Deforming grid generation and CFD analysis of variable geometry screw compressors

The most common type of twin screw machines are twin screw compressors. These normally contain rotors of uniform pitch and profile along the rotor length. However, in some cases such as in twin screw vacuum pumps with very large pressure ratios, the variable pitch rotors are often used to improve efficiency. The limited use of rotors with variable pitch and/or section profile is mainly due to manufacturing constraints. In order to analyse the performance of such machines by means of Computational Fluid Dynamics (CFD), it is necessary to produce a numerical mesh capable of calculating 3D transient fluid flows within their working domains.

An algebraic grid generation algorithm applicable to unstructured grid, Finite Volume Method (FVM) for variable pitch and variable profile screw machines is described in this paper. The grid generation technique has been evaluated for an oil free air compressor with “N” profile rotors of 3/5 lobe configuration. The performance was obtained by calculations with commercial CFD code. The grid generation procedure provides mesh of the required quality and results from CFD calculations are presented to compare performance of constant pitch rotors, variable pitch rotors and variable profile rotors. The variable pitch and variable profile rotors achieve steeper internal pressure rise and a larger discharge area for the same pressure ratio. Variable pitch rotors achieve reduced sealing line length in high pressure domains.