Effects Of Variable Diffuser Vanes On Performance Of A Centrifugal Compressor With Pressure Ratio Of 8.0

Centrifugal compressors have the advantage of producing a high pressure ratio with a smaller size when compared with the other types of compressors.In numerous applications,centrifugal compressors are required to provide high pressure ratio with good efficiency while also working in a wide operating range.It is a challenge because as pressure ratio increases,efficiency and operating range inevitably decline.Variable diffuser vane is an effective method to not only increase the operating range of the compressor but also improve the efficiency by improving the matching of the diffuser and impeller,particularly at off-design speed.This study investigates the effects of variable geometry diffuser on the performance and operating range of a centrifugal compressor with very high pressure ratio.Therefore,a transonic vaned diffuser centrifugal compressor with design pressure ratio of 8 was modelled and simulated.The numerical method employed three-dimensional viscous steady-state Reynoldsaveraged Navier-Stokes simulations.The computations were carried out by utilizing NUMECA FINE/Turbo as user interface of the EURANUS flow solver.The central scheme with Jameson type dissipation and fourth-order Runge-Kutta scheme were used for spatial and temporal discretization respectively.The Spalart-Allmaras one-equation model was selected as turbulence model.A multi-block mesh was created for the simulations and its quality and reliability was verified.An analysis of the matching of the vaned diffuser with the impeller for different working conditions and diffuser vane angles is presented.The matching analysis is done by studying the component performance of the impeller and diffuser.The results show that,improved matching of the adjusted diffuser increased efficiency by 4.5% at off-design speed.The range extension mechanism of the variable diffuser at different speed and working conditions is explained.It is showed that adjusting the vane angle by +6 to-6 degrees extended the operating range of the compressor by up to 30.0% for pressure ratios between 5.0 and 6.0.The range extension achieved by shifting both the surge and choke lines.The interaction between the diffuser and impeller was examined by studying the performance of the impeller and various flow fields at the impeller exit.It is showed that the performance and operating condition of the impeller is independent of the diffuser angle,while the mass flow rate kept constant.Through adjusting diffuser vanes,the impeller was driven to operate in a significantly broad range.This is proposed as a general method to study the behavior of the impeller over a wide range of mass flow rates.The connection between the incidence angle at the leading edge of the impeller and flow separation near the tip of the impeller is discussed.Critical mass flow rates were found on impeller characteristics,below which due to the increase of the incidence angle at impeller leading edge,flow separation emerges at near tip area.