Ported Shroud Effects on the Unstable Dynamic Characteristics of a Radial Compressor

The general trend in diesel engines is to downsize the engine by use of turbochargers that operate at higher pressure ratios. Unfortunately, increasing the rotational speed of compressors tends to decrease their range of operation due to the development of instabilities at higher minimum mass flow rate. In order to limit the occurrence of stall and eventually surge, passive control methods such as port shrouded compressors have been developed. This type of device has been proven to extend the surge margin by use of inlet recirculation but the actual characterization of functioning remains not well understood. The scope of this research is to study the effective operational range for a turbocharger with port shrouded compressor typically used in diesel engines. In order to better understand the relationship between inlet recirculation and instability control, full dynamic survey of the compressor is being performed. Measurements at the inlet of the compressor, inside the diffuser and surrounding the volute are obtained simultaneously to track down the incipience of stall. This enables to follow the pressure distribution in every section of the compressor and highlight pressure variations within the housing. By comparing the results for the same compressor tested with and without recirculation capability, focus is given on the effectiveness of the casing treatment. This yielded a better understanding of the working mechanism associated with the application of a ported shroud as an effective way of surge margin extension.