Development of New Gas Atomizer Nozzle System for the Production of Fine Metal Powder used in Power Metallurgy

在气体雾化方法中，用超音速自由射流将熔融金属雾化。然而，熔融金属液滴的破碎过程尚不清楚，工业雾化系统的设计是通过经验考虑进行的。本研究的目的是用基于激光的新诊断方法来阐明超音速喷嘴的破碎过程，开发具有特殊出口形状的超音速喷嘴，并建立数值预测超音速射流详细结构的方法。主要研究成果如下：(1)提出了一种观察水滴破碎过程的新方法。它使用的是波长为355 nm的三倍频激光。用该紫外光激光器搭建了激光全息照相系统，并用已知直径的标准粒子进行了测试，获得了10%的精度。(2)为了检验超音速小孔出口形状对水滴破碎的影响，制作了沿小孔直径方向延伸50微米的小孔。从全息测量中发现，这种线形孔板产生的水粒子直径比普通孔板小10%~20%。通过实验我们认为，通过改变喷口或喷嘴的出口形状，液滴的尺寸是可以控制的。(3)为了阐明超音速自由射流的详细结构，开发了求解欧拉方程的TVD格式，并对滞止与环境压力之比高达1500的射流进行了计算。将该格式推广到边界贴合网格计算，提高了计算精度。并开发了三维计算方案。

In the gas atomization method, the molten metal is atomized by supersonic free jets. However, the breakup process of the melted metal droplet is not known and the industrial atomizer system is designed through the empirical consideration. The aims of the present research are to clarify the breakup process with the laser-based new diagnostic method, to develop the supersonic nozzle with a special exit shape and also to establish the method for numerically predicting the detailed structure of supersonic jets. The following results are obtained.(1) To observe the breakup process of the water droplet, the novel method is developed. This uses the THG of the Nd : YAG laser (wavelength 355 nm). The inline laser holography system with this ultraviolet laser is constructed and tested with the standard particle with known diameter, and the accuracy of 10% is obtained.(2) To check the effect of the exit shape of a supersonic orifice on the breakup' of a water droplet, the orifice is made that has 50 micron-wire stretched over the diametrical direction of the orifice. It is found from the holography measurements that this wired-orifice produces water particles with the diameter 10 to 20% smaller than the normal orifice. We believe from this experiment that the droplet size is controllable by changing the exit shape of the orifice or nozzle.(3) To clarify the detailed structure of the supersonic free jets, the TVD scheme is developed for solving the Euler equations, and the jets with the ratio of stagnation to ambient pressure of up to 1500 are calculated. This scheme is extended to calculated with the boundary fitted grid to increase the accuracy. The three-dimensional calculation scheme is also developed.