A mixer is determined by multiple parameters, and it is impossible to describe a mixer using any single parameter. Shaft power (P), impeller discharge rate (Q), head (H), impeller diameter (D), and mixing speed (N) are the five basic parameters describing a mixer. The impeller discharge rate is proportional to the impeller's flow number, the first power of the impeller speed, and the third power of the impeller diameter. The shaft power consumed by mixing is proportional to the fluid specific gravity, the impeller's power number, the third power of the speed, and the fifth power of the impeller diameter. Under certain power and impeller geometry conditions, the impeller discharge rate (Q) and head (H) can be adjusted by changing the matching of the impeller diameter (D) and speed (N). That is, a mixer with a large-diameter impeller and low speed (while maintaining constant shaft power) produces a higher flow effect and lower head, while a small-diameter impeller with high speed produces a higher head and lower flow effect. In a mixing tank, the only way to cause fluid elements to collide is to provide sufficient shear rate.
From the perspective of mixing mechanisms, it is the existence of fluid velocity differences that causes the mixing of different layers of fluid; therefore, all mixing processes always involve fluid shear rate. Shear stress is a force and is the real reason for bubble dispersion and droplet breakup in mixing applications. It must be pointed out that the magnitude of the shear rate at different points in the entire mixing tank is not uniform. Studies on shear rate distribution show that there are at least four shear rate values in a mixing tank. Experimental studies show that, in the impeller region, regardless of the impeller type, when the impeller diameter is constant, both the maximum shear rate and the average shear rate increase with increasing speed. However, when the speed is constant, the relationship between the maximum shear rate and the average shear rate and the impeller diameter depends on the impeller type. When the speed is constant, the maximum shear rate of radial impellers increases with increasing impeller diameter, while the average shear rate is independent of the impeller diameter. These concepts regarding shear rate in the impeller region require special attention in mixer scaling and design. Compared to large-tank mixers, small-tank mixers typically have characteristics such as high rotational speed (N), small impeller diameter (D), and low tip speed (ND), while large-tank mixers typically have characteristics such as low rotational speed (N), large impeller diameter (D), and high tip speed (ND).
