CFD Studies of Flow in Screw and Scroll Compressors

An analysis of flow in rotary positive displacement machines has been carried out using a standard CFD package by separating out the motions of the fluid and the instantaneous volume in which it is contained. Examples are given of both twin screw and scroll compressors. Pressure-volume diagrams obtained from such analyses are compared with experimental results and good agreement shown. Longer term aims of such studies are to obtain a better understanding of the internal flow processes, especially where two component flow, such as oil/air mixtures are involved. INTRODUCTION This paper describes the first stages of a longer term project to obtain greater understanding of the nature of fluid flow within rotary positive displacement machines such as those of the screw and scroll type. The greatest need for such improved understanding is where there is more than one flow component as in oil injected compressors. In such cases, the assumptions normally made on oil and gas separation within the machines may thus be checked. Flow of fluid in a rotary positive displacement compressor is unsteady, three-dimensional, turbulent and compressible. Its analysis may be simplified if a grid is defined to describe the instantaneous volume of fluid trapped in the machine and the motion is described first in terms of the fluid relative to the grid and then of the grid itself. By this means fluid motion through all compressors of this type may be analysed with standard numerical procedures. The flow of the fluid relative to the grid may be calculated using a numerical procedure, but with the grid motion replaced by external body forces on the fluid. Although the grid is continuously in motion and changing in shape, its geometry may be defined at all times and its motion described