；研究了Laval喷嘴结构参数和工艺参数的优化值；建立了雾化压力P与冷却速度V、冷却时间t三者的定量关系式。得到以下研究成果：根据众多喷嘴的内在特点对其进行分类,再采用离散相模型(DPM)计算出全部喷嘴的的压力、速度和温度场。在此基础上,通过分析研究得出：在所研究的十一种喷嘴的范围内,雾化机理可分为四类。其中,上喷雾化和水平雾化各为一类,其他类型则根据压力云图曲率的正负值分为两类。通过进一步分析各种流场以考察雾化过程的稳定性和破碎效果；分析激波及速度大小以考察雾化器的破碎能力,结果表明Laval型喷嘴雾化能力最强。利用DPM模型对Laval型喷嘴结构几何参数进行优化。综合考虑雾化过程的稳定性、破碎效果及马赫碟对雾化过程的影响,在指定磁粉作为金属熔体条件下,研究结果表明,喷嘴的最佳结构参数应定为喷射夹角为45°,导流管直径为6mm,导流管突出高度为6mm,开口形状为弧凹形,气体通路要求平滑,最佳间隙为0.2 mm。结构参数对流场影响大小的次序依次是导流管突出高度间隙大小开口形状喷射角气体通路平滑度导流管直径。经过优化的喷嘴在相同条件下,磁粉实验产品的形貌较优化前的D50有明显地细化,D50达到14.5μm。利用DPM模型对Laval型喷嘴的雾化工艺参数进行优化。结果表明最佳工艺条件为：压力在35-70atm间选择,根据雾化介质的不同,同时结合考虑流场突变的影响,即开涡-闭涡突变,选择不同的数值；关于过热度的影响,综合考虑熔体氧化和数值分析的结果,建议控制在150-250K之间；气体种类的影响结果表明,氦气具有较突出的优势；熔体则应尽可能选择低熔点、低粘度的材料。工艺参数对流场影响大小的次序依次是雾化压力过热度气体种类熔体性质。针对上述的Laval雾化喷嘴,建立了雾化压力P与冷却速度V、冷却时间t三者的定量关系式,分别为V=57.543P0.5194、V=1.28t-1和t=0.0222P-0.5194。结合其他公式,可直接对产品的粒度在理论上作出预测。本文还使用了更先进的LES模型和VOF模型模拟雾化过程。VOF模型清楚地再现了雾化的破碎过程。在本模拟条件下,马赫碟的位置随着气路间隙的缩小离喷嘴出口的距离呈现出先变远后近的规律。三种模型的模拟结果都没发现二次破碎,这是由于速度衰减及熔体温度下降导致需要更高的韦伯数。
[Abstract]:Atomization method has been developed as the most important method to produce powder materials, but most of the research on this method is still at the level of experience, which has not yet formed a complete theory. In this paper, the numerical simulation of atomization flow field of 11 typical nebulizers is carried out by means of computational fluid dynamics, and the experimental results are verified by the preparation of magnetic powder. In this paper, the atomization capability of 11 atomizers is compared, the optimum values of structure parameters and process parameters of Laval nozzles are studied, and the quantitative relationship between atomization pressure P and cooling rate V, cooling time t is established. The following results are obtained: according to the inherent characteristics of many nozzles, the pressure, velocity and temperature fields of all nozzles are calculated by using the discrete phase model (DPMM). On this basis, the atomization mechanism can be divided into four categories in the range of 11 kinds of nozzles studied. The upper spray and horizontal atomization are classified into two categories, the others are divided into two categories according to the positive and negative values of the curvature of the pressure cloud image. Through further analysis of various flow fields to investigate the stability and crushing effect of atomizing process and the shock wave and velocity to investigate the crushing ability of atomizer, the results show that the atomization ability of Laval nozzle is the strongest. The geometric parameters of Laval nozzle are optimized by DPM model. Considering the stability of atomization process, the effect of crushing and the influence of Mach disc on atomization process, the results show that the magnetic powder is selected as the melt of metal. The optimum structure parameters of the nozzle should be 45 掳angle, 6mm diameter, 6mm projecting height, arc concave shape, smooth gas path and 0.2 mm clearance. The order of the influence of the flow field on the structure parameters is in the order of the diameter of the smooth gas path of the nozzle with the opening of the gap between the projecting height of the tube and the opening of the jet angle. Under the same conditions, the morphology of the magnetic powder experimental product was obviously finer than that of D50 before optimization, and the D50 was up to 14.5 渭 m. The atomization process parameters of Laval nozzle were optimized by DPM model. The results show that the optimum process conditions are as follows: the pressure is selected between 35-70atm, according to the different atomization medium, and considering the effect of the sudden change of the flow field, that is, the open vortex and the closed vortex catastrophe, the different values are selected, and the influence of superheat degree is also discussed. Considering the results of melt oxidation and numerical analysis, it is suggested that it should be controlled between 150 and 250K, and the effect of gas types shows that helium has outstanding advantages, and the melt should choose materials with low melting point and low viscosity as far as possible. The order of the influence of process parameters on the flow field is the melt properties of atomized pressure superheat gas. In view of the above Laval atomization nozzle, the quantitative relationship between atomization pressure P and cooling rate V, cooling time t is established, which are V _ (57.543) P _ (0.5194) V _ (1.28) t ~ (-1) and t _ (0.0222) P _ (0.5194) respectively. Combined with other formulas, the particle size of the product can be directly predicted in theory. More advanced LES model and VOF model are used to simulate the atomization process. Under this simulation condition, the position of Mach disc changes first and then approaches with the decrease of the gap between the gas path and the nozzle outlet. The simulation results of the three models show no secondary breakage, which is due to the decrease of velocity and melt temperature, which leads to a higher Weber number.
【学位授予单位】：福州大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TB44;TG14

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