Numerical investigation of the leakage flows in twin screw compressor rotors

Screw compressors are complex flow systems, but operate upon simple considerations: they are positive displacement machines consisting of meshing rotors contained in a casing to form a working chamber, whose volume depends only on the angle of rotation. Although the basic operation of twin screw compressors is well understood and the analytical methods for performance prediction are well established, the CFD analysis of such machines is still in its early days and requires more research in order to mature to the level of standardised application, as is the case with most turbomachinery applications. The performance of screw compressors is highly affected by leakages, which are dependent on various clearances and the pressure differences across these clearances. The numerical simulation of the leakage flows is the challenging aspect of running CFD in screw compressors, as the flow domain reduces in tip areas to the order of several microns. Commercial grid generators struggle in reproducing the flow path with good quality elements and user defined meshing techniques are currently available to allow the remeshing of the rotors domain to capture the modified aspect of the compression chamber at the start of each time step. This technique allows the full compressor performance prediction by means of CFD but does not allow for local mesh refinement in the tip area, where the turbulent character of the flow requires adequate boundary layer considerations. This is the area of concern in the present paper, where several means of investigating the flow in the very tight tip clearance area are proposed: 1) a three-dimensional model of all interacting compression chambers to investigate the effects of employing mesh-deforming techniques for the compressor simulation; 2) a simplified two-dimensional leakage model to identify the mesh refinement requirements for appropriate turbulence modelling. Keywords: screw compressors, CFD, leakage flows, tip seal