Performance Evaluation and Optimisation of Vaneless Diffuser Of Various Shapes for a Centrifugal Compressor

In recent years, diesel engines with reduced emissions and low fuel consumption have been developed worldwide for the purpose of environmental protection and energy conservation. Turbochargers are playing an important role in these modern engines by providing power boost to the engine. A turbocharger comprises of three major parts i.e. the turbine stage, the bearing housing and the compressor stage. Turbocharger designers are continuously seeking for compact stage designs, while maintaining the stage performance. A turbocharger’s compressor stage comprises of various parts i.e. inlet, impeller, diffuser and volute. The diffuser is an important section of the turbocharger compressor stage that plays a key role in increasing the isentropic efficiency of the stage. The diffuser converts the kinetic energy imparted to the flow by the impeller, into static pressure rise, which inturn increases the isentropic efficiency of the stage. The shape of a diffuser is conventionally simple in design. Modifications to the diffuser geometry can lead to higher efficiencies and compact designs of the compressor stage.

The present study focuses on the use of advanced computational techniques, such as Computational Fluid Dynamics (CFD), to analyse the effects of diffuser modifications on the local flow features, and the global performance parameters. A baseline diffuser configuration, consisting of a parallel wall diffuser, is numerically analysed to establish the accuracy of CFD based predictions. Various diffusers’ geometrical configurations have been analysed in the present study, both qualitatively and quantitatively. These geometrical configurations cover a wide range, such as diverging, tilting and curving of the diffuser walls. These parametric investigations aid to improve the compressor stage performance and make it more compact.

The first aim of the study is to quantify the increase in the stage performance by diverging the straight wall vaneless diffuser passage. This is carried out by diverging the shroud wall (i.e. increasing the outlet-to-inlet width ratio) and varying the location of the divergence point on the shroud wall. The results obtained depict that the effect of increasing the diffuser’s outlet-to-inlet width ratio is dominant in comparison with the location of the wall divergence point. Moreover, increase in diffuser’s outlet-to-inlet width ratio increases the downstream area ratio of the diffuser, causing the flow to separate and creating flow recirculation near the hub wall. This creates restriction to the flow and causes air blockage. Furthermore, shifting the wall divergence point towards the outlet of the diffuser relocates the flow separation point closer to the diffuser exit.

The second aim of this study is to analyse the effects of tilted diffuser walls on the flow variables within the compressor stage of the turbocharger. Tilting diffuser walls provides an increased streamwise length to the flow. Furthermore, divergence is applied to the diffuser hub wall in order to increase the outlet-to-inlet width ratio. This makes the turbocharger compressor stage compact in design, while maintaining the stage performance, which is the current requirement of the automotive sector. Design of Experiments, using Taguchi method, has been incorporated in this study to define the scope of the numerical work. The results obtained show that the diffuser with both titled and diverged walls together, performs optimally as compared to the other configurations considered.

The third aim of this study is to use curved diffuser walls in order to make the design more compact. Divergence to the hub wall is also applied to enhance the performance of the compressor stage. Various configurations of curvilinear diffuser walls have been considered for numerical analysis. The local flow field analysis has been carried out, quantifying the effects of the geometrical parameters on the stage performance. The results depict that a curved diffuser model reduces the losses within the diffuser passage, but there is negligible effect on the stage efficiency. However, when the divergence is applied to the hub wall of the curved diffuser, there is significant increase in the stage efficiency. Based on these investigations, a turbocharger’s compressor stage can be designed for a compact design and optimal efficiency.