## 4 Discussion  
By comparing the micromixedness ratio (\(\alpha\)) in Figure 7 and the mixing index (M) of the physical test in Figure 9a, the simulation results showed that the maximum mixing efficiency of the prediction was different from that of the experiment. The reasons for the deviation might be that the meshes  
Figure 11: Comparison diagram of a variation of the mass fraction of the component and the turbulent kinetic energy along the axial direction: (**a**) mass fraction of the component chart without obstacles; (**b**) turbulent kinetic energy chart without obstacles; (**c**) mass fraction of the component chart with obstacles; (**d**) turbulent kinetic energy chart with obstacles. and models were not suitable enough, the deviation dimensions and roughness of the actual internal structure of the molded parts were not considered in the simulation. Although both of them were positively correlated with the mixing efficiency, they were not uniform in dimension. Hence, only the qualitative analysis can be carried out. But the simulation results were consistent with the experimental results. The position of the turning point and the changing trend of the slope between every two points were accurately predicted, so the simulation still had sufficient credibility. [39]. Figure 12 shows how splitting-collision mechanism works. There are two main mixing patterns influencing its mixing process, collision, and swirl. Besides, continuous fluid can be divided into huge amounts of micromasses which have different physical or chemical states based on hydrodynamics theory system. When two micromasses collide, the components contained inside the micromasses will break through the surfaces of two micromasses, resulting in the mixing of two components. At the same time, a bigger micromass will turn into some smaller micromasses. The smaller micromasses have larger relative surface area, which increases the probability of contacting outer components and enhances the molecular diffusion caused by concentration differential. Besides, the fluid revolves around a low-pressure point and runs along a helical line in swirls, where the fluid micromasses are teared into many thin and long slices by the normal and tangential force from pressure difference and viscous force. The mixture slices have a larger contact area between two micromasses, so it attains stronger molecule diffusion and higher mixing efficiency. As the colliding process goes, the high-pressure zones appear at the converging zones in the central region due to the anti-force caused by colliding. Meanwhile, the obstacles do not obstruct the fluid any more after the fluid flow across the middle cross-section of obstacles. So, there are some low-pressure zones close to the back slopes. Then, the acceleration vectors point from the central region to surroundings. The acceleration vectors are against the velocity vectors resulting that the torques appear because not all acceleration vectors are on the same lines with velocity vectors. The torques cause swirls. Thus, in the paragraph after the middle cross-section of obstacles, the mixing behavior is the development of swirls as well as the collisions between the boundaries of different swirls. According to the above design principles, there are many ways to design micro reacting pipes, however, only a kind of internal structure was discussed in this paper. As the cross-section of other micro reacting pipes in Figure 13, there are other developing directions of the shape of internal structure,  
Figure 12: Scheme of splitting-collision mechanism. such as obstacles with curved face or asymmetrical structure. The internal channel structure of the micro reacting pipe has a great influence on the mixing efficiency, and the appearance of the AM technology provides a wider optimization space for the design and manufacture of the micro reacting pipe, so it is worthy of further study by researchers in various fields.