CFD Modelling in Screw Compressors With Complex Multi Rotor Configurations

Computational Fluid Dynamics (CFD) of screw compressors is challenging due to the positive displacement nature of the process, existence of very fine fluid leakage paths, coexistence of working fluid and lubricant or coolant, fluid injection and most importantly the lack of methodologies available to generate meshes required for the full three dimensional transient simulations. In this paper, currently available technology of grid remeshing has been used to demonstrate the CFD simulations of complex multi rotor screw compressors like the twin screw compressor with parallel axis and single screw compressor with cylindrical main rotor and two planar gate rotors with perpendicular axis. Presently, methodology for grid generation of constant pitch twin screw machines is available through SCORG© (Kovacevic et al., 2007) but it is currently not suitable for different topologies like that of a single screw or variable pitch rotors. It is very challenging to handle the mesh deformation that happens in the compression chambers during operations of such machines. The methodology tested for this paper uses a technique called key-frame re-meshing in order to supply pre-generated grids to the CFD solver as the solution progresses. In order to evaluate accuracy of such approach, an adiabatic compression-expansion process in a reciprocating piston cylinder arrangement is studied and compared with a diffusion equation based mesh smoothing. It has been demonstrated, that although it is possible to simulate the complex configuration of screw compressors by key-frame re-meshing technique, there are many limitations with respect to the time consumed in pre-processing, demand to computational resources, accuracy of results and general difficulty to include advanced modeling features like turbulence, multispecies or multiphase flows. Hence it was concluded that customized tools for generation of CFD grids for such complex screw machines still remain to be developed