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Research Papers

Numerical Loss Investigation of a Small Scale, Low Specific Speed 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 15, 2018; final manuscript received April 4, 2019; published online May 6, 2019. Assoc. Editor: Phillip Ligrani.

J. Eng. Gas Turbines Power 141(9), 091003 (May 06, 2019) (10 pages) Paper No: GTP-18-1697; doi: 10.1115/1.4043430 History: Received November 15, 2018; Revised April 04, 2019

Radial inflow turbines, characterized by a low specific speed, are a candidate architecture for the supercritical CO2 Brayton cycle at small scale, i.e., less than 5 MW. Prior cycle studies have identified the importance of turbine efficiency to cycle performance; hence, well-designed turbines are key in realizing this new cycle. With operation at high Reynolds numbers, and small scales, the relative importance of loss mechanisms in supercritical CO2 turbines is not known. This paper presents a numerical loss investigation of a 300 kW low specific speed radial inflow turbine operating on supercritical CO2. A combination of steady-state and transient calculations is used to determine the source of loss within the turbine stage. Losses are compared with preliminary design approaches, and geometric variations to address high loss regions of stator and rotor are trialed. Analysis shows stage losses to be dominated by endwall viscous losses in the stator. These losses are more significant than predicted using gas turbine derived preliminary design methods. A reduction in stator–rotor interspace and modification of the blade profile showed a significant improvement in stage efficiency. An investigation into rotor blading shows favorable performance gains through the inclusion of splitter blades. Through these, and other modifications, a stage efficiency of 81% is possible, with an improvement of 7.5 points over the baseline design.

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Figures

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Fig. 1

Stator geometry definition

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Fig. 2

Geometry definition of rotor: (a) meridional passage definition, (b) quasi-orthogonal passage area schedule, and (c) blade wrap angle distribution

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Fig. 3

Breakdown of loss contributions to efficiency. Losses determined though preliminary methods (TOPGEN and Rohlik) are broken down to component level only, indicated by color.

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Fig. 4

Rotor velocity triangles for CFD and TOPGEN: (a) rotor inlet and (b) rotor outlet

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Fig. 5

CFD calculated absolute flow angle as a function of radial location in stator–rotor interspace

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Fig. 6

Comparative loss breakdown of stator designs by interspace (R2,T.E./R3) size

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Fig. 7

Stator entropy rise as a function of streamwise distance from inlet for various interspace sizings, R2,T.E./R3

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Fig. 8

Stator entropy rise as a function of streamwise distance from inlet for different blade designs, both with (R2,T.E./R3)=1.05

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Fig. 9

Pressure distribution of stator blades

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Fig. 10

Plot of flux averaged rotor relative Mach number along rotor passage

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Fig. 11

Rotor tip clearance contribution to entropy rise as a function of streamwise distance from inlet

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Fig. 12

Total to static efficiency as a function of jet speed ratio (ν) for different rotor geometries, (R2,T.E./R3)=1.05, linear thickness profile bladed stator

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Fig. 13

Rotor entropy rise as a function of streamwise distance from inlet

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Fig. 14

Breakdown of loss contributions to efficiency for baseline and refined designs

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Fig. 15

Three-dimensional stage geometry. Rotor radius and axial length identical: (a) baseline and (b) refined.

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