Unsteady Performance of an Aeroengine Centrifugal Compressor Vaned Diffuser at Off-Design Conditions

As aviation fuel costs and consumption have continued to rise over recent decades, gas turbine engine manufacturers have sought methods to reduce fuel burn. Manufacturers plan to achieve this by reducing the specific fuel consumption of the machine by increasing the bypass ratio through a reduction of the diameter of the engine core. This presents an opportunity for implementing a centrifugal compressor as the final stage of the high-pressure compressor. The vaned diffuser in a centrifugal compressor stage maintains an integral role in determining the extents of the operating range as well as conditioning the flow for the downstream combustor. Thus, it is critical to understand the aerodynamics and performance of the vaned diffuser across the entire compressor operating range. This investigation focused on seven compressor operating points at the stage’s design corrected speed, which ranged from choked flow to the minimum mass flow rate before rotating stall. Steady-state and unsteady performance data were acquired to study the aerodynamics at each operating point as well as the steady-state performance of the vaned diffuser. Laser Doppler velocimetry, high-frequency pressure transducers, and additive manufacturing techniques were all implemented to acquire data in the research compressor. Unsteady velocity measurements were acquired in the vaneless space and were used to quantify the change in diffuser inlet incidence as the stage mass flow rate changes. The impeller exit jet and wake were compared at each operating point to understand the effect of these flow structures on the spanwise incidence profile. Steady-state performance metrics for the vaned diffuser were compared with the change in incidence to assess the effect of incidence on performance. Maximum static pressure recovery and minimum total pressure loss occurred at the maximum incidence operating point. The chordwise static pressure distribution is critical for health monitoring of the polymer, additive manufactured diffuser vanes. Steady-state and unsteady pressure measurements were acquired along the diffuser vane surface to assess the change in the aerodynamic lift force across the compressor operating range as well as the static pressure differential across the vane leading edge. The maximum unsteady lift on the diffuser vanes was up to 34% greater than the steady-state lift force. Unsteady static pressure differentials across the diffuser vane leading edge were similar to the steady-state values, but they were marginally greater across the entire examined operating range. These unsteady pressure measurements were acquired with high-frequency response pressure transducers installed along the diffuser vane surfaces. These transducers were also used to study the rotating stall and surge behavior of the investigated centrifugal compressor stage. This centrifugal compressor stage exhibits a spike-type rotating stall pattern at the onset of stage instability, which rapidly evolves into full flow reversal with several surge cycles. During these surge cycles, the diffuser vane leading edges are subject to a 20 psid static pressure differential. A computational model was used to predict the compressor flow at three different operating points. This model utilized the BSL-EARSM turbulence model, and it included surface roughness and an experimentally measured shroud thermal profile. The model accurately predicted the diffuser inlet flow angles near the shroud, but it predicted more radial flow near midspan. The diffuser vane leading edge static pressure differential was predicted within 1 psid at higher aerodynamic loading conditions. The differences between the computationally predicted and experimentally measured flow are attributed to difficulties associated with modelling the rate of mixing within the flow