Computational Fluid Dynamics in Rotary Positive displacement Screw MachinesAhmed Kovacevic, Sham Rane, Mohammad Arjeneh, Nikola Stosic,

City University London, United Kingdom

Long Abstract

Rotary positive displacement screw machines are used in variety of applications such as compressors, expanders, blowers, vacuum pumps, liquid and multiphase pumps. Due to technological advantage over other types of positive displacement machines, screw machines today dominate industrial applications with over 85% share. In many cases positive displacement screw machines will prove advantageous compared to turbomachines, fo example in handling multiphase fluids. Examples are industrial refrigeration, air conditioning, process gas compression etc. It is evaluated that almost 20% of world energy consumption is used for various means of gas compression and pumping. For example, in developed countries more than 25% of energy consumption in summer months is used for compression of refrigerants in air-conditioning. Depending on the application they may operate flooded by oil or another fluid or without any form of internal rotor cooling or lubrication. The typical example of a screw compressor and extracted flow domains is presented in Figure 1. Reliability and efficiency of such machines is of utmost importance for economy and ecology.

Figure 1 Screw compressor CAD geometry and the flow domain extracted for numerical analysis

Chamber thermodynamic models are traditionally used in industry for performance calculation in screw machines [1] due to their speed and relatively accurate results. However, these neglect some important flow effects that influence compressor performance. Further improvement of efficiency and reliability require use of advanced modelling techniques. Authors [2] pioneered algebraic grid generation based on rack and introduced 3D modelling of screw machines in 2000. Today CFD is becoming common tool for analysis of screw machines. This paper describes current challenges and specific requirements for grid generation and modelling of these challenging flows.

The CFD analysis of working chamber of a screw machine is transient in nature and requires a grid representing the domain deformation. One of the major factors affecting the performance prediction of twin screw compressors by use of computational fluid dynamics is the accuracy with which the leakage gaps are captured by the discretization methods. In order to represent rotor profiles accurately, it is required to use point definition of the geometry but this definition gets compromised when a numerical grid is generated for flow calculations. One of the methods of improving the profile accuracy is by increasing the number of grid points on the profile. However, this method faces limitations when it comes to the complex deforming computational domain of the twin screw compressor because the grid quality deteriorates and computational time increases tremendously. In order to address this problem, an analytical grid distribution procedure has been proposed in

this paper that can independently refine the region of high importance i.e. the interlobe space. To achieve this, a new procedure is used for generation of the initial point distribution on compressors boundaries. This procedure uses equidistant distribution of points on the outer boundary of the domain which consists of the casing and rack to calculate distribution on rotors. By this means it is possible to refine mesh in the interlobe gap to allow for a more accurate prediction of leakage flows. The total grid size can be controlled by limiting the number of cells in the region not containing interlobe gap and blow-hole areas. The paper presents the principle of this analytical grid generation and a test case to demonstrate the influence of interlobe refinement on the performance prediction. The new distribution is directed from the outer boundary to the rotor profile. With this approach it is possible to refine the grid in the interlobe space thus allowing the CFD grid to capture the rotor profile curvature more accurately. A test case was presented to compare the results from the new grids and the results have been compared with experimental measurements and calculations from a base grid. The non-conformal interface between the rotors was better resolved because of interlobe grid refinement.The results of pressure showed that there was a very small difference between the Base grid and the new grids and the internal pressure rise curve followed well with the experimental measurement. Indicated power increased by a small amount from Grid – 1 to Grid – 4 and was close to experimental value at both the speeds.There was a big influence of interlobe grid refinement on the prediction of mass flow rate and hence leakage flows. A difference between CFD calculations and experimental measurement still exists. This can be attributed to the consideration of right operational clearance gaps in the CFD model.In order to obtain higher accuracy in CFD predictions, change in leakage gap sizes due to thermal deformation need to be accounted in the CFD models. This requires employment of fluid solid interaction modelling or using experimental data. The new grid generation approach however provides a flexibility to improve the resolution of the rotor geometry and thereby reduce inaccuracy in leakage flow calculations to some extent.

References

[1] N. Stosic, I.K. Smith and A. Kovacevic. Screw Compressors: Mathematical Modeling and Performance Calculation, Springer Verlag, Berlin, ISBN: 3-540-24275-9, 2005

[2] A. Kovacevic, N. Stosic, I.K. Smith. Grid Aspects of Screw Compressor Flow Calculations, Proceedings of the ASME Advanced Energy Systems Division, 40, 83, 2000.

[3] A. Kovacevic. Boundary Adaptation in Grid Generation for CFD Analysis of Screw Compressors, Int. J. Numer. Methods Eng., 64(3): 401-426, 2005

[4] A. Kovacevic, N. Stosic and I.K. Smith. Screw compressors - Three dimensional computational fluid dynamics and solid fluid interaction, ISBN 3-540-36302-5, Springer-Verlag Berlin Heidelberg New York, 2007

[5] Voorde, V. J., Vierendeels, J. and Dick, E.: Development of a Laplacian-based mesh generator for ALE calculations in rotary volumetric pumps and compressors, Computer Methods in Applied Mechanics and Engineering, 193, 39–41, pp4401-4415, 2004