3.3.1. Numerical Simulation of Internal Airflow Field in a Three-Duct Centrifugal Fan

In order to verify whether the flux and distribution of the internal airflow field of the designed three-duct centrifugal fan could meet the cleaning requirements of Chinese milk vetch seeds, the whole flow channel model of the three-duct centrifugal fan was established by using Inventor, and ICEM-CFD was adopted to divide the grid of the threeduct centrifugal fan. Considering the large volume of the channel model and the complexity of its internal flow, tetrahedral unstructured meshing was used to divide the flow channel model. The contact areas between the fan inlet and the impeller, as well as the contact areas between the impeller and the spiral case were all set as the interfaces, and the meshings at the interfaces were encrypted [21]. The meshed centrifugal fan is shown in Figure 6a, and the total number of model grids is 3,617,588.

**Figure 6.** Mesh dividing and internal air velocity simulation of the three-channel centrifugal fan. (**a**) Mesh mode of centrifugal fan with three-channels. (**b**) Internal flow velocity vector diagram of the fan.

Then, the grid model was imported into Fluent 15.0 software, the standard *k-ε* turbulence model was used for numerical simulation and the SIMPLEC pressure-velocity coupling algorithm was used to calculate velocity and pressure of airflow in the fan. The spiral case of the fan was set as the static wall, and the meshing areas at the impeller of the centrifugal fan were set as the MRF rotation areas. Referring to the structure and working parameter design of existing centrifugal fans, rotation speed of the three-duct fan was set to be 1080 r·min<sup>−</sup>1, the impeller diameter was 385 mm, the two inlet boundary conditions of the centrifugal fan were set as the pressure inlets with the given pressure of 220 Pa, the three outlet boundaries of the model were set as the pressure outlets with the given pressure of 0 Pa, the convergence residual was set as 0.001 and the iteration steps were set as 500 [22]. The internal airflow velocity vector diagram of the three-duct centrifugal fan is shown in Figure 6b. It can be seen from the figure that the airflow velocity at the upper and lower outlets was high, while the airflow velocity at the middle outlet was slightly lower. The airflow velocity at the upper outlet (regardless of friction resistance) ranged from 7.33 m·s−<sup>1</sup> to 29.3 m·s<sup>−</sup>1, and the air volume at the outlet was about 0.55 kg·s−1; the airflow velocity at the middle outlet ranged from 5.14 m·s−<sup>1</sup> to 24.9 m·s−1, and the air volume was about 0.42 kg·s−1; the wind speed of the lower outlet was between 6.6 m·s−<sup>1</sup> and 25.6 m·s<sup>−</sup>1, and the air volume was about 0.57 kg·s<sup>−</sup>1.
