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Research Papers

Design of an Annular-Radial Diffuser for Operation With a Supercritical CO2 Radial Inflow Turbine

[+] Author and Article Information
Joshua A. Keep

School of Mechanical and Mining Engineering,
The University of Queensland,
Brisbane, Queensland 4072, Australia
e-mail: j.keep@uq.edu.au

Ingo H. J. Jahn

School of Mechanical and Mining Engineering,
The University of Queensland,
Brisbane, Queensland 4072, Australia
e-mail: i.jahn@uq.edu.au

Manuscript received November 19, 2018; final manuscript received April 4, 2019; published online May 2, 2019. Assoc. Editor: David Sánchez.

J. Eng. Gas Turbines Power 141(8), 081020 (May 02, 2019) (12 pages) Paper No: GTP-18-1701; doi: 10.1115/1.4043431 History: Received November 19, 2018; Revised April 04, 2019

Radial inflow turbines are a relevant architecture for energy extraction from supercritical CO2 power cycles for scales less than 10 MW. To ensure stage and overall cycle efficiency, it is desirable to recover exhaust energy from the turbine stage through the inclusion of a suitable diffuser in the turbine exhaust stream. In supercritical CO2 Brayton cycles, the high turbine inlet pressure can lead to sealing challenges at small scale if the rotor is supported from the rotor rear side in the conventional manner. An alternative is a layout where the rotor exit faces the bearing system. While such a layout is attractive for the sealing system, it limits the axial space claim of the diffuser. Designs of a combined annular-radial diffuser are considered as a means to meet the aforementioned packaging challenges of this rotor layout. Diffuser performance is assessed numerically with the use of Reynolds-averaged Navier--Stokes (RANS) and unsteady Reynolds-averaged Navier--Stokes (URANS) calculations. To appropriately account for cross coupling with the stage, a single blade passage of the entire stage is modeled. Assessment of diffuser inlet conditions, and off-design performance analysis, reveals that the investigated diffuser designs are performance robust to high swirl, high inlet blockage, and highly nonuniform mass flux distribution. Diffuser component performance is dominated by the annular-radial bend. The incorporation of a constant sectional area bend is the key geometric feature in rendering the highly nonuniform turbine exit flow (dominated by tip clearance flows at the shroud) more uniform.

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Figures

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Fig. 1

Cantilevered rotor layouts: (a) conventional arrangement and (b) the proposed arrangement

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Fig. 2

A diffuser with arbitrary annular geometry

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Fig. 3

Sectional view of proposed geometry for analysis

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Fig. 4

Section view of diffuser geometries

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Fig. 5

Section view of a typical diffuser mesh

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Fig. 6

Mass flow averaged diffuser inlet pressure versus number of blade passes for different time-step values

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Fig. 7

Average diffuser inlet pressure versus number of blade passes for the selected time-step value

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Fig. 8

Diffuser inlet streamwise velocity for selected geometries

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Fig. 9

Diffuser on-design performance as a function of geometry: (a) Cp versus a and (b) K versus a

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Fig. 10

Diffuser pressure rise coefficient (Cp) along diffuser passage for selected geometries: (a) a = 0.075, (b) a = 0.1, (c) a = 0.15, and (d) a = 0.175

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Fig. 11

Diffuser total pressure loss coefficient (K) along diffuser passage for selected geometries: (a) a = 0.075, (b) a = 0.1, (c) a = 0.15, and (d) a = 0.175

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Fig. 12

Diffuser streamwise velocity (hub to shroud) at inlet (A), start of radial section (B), and end (C) for selected geometries. Stations shown in Fig. 3: (a) a = 0.075, (b) a = 0.1, (c) a = 0.15, and (d) a = 0.175.

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Fig. 13

Diffuser off-design performance for geometry a =0.15. See Table 4 for conditions: (a) Cp and (b) K.

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Fig. 14

Turbine stage efficiency at nominal flow conditions as a function of geometry, measured at locations shown in Fig. 3

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Fig. 15

Diffuser performance comparison for geometry a =0.15 by simulation type. Performance calculated at inlet (A), start of radial section (B), and end (C): (a) diffuser streamwise velocity and (b) diffuser pressure rise coefficient (Cp).

Tables

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