The role of impeller outflow conditions on the performance and stability of airfoil vaned radial diffusers

Abstract

This thesis quantifies the relative importance of the impeller outflow angle, Mach number, non-uniformity and unsteadiness on diffuser performance, through diffuser experiments in a compressor stage and in a swirling flow test rig combined with steady and unsteady Reynolds-Averaged Navier Stokes computations. The test article is a pressure ratio 5 tur-bocharger compressor with an airfoil vaned diffuser. The swirling flow rig is able to generate rotor outflow conditions representative of the compressor except for the periodic pitchwise unsteadiness, and ts a 0.86 scale diffuser and volute. In both rigs, the time-mean impeller outflow is mapped across a diffuser pitch using miniaturized traversing probes developed for the purpose. Diffuser performance is well correlated to the average impeller outflow angle when the metric used is effectiveness, which describes the pressure recovery obtained relative to the maximum possible given the average inflow angle, Mach number and the vane exit metal angle. This is reflective of the strong mixing processes that occur in the diffuser inlet region. Utilizing effectiveness captures density changes through the diffuser at higher Mach numbers; a 10% increase in pressure recovery is observed as the inlet Mach number is increased from 0.5 to 1. Further, effectiveness is shown to be largely independent of the unsteady pitchwise non-uniformity from the rotor. The observed exception is for operating points with high time-averaged vane incidence. Here, it is hypothesized that excursions into high-loss flow regimes cause a non-linear increase in loss as large unsteady angle variations pass by from the rotor. Given that straight-channel diffuser design charts typically used in preliminary radial vaned diffuser design capture neither streamtube area changes from impeller exit to the diffuser throat nor vane incidence effects, their utility is limited. An alternative approach, utilizing effectiveness and vane leading edge incidence, is proposed. Finally, strong pressure fluctuations downstream of the diffuser vane leading edge, but not upstream, are strongly suggestive of a leading edge vortex previously proposed as the mechanism for short-wavelength stall inception. A preferential location for the stall onset is observed due to the circumferential pressure non-uniformity imposed by the volute.