Full 3D numerical analysis of a roots blower with open-source software

In recent years, computational fluid dynamics (CFD) has been applied for the design and analysis of positive displacement machines (both compressors and expanders) for vapor compression and power generation (e.g., ORCs) applications. In particular, twin screw compressors are widely employed in industrial vapor compression systems because of their high efficiency compared to other compressor types. The numerical modeling of the operation of such machines is challenging: the dynamics of the compression (or expansion) process and the deforming working chambers make the simulation process a not-trivial task. The relative motion of the rotors and the variation of the gaps during machine operation are few of the major challenges towards the implementation of reliable CFD models. Furthermore, the elaborated working fluid (i.e. the refrigerant) operates in many cases either close to the critical point or to the saturated-vapor line. Under such conditions, the ideal gas model does not hold and, thus, a compressible real gas solver is required. Among the several numerical techniques that have been developed throughout the years, the custom predefined mesh generation is one of the most used techniques. In such an approach, a set of meshes (one for each time step) is generated in advance before running the CFD simulation. The solver is fed with the mesh for each time step retaining the configuration of the mesh unchanged. In this work, SCORG-V5.2.2 was used to generate the meshes of the deforming domain around rotating parts of the machines. This was interconnected with OpenFOAM-v1606+, which is used to compute the flow field associated with the operation of the twin screw machine. It was demonstrated that the proposed methodology allows for a fast simulation and to achieve a good agreement with experimental test results.