Experimental investigation of flow distortion effects on the performance of radial discrete-passage diffusers

Abstract

A swirling-radial-flow generator has been developed for the study of fluid-dynamic phenomena in radial passage- and vaneless-diffusers. A unique feature of the swirling-flow generator is the capability of providing a wide range of diffuser inlet flow conditions. This is accomplished by means of a very-high-solidity rotating radial-outflow nozzle cascade in conjunction with annular cross-flow injection/suction slots in the flow-path walls immediately upstream and downstream of the rotor blading. The rotor generates a shockless and weak-wake axisymmetric transonic swirling flow which can be tailored to provide a desired level of diffuser inlet flow-field axial distortion by means of cross-flow injection and/or suction through the annular slots.

A complete test-facility was designed and constructed based on this concept and was utilized to study effects of inlet flow-field axial distortion on the pressure-recovery performance and stability of a modem high-performance gas-turbine-engine radial discrete-passage diffuser. It was shown that the diffuser pressure-recovery coefficient, if based on the inlet availability-averaged total pressure, correlates well with the diffuser inlet momentum-averaged flow angle independent of flow-field axial distortion and Mach number over the wide flow parameter range investigated. It was argued that the generally accepted high sensitivity of diffuser pressure recovery performance to inlet flow distortion and boundary-layer blockage is largely due to inappropriate quantification of the diffuser inlet flow-field parameters.

Time resolved pressure measurements in the vaneless space between the rotor and diffuser showed that the diffuser operating range is limited by the onset of rotating stall triggered by the loss of flow stability in the diffuser, independent of the rotor operating point (if overall compression system instability did not occur first). It was found that the loss of flow stability in the diffuser occurred at a critical value of the diffuser inlet momentum-averaged flow angle and corresponding overall-diffuser pressure recovery coefficient (based on the availability-averaged total pressure), independent of inlet flow-field distortion and Mach number, over the wide flow parameter range investigated.

A simple analytical consideration of an idealized diffuser consisting of a constant-area mixing duct followed by an isentropic-flow diffuser was used to show that the observed insensitivity of the diffuser pressure-recovery performance to inlet flow-field distortion can be attributed to rapid mixing of the flow. It was shown that measurements of the static pressure distribution along the centerline of an individual diffuser-passage support this hypothesis.