This study examines the effects of periodic inflow unsteadiness on the flow development through fishtail-shaped diffusers utilized on small gas-turbine engines. In this application, periodic unsteadiness is caused by a jet-wake type of flow discharging from each passage of the centrifugal compressor impeller. The study consists of detailed measurements in a large-scale fishtail diffuser rig with a geometry that is typical of those used in small gas-turbine engines. Measurements of the transient velocity field have been performed at five cross-sectional planes throughout the diffuser using a miniature hot-wire probe with four wires. These measurements involve frequencies of inflow unsteadiness corresponding to design as well as off-design operating conditions. Results indicate significant effects of inflow unsteadiness at the low end of the tested frequencies on the time-averaged streamwise and cross-flow velocity fields in the diffuser. This is shown to translate into a notable impact on the pressure recovery. In addition to providing insight into the physics of this flow, the experimental results presented here constitute a detailed and accurate data set that can be used to validate computational-fluid-dynamics algorithms for this type of flow.

No abstract

This study examines the effects of periodic inflow unsteadiness on the flow development through fishtail diffusers utilized on small gas-turbine engines. The periodic unsteadiness is due to the distortion of the flow in the peripheral direction at the exit of the centrifugal compressor impeller, caused by the jet-wake type of flow discharging from each passage of the impeller. The study consists of detailed measurements in a large-scale fishtail diffuser rig with a geometry that is typical of those used in small gas turbine engines. An unsteady-flow generator is used to approximate the type of diffuser-inflow conditions that exist at the exit of centrifugal compressor impellers. Detailed measurements of the transient velocity field have been performed at the inlet and at four cross-sectional planes throughout the diffuser using a miniature 4-wire probe. These measurements involve frequencies of inflow unsteadiness corresponding to design as well as off-design operating conditions. The measured velocity field is analyzed in a time-averaged sense and in the context of previously-performed steady-flow measurements. Results show significant effects of inflow unsteadiness at lower frequencies on the flow field in the diffuser, which translates into a substantial impact on the pressure recovery of the diffuser. In addition to providing insight into the physics of this flow, the experimental results presented here constitute a detailed and accurate data set that can be used to validate computational-fluid-dynamics codes for this application.