Three dimensional numerical analysis for flow prediction in positive displacement screw machines
A substantial proportion of all industrial compressors now produced are of the twin-screw type. These are rotary positive displacement machines, which operate at high efficiency over a wide range of speeds and pressure differences. Currently their performance is estimated by assuming simplified one-dimensional flow through passages with dimensions that are invariant with temperature and pressure. As manufacturing accuracy increases, clearances can be reduced and compressors thereby made smaller and more efficient. However to obtain full advantageo f this at the design stage,it must be possible to estimate accurately internal fluid flow patterns, pressure and temperature distribution and their effects on the working process. An interface has therefore been developed in order to generate a 3-D numerical grid for this purpose. This employs a procedure to produce rotor profiles and an analytical transfinite interpolation method with adaptive meshing to obtain a fully structured 3-D numerical mesh, which is directly transferable to a Computational fluid dynamics (CFD) code. Robust calculations can then be performed while allowing for moving, stretching and sliding between the rotors with large variations in the chamber shape and proportions. Changes in the solver functions have improved convergence and increased the speed of solution. One of these is to include a means to maintain constant pressure within the inlet and outlet ports throughout the calculating procedure. Also, the CFD procedure has been modified to enable fast calculations to be made with real working fluids and to estimate two-phase flow effects due to phase change and oil injection in the working chamber. The interface, pre-processing code and calculating procedures have been used with a commercial CFD solver to estimate the performance of three different compressor applications. The predicted results for one of these were compared with those obtained from the author's measurement in an experimental test rig and good agreement was obtained.