###### Abstract  
The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanets of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier-Stokes equations. Diffuser vanets with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanets with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor. captridgat compressor; inclined leading edge; vaned diffuser; stall margin; blade passing frequency +
Footnote †: This paper is an extended version of paper published in ASME Turbo Expo 2017 (GT2017), Charlotte, NC, USA, 26-30 June 2017. Paper No. : GT2017-65117. causes complicated, unsteady flow structures between the exit of impeller and the throat of the diffuser, in which unsteady transonic flow structures are believed to impact the operating range and the aerodynamic performance of centrifugal compressors [9; 10; 11; 12]. Therefore, understanding of the flow variations in the impeller-diffuser interaction region with respect to the circumferential and spanwise directions is a fundamental factor to enhance the compressor efficiency in various operating points. More recently, several theoretical and experimental researches have been carried out to realize the transient flow-field and radiated noise mechanism within the impeller and diffuser passages of centrifugal compressors [13; 14; 15; 16; 17]. [PERSON] et al. [5; 6] revealed that the interactions between the impeller and diffuser play important roles in the aerodynamic performance of centrifugal compressor. They showed that in most cases, smaller radial gaps between the impeller exit and diffuser vane inlet lead to a more uniform flow-field and higher pressure recovery coefficient at the diffuser outlet, which results in improvement in compressor performance. [PERSON] et al. [18] experimentally and numerically investigated the pressure variations in a centrifugal compressor with a vaned diffuser. They showed that these variations exist from the leading edge toward the trailing edge of the diffuser channels, and the maximum value of the pressure oscillations is reached to 30% of the pressure at the compressor inlet. Nowadays, numerical simulations have been extensively used to investigate the complex flow characteristics within the turbomachines, due to development of high-performance computers and advanced computational fluid dynamics algorithms [19; 20; 21; 22; 23]. [PERSON] et al. [19] quantified the role of outflow from the impeller exit in the compressor performance with vaned diffusers. They found that the effectiveness is well correlated to the mixed-out average flow angle.