Grid deformation strategies for CFD analysis of screw compressors

• Key-frame grid re-meshing investigated for screw compressor CFD modelling.
• Accuracy of calculation evaluated through analysis of an isentropic process in a piston-cylinder.
• Successful use of customized grid generator demonstrated for twin screw compressor CFD analysis.
• The need for development of customized grid generators for multi-rotor screw machines highlighted.

Customized grid generation of twin screw machines for CFD analysis is widely used by the refrigeration and air-conditioning industry today, but is currently not suitable for topologies such as those of single screw, variable pitch or tri screw rotors. This paper investigates a technique called key-frame re-meshing that supplies pre-generated unstructured grids to the CFD solver at different time steps. To evaluate its accuracy, the results of an isentropic compression-expansion process in a reciprocating piston cylinder arrangement have been compared. Three strategies of grid deformation; diffusion equation mesh smoothing, user defined nodal displacement and key-frame remeshing have been assessed. There are many limitations to key-frame re-meshing. It requires time consuming pre-processing, has limited applicability to complex meshes and leads to inaccuracies in conservation of calculated variables. It was concluded that customized tools for generation of CFD grids are required for complex screw machines.