Numerical Simulation of Fluid Flow and Solid Structure in Screw Compressors

Screw compressor performance is usually predicted by thermodynamic and fluid flow calculations based on a dimensionless flow model. However factors, which affect the power consumption, delivery and reliability of these machines, such as dynamic flow losses, leakage and oil flow drag, are not estimated accurately by such procedures. By the use of 3-D numerical modelling of the compressor fluid flow and structure deformation, these factors can be predicted more precisely and hence losses within the machine can be minimized at the design stage. The authors have developed a CAD-CCM (Computer Aided Design to Computational Continuum Mechanics) interface program to transfer the screw compressor geometry directly into a commercial numerical solver, which simultaneously solves the compressor fluid flow and structural deformation of its parts. All necessary initial and boundary conditions, a procedure for moving of the mesh and additional functional features of the commercial solver are introduced through the user functions automatically generated by the interface program. Results have been obtained for an oil-flooded screw compressor and some of them are presented in this paper. They are presented as overall compressor parameters in the form of power and volume flow. Additionally, pressure-time diagrams of the compression process, flow and pressure patterns in the compressor chambers and rotor deformation are illustrated. INTRODUCTION Screw compressors comprise a meshing pair of helical rotors on parallel axes, contained in a casing. Together, these form a succession of working chambers whose volume depends on the angle of rotation. An outline of the main elements of a screw compressor is presented in Figure 1, where it is shown how the two rotors are contained in the casing, by means of views from opposite ends and sides of the machine. The dark shaded portions show the enclosed region where the rotors are surrounded by the casing and compression takes place, while the light shaded areas show the regions of the rotors which are exposed to external pressure. The large light shaded area in Fig 1a) corresponds to the low pressure suction port. The small light shaded region between shaft ends B and D in Fig 1b) corresponds to the high pressure discharge port. Admission of the gas to be compressed occurs through the low pressure port. After a certain angle of rota tion, the port is cut off and further rotation leads to a progressive reduction in the trapped volume. This causes the pressure of the contained gas to rise. The compression process continues until the rear ends of the passages between the rotors are exposed to the high pressure discharge port through which gas flows out at approximately constant pressure. The design parameter which influences screw compressor performance most strongly is the rotor profile. A difference in shape, which can hardly be detect ed by the eye, can effect significant changes in flow rates delivered