###### Abstract
Incidence angle was considered as the main effective factor on the compressor performance, which increases the separation regions inside the diffuser passages at off-design operating points, leading to an increase in loss and decrease in diffuser pressure recovery. [PERSON] et al. [24] numerically analyzed the influence of impeller-diffuser interaction on the performance of a centrifugal compressor stage. The unsteady effects of impeller exit on diffuser loss and pressure recovery coefficients are similar to those from axial distortion at the inlet of the diffuser. It was found that both have negligible effects in comparison with the spatially and time-averaged flow incidence at the diffuser throat. [PERSON] et al. [25] studied the time-dependent interaction influences on the diffuser efficiency of a centrifugal compressor. They showed that the circumferential variations of flow angle at the leading edge of the diffuser blade affect the diffuser efficiency. [PERSON] et al. [26] numerically studied the acoustic field of a centrifugal compressor. They found that the experimental whistle at the compressor inlet near the compressor surge is caused by the flow pulsations, detached and re-attached flow regions within the compressor passages. Although numerous investigations have been conducted to predict the noise generation and acoustic behaviors in turbomachines, most of them performed to the axial turbomachinery components because of their wide range applications in the aircraft engine industry. Additionally, most of the measurements and computational researches on the centrifugal compressors were performed to low-speed compressors, and few publications are provided for high-speed radial compressors [27; 28]. One of the primary and complicated aerodynamic flow behaviors within the turbomachinery components is unsteadiness, which is also known as the key parameter in noise generation. Both tonal (discrete) and broadband noise are typically generated in the centrifugal compressors. The generated broadband noise in the centrifugal compressors is caused by the interaction between the three-dimensional turbulent structures and separation regions with the impeller and diffuser blades, and the secondary flow-field within the compressor channels. On the other side, the interaction between the impeller trailing edge and diffuser vane generates the tonal noise, which is known as the dominant noise in the centrifugal compressors. This tonal noise is related to the blade passing frequency, and depends on the impeller rotational speed, as well as the blade counts. Therefore, the effect of interaction between the impeller and diffuser on the compressor efficiency and acoustic performance is an important factor which needs to be taken into account. Few approaches have been proposed in the literature to reduce the tonal noise in the centrifugal compressors by changing the geometry of the impeller and diffuser components. [PERSON] et al. [12] showed that the hub cavity affects the aero-acoustically generated noise in a compressor using large eddy simulation and the Ffowcs Williams-Hawkings model, for which expensive computational resources are required. [PERSON] et al. [14] investigated the influences of the tapered diffuser blade of the centrifugal compressor on the flow structure and noise generation. Although the tapered vanes can attenuate the level of the tonal noise, producing the tapered vanes is complicated and expensive. In a previous study by the authors [11], applying the inclination angle in diffuser vane leading edges was found to enhance the pressure recovery coefficient and reduce the loss characteristics. However, in that study, a steady Reynolds-averaged Navier-Stokes approach (RANS) was only considered to analyze the aerodynamic performance at the design point, and the influences of inclined leading edges on the unsteady interactions between the trailing edges of the impeller blades and leading edges of the diffuser vanes were neglected.