Screw Compressor with Variable Geometry Rotors-Analysis of Designs by CFD

Twin screw compressors are positive displacement machines commonly used in industrial applications. Due to constraints of conventional manufacturing techniques these are usually produced with helical rotors of uniform pitch. However, other twin screw machines, such as vacuum pumps more often use variable pitch rotor for improved efficiency. Single screw machines are of nonparallel axes and cross sectional profile which changes along the rotor axis. Their manufacturing is more challenging compared to twin screw compressors. With advancements in manufacturing technologies, the new generation of screw machines may see rotors with variable pitch and variable profile which could lead to improved efficiency and extension of operating range of such machines. The performance analysis of such machines by use of chamber models is difficult and in some cases impossible. The alternative is Computational Fluid Dynamics (CFD). Grid generation of twin screw compressors pioneered by authors in late 1990’s allowed use of CFD for analysis and improvements in this technology. This technique is widely used for machines with parallel rotors, uniform profile and constant lead, such as twin screw compressors and expanders, gear pumps, multiphase pumps etc. In order to analyse novel configurations of screw machines with non-parallel axes and variable geometry profile by use of CFD it is necessary to produce suitable numerical mesh capable for reliable calculation of 3D transient fluid flows within domains with sliding and stretching interfaces. Performance of an oil-free 3/5 lobed twin screw compressor with standard constant pitch; variable pitch rotors and uniform ‘N’ profile; and variable profile rotors has been compared using CFD models. The variable pitch and variable profile rotors achieve steeper internal pressure rise. Variable lead rotors additionally have both sealing line length and blow-hole area reduced in the high pressure region. CFD calculations confirmed that variable geometry rotors are more suitable for high pressure ratio applications.