Numerical Studies on the Effect of Impeller Blade Skew on Centrifugal Compressor Performance

Centrifugal compressors are designed for a given operating pressure and mass flow rate. But often these machines are required to be run at various off design operating conditions. The need to maintain reasonably high efficiency under off design conditions with acceptable stall margin makes the compressor design more challenging. These requirements call for improvemnet in the flow quality through the impeller by incorporating suitable changes in the vane shape. This paper examines the effect of introducing tangential skew near the trailing end of a high pressure ratio centrifugal compressor impeller. The blade skew was introduced in such a way that the blade angle distribution varied from hub to shroud near the trailing end only. Detailed, steady state, numerical simulations were carried out on the compressor, using a commercially available code, FLUENT. Results of the baseline CFD analysis on the original impeller were validated through comparison with the available experimental data. Subsequently, the impeller geometry was modified by incorporating different positive and negative skew angles, ranging from –45° to +45° and each configuration was analysed in terms of compressor overall performance and flow behaviour through the impeller blade passages. In general, the skewed impellers showed varying improvement in stall margin, pressure ratio and efficiency. It was observed that the impeller with +45° skew produced highest pressure ratio with reduced stall margin compared to baseline 0° skew impeller. On the contrary, the impeller with -45° skew produced a lower pressure ratio with increased stall margin compared to the baseline 0° skew impeller. It is concluded that a proper blade skew to the impeller needs to be imparted based on the requirement of higher pressure ratio or higher stall margin. The results have also suggested a need for further optimisation of skew angle at the impeller trailing end.