Advances in Numerical Modelling of Helical Screw Machines

Screw compressors are mature products used broadly in industrial applications. Similar technology is applied for industrial expanders, pumps, vacuum pumps… Mathematical models of various complexities have been developed through decades. Computational Fluid Dynamics (CFD) is regarded the most modern technique which proved suitable for fundamental and applied research of these machines. This paper is review of the work in this area. Case studies show the scope and applicability of CFD in screw machines. Examples include prediction of flow generated noise in screw machines, cavitation modelling in gear pumps, and flow in multiphase oil and gas pumps. Additionally, test programme carried out with Laser Dopler Velocimetry to measure velocity distribution in screw compressor flow domains is presented. It provided data for verification of CFD predictions and suggested areas of research including evaluation of turbulence modelling can provide more accurate and faster CFD calculations. n = Constant. The advent of digital computing made it possible to model the compression process more accurately and, with the passage of time, ever more detailed models of the internal flow processes have been developed, based on the assumption of one-dimensional non-steady bulk fluid flow and steady one dimensional leakage flow through the working chamber. Together with suitable flow coefficients through the passages, and an equation of state for the working fluid, it was thus possible to develop a set of non-linear differential equations which describe the instantaneous rates of heat and fluid flow and work across the boundaries of the compressor system. These equations can be solved numerically to estimate pressure-volume changes through the suction, compression and delivery stages and hence determine the net torque, power input and fluid flow, together with the isentropic and volumetric efficiencies in a compressor. In addition, the effects of oil injection on performance can be assessed by assuming that any oil passes through the machine as a uniformly distributed spray with an assumed mean droplet diameter. Such models have been refined by comparing performance predictions, derived from them, with experimentally derived data. A typical result of such modelling is the suite of computer programs described by Stosic et al, 2005. Similar work was also carried out by many other authors such as Fleming and Tang 1998 and Sauls, 1998. Despite the speed and relatively accurate results, these models neglect some important flow effects mainly in the suction and discharge ports which could influence compressor performance. Screw compressor performance can be estimated more precisely by a three dimensional Computational Fluid Dynamics (CFD). Nonetheless there are few publications available that describe its successful application in this field. Kovacevic et al published a number of papers between 1999 and 2005 which described 3D numerical analysis of the entire machine domain. These were followed by a monograph on CFD applied to screw machines by Kovacevic et al, 2006, which gives a comprehensive overview of the methods and tools used for the analysis of flow in these machines. A number of commercial CFD software packages are currently available which can both analyse the flow through screw machines and easily integrate the results with CAD systems. However the moving, stretching and sliding mesh required for mapping the working chamber cannot be produced within their grid generator packages. Additionally, the time required for calculation of the flow through the entire machine by use of these codes is excessively long. Therefore development of the grid generation method proved to be the key for success for application of CFD in analysis of screw machines.