Combination of CFD and Experimental Techniques to Investigate the Flow in Centrifugal Compressors Near the Surge Line

Surge phenomenon is becoming a limitation of the low end torque for downsized turbocharged Diesel engines. The stable operation of centrifugal compressors is limited at low flow rates because of the occurrence of surge. In the present work a CFD analysis of the flow inside automotive centrifugal compressors near the surge line is carried out. For this purpose, the actual geometry of a compressor -including its inducer, rotor, diffuser and volute- has been modelled with a CFD code. Initially, steady calculations have been performed to have a first approximation of the flow characteristics inside the compressor. In these calculations, a source term is imposed in the momentum equations to simulate the movement of the rotor and its effects on the air when it passes through it. In order to get a better understanding about the phenomena that occur inside the compressor when surge starts, the same compressor has been calculated for different cases with the same rotor speed and different mass flow rates. The geometry of the inlet duct has also been varied.

Some calculations in transient conditions, including moving meshes (rotor) have then been performed for very low mass flow rates in order to simulate in more realistic conditions the beginning of stall that leads to surge.

CFD results have been compared with experimental results and good agreement has been found for the compressor map points calculated. Hence, a complete 3D analysis can be performed to help to understand the stalled flow and the surge phenomenon.