# Mechanisms of Sweep on the Performance of Transonic Centrifugal Compressor Impellers
## 1 Introduction
Most designs with forward sweep showed better efficiency than that of the unswept or the aft-swept ones. Nevertheless, cases do exist that aft sweep also has the potential to improve the efficiency [21]. In radial turbomachines, although there are applications of sweep to reduce the stress levels [22], control the blade root separations [23], and adjust the flow capacity [24], a parametric study of sweep has relatively seldom been reported. When sweep is applied to the impeller main blade, similar effects on the flowfield are expected because the local flow patterns of the axial part of the impeller are much similar to that of the axial compressors. [PERSON] and [PERSON] [25] confirmed the aerodynamic effects of the main blade sweep on controlling the leading edge loading, the shock structure and the accumulation of low-momentum flow in a turbocharger impeller case. It was found that an increase in sweep angle led to an increased efficiency due to the weaker shock-boundary layer and shock-leakage interaction loss. Later, the numerical results of [PERSON] and [PERSON] [26] implied that forward sweep only increased the stable operating range while aft sweep increased the efficiency. When sweep is applied to splitters, its effects remain uncertain because its leading edge locates further downstream, where the local flow patterns are much more complex than that of the main blade. Cases with swept splitters and unswept main blades were reported by [PERSON] and [PERSON] [27]. The impact on the aerodynamic performance was comparatively limited, and the aerodynamic effects on the flowfield have not been well understood. Because of limited studies of impeller sweep, there are still some remaining questions listed as follows:  
1. The impacts of blade sweep on the aerodynamic performance of the centrifugal impeller have not been systematically studied yet. It is hard to tell the general trend of aerodynamic performance parameters (i.e., choke mass flow rate, efficiency and pressure ratio) with changing sweep feature. Such understandings are important to guide future optimization designs. 2. The aerodynamic effects of sweep on the flowfield have not been fully understood. Some of the effects in the axial part of the impeller were reported in previous research, but few studies have addressed the axial-to-radial part of the impeller, which differs between axial and radial turbomachines. 3. The underlying mechanism for the optimal sweep angle with best efficiency has not yet been determined. To answer these remaining questions, mechanisms of sweep on the aerodynamic performance of a transonic impeller with twin splitters have been numerically investigated in this paper. Sweep is defined as scaling up or down the shroud chord while keeping the blade angle and the thickness distribution along the relative chord length unchanged. The variation range of the sweep angle has been chosen from \(-25^{\circ}\) to \(+25^{\circ}\). Performances of both datum and swept cases have been compared in detail, and then aerodynamic effects of sweep have been analyzed and discussed.