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Research Papers: Flows in Complex Systems

Mechanism of the S-Shaped Characteristics and the Runaway Instability of Pump-Turbines

[+] Author and Article Information
Linsheng Xia

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: xialinsheng@whu.edu.cn

Yongguang Cheng

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: ygcheng@whu.edu.cn

Jianfeng You

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: youjf@whu.edu.cn

Xiaoxi Zhang

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: zhangxiaoxi@whu.edu.cn

Jiandong Yang

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: jdyang@whu.edu.cn

Zhongdong Qian

State Key Laboratory of Water Resources and
Hydropower Engineering Science,
Wuhan University,
Wuhan 430072, China
e-mail: zdqian@whu.edu.cn

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 6, 2015; final manuscript received October 11, 2016; published online December 7, 2016. Assoc. Editor: Kwang-Yong Kim.

J. Fluids Eng 139(3), 031101 (Dec 07, 2016) (14 pages) Paper No: FE-15-1721; doi: 10.1115/1.4035026 History: Received October 06, 2015; Revised October 11, 2016

Understanding the formation mechanism of the S-shaped characteristics (SSCs) and the relationship between flow structures and the runaway instability (RI) is the prerequisite for optimizing runner design to promote operational reliability and flexibility. In this study, a new turbine equation is derived to reveal the prime cause of the SSCs, and the influence of geometric parameters on the SSCs is analyzed. Moreover, the flow patterns in three model turbines of different specific-speeds are simulated by unsteady Computational fluid dynamics (CFD), and the correlation between inverse flow vortex structures (IFVSs) and the RI in the SSCs region is identified. Theoretical analysis shows that the turbine equation can theoretically predict the change trend of the first quadrant SSCs curves of the pump-turbines; the flow losses caused by small blade inlet angle, instead of the diameter ratio, are the primary cause of the SSCs. The numerical simulation results reveal that the IFVSs at the hub side of the runner inlet are the origin of the RI; when operating points are far away from the best efficiency point (BEP), the IFVS locations change regularly. For large guide vane openings (GVOs), the IFVSs first incept at the shroud side, and then translate to the hub side, and further back to the midspan, when the discharge decreases. The inception points (IPs) of the SSCs correspond to the onset of the IFVSs at the hub side, which are in advance of the zero-torque operating points (ZTOPs); therefore, the ZTOPs are located in the positive slope region, leading to RI. For small GVOs, however, the IFVSs only locate at the midspan; the IPs of the SSCs, having no definite correlation with the IFVSs, are coincided with or are below the ZTOPs, because the ZTOPs are in the negative slope region and RI disappears. It is also found that the IPs of SSCs are the turning points of the predominant states between the turbine effect and pump effect. These results are valuable for design and optimization of pump-turbine runners.

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References

Figures

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Fig. 1

Velocity triangles at the runner inlet and outlet

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Fig. 2

Diagram for the formation of the S-shaped characteristic curve

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Fig. 3

Speed-discharge characteristics by the theoretical turbine equations: (a) pumping effect neglected (Eq. (8)) and (b) pumping effect considered (Eq. (10))

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Fig. 4

Comparisons between the predicted and measured characteristics: (a) BQ, (b) XJ, (c) BLH, and (d) SBY

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Fig. 5

Loss coefficients for different turbines

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Fig. 6

Influence of the inlet blade angle β1b

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Fig. 7

Influence of the diameter ratio D1/D2

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Fig. 8

Computational domain and grid: (a) geometry and (b) grid

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Fig. 9

Grid dependency test at normal operating points for the three turbine models

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Fig. 10

Characteristic curves of model 1 for GVO 24 deg: (a) n11Q11 and (b) n11T11

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Fig. 11

Characteristic curves and flow behavior of model 2 for GVO 20 deg: (a) speed-discharge characteristics and (b) average flow rate distributions along the spanwise at the runner inlet

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Fig. 12

Characteristic curves and flow behavior of model 1 for GVO 6 deg: (a) speed-discharge characteristics and (b) average flow rate distributions along the spanwise at the runner inlet

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Fig. 13

Characteristic curves and flow behavior of model 2 for GVO 6 deg: (a) speed-discharge characteristics and (b) average flow rate distributions along the spanwise at the runner inlet

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Fig. 14

Characteristic curves and flow behavior of model 3 for GVO 20 deg: (a) speed-discharge characteristics and (b) average flow rate distributions along the spanwise at the runner inlet

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Fig. 16

Flow vortex structures at the runner inlet of model 1 for GVO 24 deg: (a) separations emerging Q11 = 0.167 m3/s, (b) inverse flow at shroud side Q11 = 0.139 m3/s, (c) inverse flow at hub side Q11 = 0.094 m3/s, and (d) inverse flow at midspan Q11 = 0.023 m3/s

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Fig. 17

Flow vortex structures at the runner inlet of model 1 for GVO 6 deg

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Fig. 18

Variations of the pressure coefficients along the streamwise of model 1 for GVO24 deg: (a) total pressure coefficient Ctp, (b) partial enlarged view of Ctp, (c) static pressure coefficient Csp, and (d) kinetic pressure coefficient Ckp

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Fig. 19

Variations of the pressure coefficients along the streamwise of model 1 for GVO 6 deg: (a) total pressure coefficient Ctp, (b) partial enlarged view of Ctp, (c) static pressure coefficient Csp, and (d) kinetic pressure coefficient Ckp

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