Analysis of the stability of centrifugal compressors for turbocharging application. Identification of criteria for the prediction of the stable operating limit

A centrifugal compressor for turbocharging application often has to work in conditions far from optimal conditions, due to the different operating conditions it is subjected in an automotive application. Therefore, the knowledge of the operating limits of a centrifugal compressor is fundamental in a first design phase. In particular, can predict the surge limit range, at a given speedline, is a serious challenge. In the literature there are extensive studies on the phenomena that lead the compressor to work in unstable operation, in particular the rotating stall. However, there is a lack of precise methods that can establish with certainty the correct surge massflow rate. Nowadays with the ever increasing computational resources a large contribution can be provided by the CFD techniques. In this regard, in this thesis the main goal is to able to identify some criteria to predict the surge line, through simplified models, which do not require excessive computational resources. For the compressors with a vaned diffuser, a stability parameter is developed, which shows excellent reliability in this type of turbomachinery. However, in the case of compressors with vaneless diffuser, has been necessary to develop further criteria due to the loss of effectiveness of the previous criterion. In more details for the centrifugal compressors with vaneless diffuser, through a deep fluid dynamic analysis, has been possible to identify two different fields of application to develop the appropriate criteria, based on the fluid-dynamic phenomena that lead the compressor to unstable operation. In particular in this work, for the case with vaneless diffuser, the following criteria have been developed at the low rotational speed: the critical angle (a modified version respect to the Senoo’s criterion) and the recirculation zone at the diffuser, while for the impeller the criterion of the diffusion ratio. Finally, it has been shown how, increasing the rotational speed, the volute acquires a key role in the inception of unstable machine phenomena; so a criterion based on this last component has been developed for high rotational speed.