【正文】 ,50176。,45176。,30176。 μ is the dynamic viscosity of the fluid。 C1, C2, and Cm are the experience constant。 YM is the contribution of pulse expansion in pressive turbulence。 GK is the rate of production of kiic energy of turbulence。 (3) inertia force and gravity are included, while mass force and lifting force are ignored。 the focus radius is smaller, the focal distance is also smaller, and the gathering characteristic is better. When the cone ring gap of the coaxial powder nozzle is invariable, the cone angle getting too big or too small is harmful for powder gathering. When other conditions are invariable, extreme protective gas velocity (too large or too small) is harmful for powder gathering. When the protective gas velocity approaches to 6 m/s, the gathering characteristic of the coaxial nozzle achieves its best performance. Keywords Laser metal direct manufacturing . Coaxial nozzle . Numerical simulation . Flow field 1 Introduction Laser metal direct manufacturing (LMDM) technology is an advanced manufacturing technology arising over the last decade [1–3], in which highenergy beam such as laser or electron beam is often used as its heat source, and metallic parts can be fabricated layer by layer on the basis of rapid prototype technology. The coaxial powder nozzle is one of the key technologies in LMDM, which feeds the metal powder uniformly and steadily into the melting pool mainly depending on kiic energy of the gas [4]. Powder and laser beam export simultaneously in coaxial nozzle, which can disperse the metal powder as circularity and then converge into the melting pool, and overe the fault that side powder feeding could be only adopted in linear movement rather than plex track. Furthermore, this nozzle can fit the variation of the scanning direction well, and the isotropy property can be achieved in LMDM technology. Much research work has been carried out in this field. Mazumder et al. adopted the closedloop control to improve the manufacturing accuracy in LMDM process [5]. Smurov applied the direct metal deposition technology to fabricate functionally graded object from metallic powder [6]. Costa et al. established a thermokiic LPD model coupling finite element heat transfer calculations with transformation kiics and quantitative property– 沈陽理工大學學士學 位論文 36 structure relationships to the study of the influence of substrate size and idle time on the microstructure and hardness of a tenlayer AISI420 steel [7]. Das reviewed the mechanism of oxidation, atmosphere control, wetting, epitaxial solidification, metal vaporization under vacuum, and oxide purification to understand these physical mechanisms for the design of direct selective laser sintering machines, process control, and materialsspecific selective laser sintering process development [8]. Wen et al. presented a prehensive model to predict the powder flow structure and laser particle heating process in terms of multiparticle behavior and reveal the characteristics of powder supply for the coaxial deposition process [9]. Miranda et al. used the highpower fiber lasers due to the flexibility in beam position and manipulation to control surface finishing and the smoothness of the part [10]. More attention will be paid on developing the applied temperature field model in direct laser fabrication, which is required to describe not only plicated nonlinear phenomenon involved with heat conduction, convection and diffusion, heat radiation and phase change problems, but also give a 3D visualexpressing method to exhibit the feature of material additive fabrication [11, 12]. The effect of cone angle, cone ring gap, and shield gasvelocity of the coaxial nozzle on the gathering property of powder flow field is investigated in this paper. A disperse phase model is adopted to conduct the numerical simulation of the powder flow field, including the distributing rules of the powder flow field and analysis of its gathering property, The gathering property of the powder flow field, the distributing rules of the flow field velocity, the rules of the flow field parameters (focus (f), focus radius (r), and powder concentration (CF)) are obtained by FLUENT, which can be referred in the design and optimization of the coaxial powder nozzle. 2 Numerical model of carrier gas powder in nozzle flow field In LMDM, the coaxial powder nozzle has the forming powder orbit of two coaxial cone ring gaps that are coaxial with laser beam, and then concentrate at the laser focus (see as Fig. 1). Lin has studied the gas–solid twophase flow in coaxial powder nozzle when the Reynolds number is 2,000 [13], and the result shows that the metal concentration will reach the maximum kg/m3 when it is 5 mm under the bottom of the nozzle. Energy, momentum, and mass transmission process exist in coaxial powder feeding system, which directly determines the size, precision and performance of the part made by LMDM. Thus, it is necessary to conduct some research on its powder flow field, but until now, there is only limited relevant research on this subject [14, 15]. 沈陽理工大學學士學 位論文 37 Fig. 1 Working principle of the coaxial powder nozzle, α is the anglebetween fed powder and the horizon, β is the angle between shield gasand the horizon In the discrete phase model in this paper, the gas phase is puted by the standard k–ε turbulent flow model, and the discrete phase is puted by building particle track model. There are assumptions as below in the model: (1) the particle’s movement is steady, and the powder and gas have the same velocity。Gangxian Zhu amp。 Dichen Li amp。 沈陽理工大學學士學 位論文 33 在成文過程中，我還得到了來自同學的寶貴建議，同時還得到了朋友的支持和幫助 ，在此一并致以真誠的謝意。徐老師的嚴謹?shù)慕虒W態(tài)度，高尚的治學精神和精益求精的工作作風，深深地感染著和激勵著我。 時，粉末的匯聚性能較好。 時，粉末匯聚的速度矢量圖 圖 85%~90%之間，圖 的體積百分數(shù)在 95%以上，圖 80%~85%之間 。 時，粉末匯聚的速度矢量圖 沈陽理工大學學士學 位論文 31 圖 75176。 時，粉末匯聚