A Preliminary Design Tool for Radial Compressors

Abstract: The aim of this thesis has been to implement a computer program capable of modelling radial compressors. The work was conducted at the Division of Thermal Power Engineering, Department of Energy Sciences at the Faculty of Engineering, Lund University. With such a large field as radial compressors, some decomposition of the subject was necessary. This thesis focuses on modelling the impeller and diffuser of the compressor, where the components are individually studied. By doing this division between different parts of the compressor, the individual components can be designed in isolation. If, for instance, only a vaned diffuser is to be designed, no impeller design must first be found. The resulting computer program called LURC (Lund University Radial Compressor) have two different design tools for the impeller, one in-depth impeller flow analysis, one vaneless diffuser design tool and one vaned diffuser design tool. The compressor designer can easily perform different analyses with LURC, all design tools being easily accessible at all times. Also, several of the different analysis tools are interconnected, allowing easy data exchange. The so-called preliminary impeller design tool requires minimum input data from the user, quickly yielding a promising impeller design. It is a one-dimensional analysis method, where most parameters are specified on the mean streamline. The complete impeller geometry is directly generated, making it a perfect tool for the non-experienced compressor designer. Further, the result can be used as input data in any of the two diffuser analyses. In the detailed impeller design tool more input data must be specified. Also, the geometry and blade shapes are under full control of the designer. This analysis is also one-dimensional but has the capability of creating three-dimensional impeller geometries. Such a tool is well suited to the more experienced designer, knowing what he or she want to achieve. The result can be used in both diffuser analyses, but also in the in-depth impeller flow analysis. The in-depth impeller flow analysis is based on a quasi three-dimensional calculation method. In this case a streamline curvature method is coupled with a stream function flow analysis, allowing the three-dimensional problem to be composed of two two-dimensional ones. From this analysis tool the velocity distributions along the blades may be found, giving the experienced designer a vital piece of information. Vaneless diffusers are very common in radial compressors, making it important to incorporate these in LURC. Input data may be entered manually or imported from a previous impeller analysis. The analysis is based on one-dimensional theory, including effects of wall friction. The complete geometry is generated and the key results, such as discharge pressure and velocity, are calculated. The vaned diffuser design tool are focused on channel diffusers of the wedge type. Also included in the analysis are the vaneless space right after the impeller discharge. As was the case for the vaneless diffuser design tool, inlet data can be entered either manually or fetched from an impeller analysis conducted earlier. The whole diffuser geometry is generated, that is, both the vaneless space and the channel diffuser part. One-dimensional compressible channel flow is the basis for this analysis. LURC fulfils its main objective; being a fast design tool for establishing promising compressor stage designs. Although mainly based on one-dimensional analyses, it is a very efficient and powerful tool for determining important design parameters in a timely fashion.