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Research Papers

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

[+] Author and Article Information
Jonathan N. Everitt

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: jonathan.everitt@alum.mit.edu

Zoltán S. Spakovszky

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zolti@mit.edu

Daniel Rusch

Compressor Development (Dept. ZTE),
ABB Turbo Systems Ltd.,
Bruggerstrasse 71a,
Baden CH-5401, Switzerland
e-mail: daniel.rusch@ch.abb.com

Jürg Schiffmann

Laboratory for Applied Mechanical Design,
Ecole Polytechnique Fédérale de Lausanne,
Neuchâtel 2000, Switzerland
e-mail: jurg.schiffmann@epfl.ch

1Corresponding Author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 3, 2016; final manuscript received October 11, 2016; published online January 4, 2017. Editor: Kenneth Hall.

J. Turbomach 139(4), 041004 (Jan 04, 2017) (10 pages) Paper No: TURBO-16-1224; doi: 10.1115/1.4035048 History: Received September 03, 2016; Revised October 11, 2016

Highly loaded impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is nonuniform 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, nonuniformity, 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 (RANS) 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 and 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 time-averaged spanwise and unsteady pitchwise nonuniformity 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 time-averaged 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.

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References

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Figures

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Fig. 1

Experimentally determined pressure recovery coefficient for the datum research diffuser, as a function of momentum averaged angle as defined by Filipenco et al. [3] showing relatively poor correlation

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Fig. 2

Project diffusers showing how the camber line is altered to vary the throat area. This modifies the leading edge angle. The trailing edge angle is invariant for volute matching. “D” is short for “diffuser” and D2 is the datum geometry.

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Fig. 3

Static pressure taps within a diffuser passage used to break down pressure recovery into subcomponents, together with traverse probe locations A–D (shown in adjacent passage only for illustration)

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Fig. 4

Sketch of swirl rig cross section and blading

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Fig. 5

Spanwise profiles of Mach number and radial component at the diffuser inlet, at midpitch, as measured in the compressor (“Comp”) versus URANS (a) 58% speed, ηmax and (b) 100% speed, ηmax

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Fig. 6

Spanwise profiles of flow angle at the diffuser inlet, at midpitch, as measured in the compressor versus URANS (a) 58% speed, ηmax and (b) 100% speed, ηmax

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Fig. 7

Comparison of diffuser subcomponent characteristics recorded in the compressor tests and calculated in URANS at 58% design corrected speed

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Fig. 8

Comparison of diffuser subcomponent characteristics recorded in the compressor tests and calculated in URANS at 100% design corrected speed

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Fig. 9

Results from the compressor test, showing diffuser effectiveness correlates well with mixed-out average flow angle

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Fig. 10

Results from URANS simulations added to the results from the compressor tests at three different compressor design corrected speeds

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Fig. 11

Spanwise profiles of flow angle at midpitch for three URANS cases at the same mixed-out average angle, showing a variety of blockage and skew which obtain comparable pressure recoveries

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Fig. 12

Total pressure loss coefficient (Cp,t) as a function of diffuser inlet incidence, showing a classic “loss bucket” shape similar to cascade airfoil data; diffuser inflow Mach numbers > 0.8, and impeller/diffuser combinations as indicated by the legend

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Fig. 13

Cp,t as a function of diffuser inlet incidence for diffuser inflow Mach numbers from 0.5 to 0.8, showing greater sensitivity to secondary effects such as diffuser loading distribution

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Fig. 14

Diffuser effectiveness in the compressor tests and the swirl rig, showing generally good correlation. An exception is where the swirl rig rotor is choked, leading to supersonic diffuser inlet flow (“SS”).

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Fig. 15

Diffuser and volute Cp,t in the compressor tests and the swirl rig. A clear difference exists between the two machines at high angles.

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Fig. 16

Spanwise profiles of flow angle (top) and Mach number including radial component (bottom), for a matched operating point representative of the compressor 58% speed, ηmax operating point. “A” and “C” represent pitchwise positions per Fig. 3.

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Fig. 17

Angle variations at rotor exit (1.02% r2) for the compressor impeller and the swirl generator, for vaneless diffuser RANS simulations, i.e., isolated rotors, at average conditions representative of the compressor 58% speed ηmax operating point

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Fig. 18

Sketch indicating the hypothesized mechanism by which unsteady angle variations increase total pressure loss relative to a steady flow

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Fig. 19

Diffuser and volute Cp,t for the compressor and swirl rigs, and diffuser Cp,t for stage URANS and diffuser-only RANS CFD. Stage CFD results, once adjusted for the losses inherent in the application of a mixed-out inlet condition, agree with the diffuser-only CFD across the operating range.

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