CFD grid generation and analysis of screw compressor with variable geometry rotors

This paper presents development of an algebraic grid generation algorithm applicable to Finite Volume Method for Computational Fluid Dynamics (CFD) calculation of variable pitch twin screw machines. It is based on the principles developed for the uniform pitch rotors with constant cross-section profile. The same algorithm could be also used for rotors with variable profile geometry. Performance predictions are obtained by ANSYS CFX for an oil-free 4/5 lobed twin screw compressor with variable pitch rotors and uniform 'N' profile. A comparison with the performance of a compressor of the same rotor size and wrap angle, but with the uniform pitch rotors showed that the variable pitch rotors give better compression characteristics. This is achieved by reduced throttling losses, reduced length of the sealing line towards the high pressure end and a larger discharge area for the same pressure ratio.