Comparison of Flow Characteristics of Centrifugal Compressors by Numerical Modelling of Flow

The centrifugal impellers are used in a wide variety of turbo-machineries, ranging from low pressure fans for cooling of electric motor to high pressure ratio gas turbine compressors, from tiny cryo-cooler compressors to large industrial petrochemical compressor stations. The relative flow in centrifugal impeller is very complex due to different fluid dynamics phenomenon and their interactions. It is subjected to the complex secondary flows and significant separation of the boundary layers, which result in non-uniformity of outlet flow both in tangential direction (e.g. « jet-wake» flow) and axial direction. By the development of experimental and computational techniques the considerable progress in understanding of complex flow effects in impeller is made. The wide-spread use of reactive impellers with backswept blades in all state-of-the-art designs of compressors has resulted in significant improvements of aero and thermodynamics characteristics of impeller (e.g. distribution of outlet flow velocities). The increasingly present appearance in the several recent years of calculations by means of verified and validated users software based on the engineering numerical methods enables an increasingly wide replacement of time-consuming experimental investigations (i.e. of physical modelling), which give a limited number of data of integral character, by mathematical modelling which gives a large number of data over a short time, enabling not only integral but also structural analysis. This results in rapid and significant improvements of aero and thermodynamics characteristics and in the end in energy conversion efficiency. The aim of the paper is to apply expert knowledge in carrying out numerical modelling of the flow in centrifugal compressor by means of users software. The algorithm is presented on the comparison of aero and thermodynamics characteristics of centrifugal compressors with reactive impellers with backswept normal and « S» -shaped blades. The comparison shows that the compressor with impeller with « S» shape characteristics has better aero and thermodynamics characteristics and thus also the energy conversion efficiency. Since there are experimental measurements of aero and thermodynamics characteristics of numerical modelling of the investigated compressors in literature, the verification and validation of the users software have been performed simultaneously. Also the alternating modelling of the flow and change of impeller blades geometry with the aim of improving the aero and thermodynamics characteristics up to attaining of maximum possible efficiency of energy conversion presents the algorithm of geometry optimisation of the impeller design with backswept blades.