Effect of Runner Solidity on Performance of Elbow Draft Tube

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Procedia Environmental Sciences 00 (2011) 000–000

Energy Procedia 14 (2012) 2054 – 2059

 

Procedia Environmental Sciences www.elsevier.com/locate/procedia

Effect of Runner Solidity on Performance of Elbow Draft Tube Ruchi Kharea, Dr. Vishnu Prasadb, Dr. Sushil Kumar Mittalc a* a,b,c Department of Civil Engineering, M.A. National Institute of Technology, Bhopal – 462 051, India

Abstract The significant amount of kinetic energy comes out of runner in reaction turbines and the draft tube is used to recover this kinetic energy into useful pressure energy. The energy recovery in draft tube depends on its design and magnitude of velocity components at runner exit. Further the amount of kinetic energy coming out of runner depends on the solidity of runner as well as its rotational speed. Hence draft tube is important component of turbine next to runner. In this paper, 3D viscous turbulent flow simulation has been done in the complete flow passage of Francis turbine using commercial Computational Fluid Dynamics (CFD) code for three runner solidities at different rotational speeds. The draft tube performance parameters in non-dimensional form are computed from simulation results and the effects of runner solidity and operating speed on draft tube performance are discussed. The simulation results are also compared with the experimental results for validation and are found to be within reasonable accuracy.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee 2011International Published byConference Elsevier Ltd.on Selection and/or peer-review under responsibility of©2nd Advances in Energy Engineering (ICAEE). of [name organizer] Keyword: Draft tube; solidity; Francis turbine; efficiency; computational fluid dynamics. Nomenclature velocity at draft tube inlet (m/s) C3 velocity at draft tube outlet (m/s) C4 D runner diameter (m) net head of turbine (m) Hn n rotational speed of runner (rpm) V velocity (m/s) g acceleration due to gravity( m/s2) Δhd head loss in draft tube (m) draft tube efficiency (%) ηd υ energy recovery in draft tube (%)

1.

Introduction

Hydraulic turbines are used to extract energy from the continuously flowing water by dynamic action of water on the blades on runner. The flow passage has stationary and moving blades, thus the flow inside the turbine is highly complicated [1] and a well designed water path can only give the best performance. Every part of the turbine is designed independently and the performance of components individual as well as complete assembly needs to be assured. The design and performance of draft tube and runner affect the overall performance of the turbine significantly [2,3,4]. Hence the efficient design of runner blade as well as draft tube both needs detailed flow information. The energy transfer takes place in runner and solidity of runner not only affects the performance of runner but also of distributor and draft tube. The solidity may either be changed by

________________  *a: Tel+91- 0755 -405-3045 E-mail address: [email protected],[email protected]

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International   Conference on Advances in Energy Engineering (ICAEE). doi:10.1016/j.egypro.2011.12.1207

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changing the cord length or by changing the number of blades. The experimental study by changing the solidity is a complicated, lengthy and costly process. The numerical simulation of the complete turbine and its analysis is more informative and cost effective. Thus study of the performance of draft tube by changing the solidity of runner is an important aspect of the turbine design. In the present paper, the CFD analysis of a mixed flow Francis hydraulic turbine is carried out for three solidities of runner at constant guide vane opening using commercial Ansys CFX code at different rotational speeds of runner. The draft tube performance parameters like efficiency, head recovery and relative losses are computed and plotted against speed. The stream line and pressure plots in the draft tube are also compared for different solidities of runner. The overall performance of the turbine at the best operating regime is validated with the experimental results and found to be in close comparison. 2.

Geometry and mesh generation

The 3D geometry of complete turbine space of an experimentally tested Francis turbine consisting of casing, stay ring, distributor, runner and draft tube has been used for numerical simulation. Out of these components, runner is rotating and all other components are stationary. Therefore, each component of turbine has been modeled separately in Ansys Workbench and then assembled through proper interfaces for simulation of turbine as a whole. The diameter of runner is 1010 mm. The runner has been modeled with 11, 13 and 15 number of blades to get variation in solidity shown in fig.1. The elbow draft is used in this turbine with inlet and outlet areas as 1.028 m2 and 4.789 m2 respectively. The meshes of all components are generated in Ansys ICEM CFD. The tetrahedral elements are used for flow domain and triangular elements for 2D surfaces. The flow space of runner has 455749 nodes and 2209472 tetrahedral elements while draft tube has 273923 nodes and 1425659 elements.

(a)

(b)

(c)

Fig.1. Three dimensional view of runner with (a)11 blade; (b) 13 blades; (c) 15 blades

Fig. 2. Three dimensional view of draft tube 3.

Boundary conditions

The flow simulation is performed for constant guide vane opening of 80.93 mm and six rotational speeds of runner varying from 400 rpm to 900 rpm at an interval of 100 rpm. Mass flow rate at inlet is specified as 7200 Kg/s and kept constant throughout the simulation. As the flow is highly rotational, shear stress transport (SST)

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κ-ω turbulence model has been used for simulation. The walls of all domains are taken as stationary with no slip condition. All domains set stationary except runner. 4.

