The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers

Abstract

Highly-loaded impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is non-uniform across the span and pitch and swirling at angles approaching tangential. With the exception of the flow angle, conflicting data exist regarding whether these attributes have substantial influence on the performance of the downstream diffuser. This paper quantifies the relative importance of the flow angle, Mach number, non-uniformity and unsteadiness on diffuser performance, through diffuser experiments in a compressor stage and in a rotating swirling flow test rig. This is combined with steady and unsteady Reynolds-Averaged Navier Stokes computations. The test article is a pressure ratio 5 turbocharger 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 fits 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.Across approximately two-thirds of the stage operating range, 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. 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 timeaveraged spanwise and unsteady pitchwise non-uniformity from the rotor; this independence is reflective of the strong mixing processes that occur in the diffuser inlet region. The observed exception is for operating points with high timeaveraged vane incidence. Here, it is hypothesized that temporary excursions into high-loss flow regimes cause a nonlinear 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. INTRODUCTIONModern high pressure ratio centrifugal compressors typically utilize a high speed impeller with backswept blades and a vaned diffuser. As part of ongoing efforts for a systemwide reduction in weight, increase in efficiency and/or reduction of emissions, designers seek to improve the pressure ratio, operating range and efficiency of the centrifugal compressor, which is often limited by the diffuser. Despite the relative simplicity of its geometry, the flow in the diffuser is complex and there is no well-established, reliable approach to gui...

The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers

“…Only a sample of this data is shown here; further validation is shown in [15]. Figures 5 and 6 show the good agreement in spanwise profiles observed: the maximum angle difference between experiment and URANS is 5° and the maximum Mach discrepancy is 11%.…”

“…This is sketched in Figure 18. 5 RANS simulations of the isolated impeller are shown in [15] to provide a similar impeller outflow as in the stage URANS simulations, at low speeds. A mixing analysis supports this hypothesis and reveals that the increased magnitude of the pitchwise non-uniformity from the rotor encountered toward stall does not lead to sufficiently increased loss to explain the discrepancy, noting that except at high angles, the two machines are in close agreement.…”

“…The design is a modification of that used by Filipenco, described in [3,14]. Details of the modifications can be found in [15]; these included modification of span-to-radius ratio and the fitting of a turbocharger volute.…”

“…The exit boundary condition enforces a uniform static pressure at diffuser exit. Further details on the numerical approach can be found in [15].…”

“…The probe is calibrated for temperature recovery factor per [16]. Further information on both traversing probes and their calibration can be found in [15].…”