Optimal Vaneless Diffuser Design For A High-End Centrifugal Compressor

Turbochargers are widely used in the automotive industry to reduce engine emissions and to increase the power. Centrifugal compressors are an integral part of turbochargers. Centrifugal compressors comprises primarily of inducer, impeller, diffuser and volute. The diffuser has an important role in the isentropic efficiency of the compressor. Over the past few decades, researchers have been trying to increase the total-to-total compressor stage efficiency by altering the diffuser’s geometries. Many different methods have been adopted for this purpose, like pinching the diffuser walls, tilting the diffuser walls etc. Pinching increases the outer width of the diffuser while tilting provides an increased radial length. In the present study, both these methods have been used simultaneously. The primary benefit of doing so is to make the turbocharger compressor stage more compact in design, which is the current requirement of the automotive market. In order to investigate the effect of pinching and tilting of diffuser walls, a Computational Fluid Dynamics based solver has been used to predict the flow phenomena within the compressor, especially in the vaneless diffuser. Design of Experiments, using Taguchi’s method, has been incorporated in this study to statistically define the scope of the numerical work, and to obtain the optimal configuration of pinching and tilting that leads to maximum total-to-total compressor stage efficiency. The results depict that the compressor stage efficiency increases up to a tilt angle of 6.25º, after which it starts to decrease. Furthermore, the stage efficiency increases with increase of diffuser outlet width i.e., pinching the diffuser passage, however, this increasing trend has been observed up to an outlet width ratio of 1.23, after which the stage efficiency starts to decrease. Hence, the optimal diffuser model, based on the combined tilting and pinching results obtained, which leads to the maximum total-to-total compressor stage efficiency, has been identified and analysed.