**3. Results and discussions**

#### **3.1 Mesh**

The tank geometry and the mesh are done on OpenFOAM and the impeller geometry on Sketch Up. Then, the OpenFOAM snappyHexMesh command is used to obtain the final mesh. The mesh is refined to meet the quality criteria of mesh such as cell skewness and mesh non-orthogonality. To meet these criteria, we reach a very large number of cells and thus long computing times. Consequently, we do not perform grid independence tests.

The surface mesh of the digester tank and the cross section of the mesh at the impeller level are shown in **Figures 3** and **4**.

The characteristics of the mesh are shown in **Table 2**. In total, this mesh is composed of 2,841,535 cells with mostly hexahedra. The total mesh faces is 9,150,640. The impeller edge has the larger amount of faces with a value of 465,732 due to its geometry. Concerning the interface between the moving zone and the stationary zone (AMI1 and AMI2), the two surface meshes must be identical with the same faces number in order to avoid numerical errors. Globally, the maximum cell skewness is 3.4 and the maximum mesh non-orthogonality is 65.0, which reflect an acceptable mesh quality. The dimensionless local Reynolds number *y*<sup>þ</sup> is below 1.

**Figure 3.** *Surface mesh of the digester: top (left) and wall (right).*

#### **Figure 4.**

*Cross section of the mesh at the impeller level.*


**Table 2.** *Mesh characteristics.* *CFD Simulations in Mechanically Stirred Tank and Flow Field Analysis: Application… DOI: http://dx.doi.org/10.5772/intechopen.93926*
