An integrated model for the performance calculation of Screw Machines Ahmed Kovacevic, Elvedin Mujic, Nikola Stosic, Ian K. Smith Centre for Positive Displacement Compressor Technology, City University, London, United Kingdom ABSTRACT There is a need to develop improved analytical procedures in order to improve performance, reduce noise emission and reduce the manufacturing costs of screw compressors. Most mathematical models, used by industry for screw compressor performance estimation and optimisation, are based on quasi one dimensional calculation of the governing flow equations in a control volume. Despite being fast and accurate for general compressor performance calculations, these do not take full account of flow losses induced in the compressor inlet and outlet ports although the effects of these are significant. Three dimensional fluid flow calculations take account of these phenomena but require significant time and effort to be performed due to the complex geometry handling procedures and large numerical meshes required. An alternative approach is to combine both methods in a single integrated procedure. This paper describes how a three dimensional Computational Fluid Dynamics model of flow in the compressor ports has been integrated with a one dimensional mathematical model of flow in the working chamber through a common integral management system. By this means, solutions may be obtained faster than with a full 3D approach with results that are more accurate than from a 1D model. The transmission of the boundary conditions from one region to the other has been established through the user coding of a CFD system. The methods described are of considerable scope and can be applied, not only to screw compressors but also to any other type of twin rotor rotational machine with parallel axes, such as gear pumps, vacuum pumps and roots blowers. A comparison of measured and predicted pressure oscillations in the discharge port of an industrial oil injected compressor is given as an example of the use of this procedure. NOMENCLATURE A - area m - Mass in the chamber m & - mass flow U - Internal energy V - volume v - velocity P - pressure T - temperature