Formulae used The following formulae are used for computation of non-dimensional parameters: Velocity coefficient

v=

Draft tube loss

ξ=

V 2 gH

(1)

Δhd Hn

Energy recover in draft tube = υ

(2)

⎞ 1 ⎛ C 32 − C 24 − ξH n ⎟ × 100 ⎜ H n ⎝ 2g ⎠

(C

2 3

Draft tube efficiency

ηd =

Speed factor

nED =

− C42 − 2 gξ H n ) C32

(3)

×100

(4)

nD gH n

(5)

5. Results and discussions It is seen from fig.3 that pattern of turbine efficiency variation from experimental and CFD results is closely matching and the best operation regime is same in both the approaches. The energy recovery from draft tube depends on the design of draft tube and operating regime of turbine [5]. The absolute velocity at runner outlet in fig.4 initially found to be gradual decreasing but after 20 speed factor value, it starts increasing. It is also observed that more is the solidity, less is the absolute velocity. 1.0

94 CFD

92

0.9 Experimental

Absolute velocity coefficient

90

Efficiency (%)

88 86 84 82 80 78 76 74 16

At outlet for Z =11 At outlet for Z =13 At outlet for Z =15

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

20

24

28

32

Speed factor

36

40

0.0 15

20

25

30

Speed factor

Fig.3. Comparison of CFD and experimental results; Fig.4. Variation of absolute velocity

35

40

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0.7

0.5

0.5 0.4

Whirl velocity coefficient

Meridional velocity coefficient

0.6

0.3

0.2

0.1

At outlet for Z = 11 At outlet for Z = 13 At outlet for Z = 15

0.4 0.3 0.2 0.1 0.0 -0.1 At outlet for Z = 11 At outet for Z = 13 At outet for Z = 15

-0.2 -0.3 -0.4

0.0 15

20

25

30

35

40

-0.5 15

20

25

30

35

40

Speed factor

Speed factor

Fig. 5. Variation of meridional velocity; Fig. 6. Variation of whirl velocity

The meridional and whirl component of the velocity coming out of the runner at all three solidities are shown in fig. 5 and fig.6. It is observed that meridional velocity has increasing pattern with rotational speed but rate of increase is more at lower solidity. The whirl velocity also increases with speed but very less affected by solidity. It is also seen that at low speed, its direction is opposite to peripheral velocity of runner. It is depicted from fig.7 that relative velocity increases gradually with rotational speed of runner and also affected a little by solidity. The draft tube efficiency in fig. 8 has parabolic variation at all solidities and it is observed that maximum efficiency points are close to design speed factor i.e. 27.4 of turbine but it slightly shifts towards higher speed factor as solidity increases. The losses in draft tube again have parabolic variation as shown in fig.9 but variation is reverse to that of efficiency which is obvious because efficiency will be maximum where losses are minimum. The energy recovery in draft tube increases with speed at all solidities up to speed factor value of 30 to 35 and then starts decreasing with further increase in speed as seen from fig.10. The head recovery is the highest at designed solidity. Major part of head recovery in draft tube occurs from meridional velocity [8] and hence head recovery increases with speed at all solidities due to increased meridional velocity. The decrease of head recovery at higher speed factor may be due to much increased whirl velocity and its change of direction at outlet of runner as compared to meridional velocity.

1.0

90 Z = 11 Z = 13 Z = 15

80

0.8 0.7 0.6 0.5 0.4 0.3

At outlet for Z = 11 At outlet for Z = 13 At outlet for Z = 15

0.2

70 60 50 40 30 20

0.1 0.0 15

Draft tube efficiency (%)

Relative velocity coefficient

0.9

20

25

30

35

40

10 15

20

Speed factor

Fig. 7. Variation of relative velocity; Fig. 8. Variation of draft tube efficiency

25

30

Speed factor

35

40

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Fig. 9. Variation of draft tube losses; Fig. 10. Variation of draft tube recovery

Fig.11 – Stream lines pattern in draft tube at three solidities

Fig.12 – Pressure contours in draft tube at three solidities

The streamline patterns in draft tube at constant speed of 600rpm for three solidities in fig.11 and whirl velocity variation in fig.6 indicate that the amount of whirl coming out of runner decreases with increase in solidity.

Ruchi Kharea et al.\ / Energy Procedia 14 (2012) 2054 – 2059

The maximum velocity zone is observed at the inner side of elbow in all three solidity configuration and consequently the low pressure zone at this location in fig. 12. The velocity reduces and pressure increases from inlet to outlet of draft tube leading to conversion of kinetic energy into pressure energy. The stream line pattern is smooth at designed solidity i.e, Z=13 in comparison to other two solidities. The range of pressure variation in draft tube decreases with increase in solidity as seen from fig.12 and hence occurrence of cavitation in draft tube may be minimum at higher solidity . 6.

Conclusions

It is seen that the meridional and whirl components of velocity coming out of runner are dependent on the operating regime of turbine and it is found that these velocities in turn affect the losses and energy recovery in draft tube. The energy recovery increases up to certain speed factor and after that it starts decreasing but it is high at low solidity of the runner. It is found that the draft tube loss and efficiency have parabolic variation with speed factor and the point of maximum efficiency or minimum loss shifts towards higher speed factor with decrease in solidity. The loss, efficiency and recovery characteristics of draft tube using CFD will be useful for design optimization of draft tube geometry to give the best performance. References [1]

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