Fluid-Solid Interaction In Screw Compressors

Pressure and temperature differences within screw compressors cause the various components to distort. The analytical methods currently used to predict the distortion of the components are insufficiently accurate to account for the complexity of the processes within these machines. Accordingly, in order to avoid rotor contact and seizure, the design clearances normally selected are larger than those required to satisfy manufacturing tolerances. The aim of this study was therefore to use full 3-D numerical modelling of solid-fluid interaction within the machine to determine minimum safe clearances at the design stage and thereby maximise the compressor efficiency. To this end, the authors have developed an independent stand-alone CAD-CCM (Computational Continuum Mechanics) interface program in order to transfer the screw compressor geometry directly into a commercial CCM solver. The interface program employs a rack-generating procedure and an analytical transfinite interpolation method to obtain a fully structured, block oriented, hexahedral 3-D numerical mesh of the screw compressor rotors and their working chambers. All necessary initial and boundary conditions, a deformation procedure for the moving mesh and additional functional features for the commercial solver are introduced through the user functions automatically generated by the interface program. By this means, the interaction between fluid flow and deformed solid body are predicted for screw compressor. Results obtained for a dry air compressor are presented in this paper and it is shown, by means of flow and distortion diagrams and pressure-angle diagrams of the compression processes, how rotor profile clearances vary in the machine.