Performance Analysis of 70 kWMicro-class Francis Hydro Turbine by CFD

1

Anup KCFlow Informatics Lab, Korea Maritime and Ocean University, Busan, South Korea

Current AffiliationDepartment of Mechanical Engineering, Kathmandu University, Nepal

4/7/2014

International Symposium on Current Research in Hydraulic Turbines IV

Supervisor

• Prof. LEE, Young Ho, Ph.DFlow Informatics LabDepartment of Mechanical and Energy System EngineeringSchool of EngineeringKorea Maritime and Ocean UniversityBusan, South Korea

4/7/2014 2

Contents• Steady state performance analysis of 70kW Francis Turbine by variable

flow rates- steady state analysis

• Transient analysis of the turbine at full load

• Vortex shedding at part load operation of the turbine

4/7/2014 3

Francis Turbine for Micro hydropowerHigh head,low discharge

Low head,high discharge

Hydro Turbine

Impulse Type

Reaction Type

Francis

Cross Flow

Turgo

Pelton

Propeller/Kaplan

Bulb

Francis

4/7/2014 4

Design Parameters• Hydraulic Head (Hn): 18m• Discharge(Q): 0.5m3/s• Chosen Rotational Speed (n): 900 rpm• Specific speed (Ns): 203.096

• Theoretical power of the turbine • Considering 80% efficiency of the turbine,

kWQhHP nt 88�� �

kWQhHP nr 70����45

H

PnNs �

4/7/2014 5

Turbine Components

CFD Domains1. Spiral casing

• 16 guide vanes, 35• 8 stay vanes

2. Runner• 13 runner vanes

3. Draft tube• Single channel draft

tube

4/7/2014 6

Spiral casingGuide vanes

Stay vanes

Runner

Draft Tube

Draft tube cone

4/7/2014 7

Grid Discretization

Meshing

• Tetra elements with prism layers, Hexa for draft tube• Total no. of nodes: 5.4Millions

4/7/2014 8

CW from top: Spiral case, Runner, Draft Tube and Inlet Pipe

Numerical Analysis: Boundary Conditions

Input Parameters:• Working Fluid: Water @25C, Water vapor at 25C• Physical Timescale: 0.0012 (1/ɷ)• Turbulence Model: k-ɷ SST• Reference Pressure: 1 atm.• Simulation Type: Steady state• Solver: Ansys CFX v.13

4/7/2014 9

inlet

outlet

t

etetleleleoututt

Inlet: 18m Pressure head (Hn)Outlet: mass flow rate (Q)Rotational Speed: 900 rpm (fn=15Hz)

Performance Evaluation

• Evaluation by varying discharge• At full load, P=69.5kW, 88% efficient• Power and efficiency drops at part loads due to loss in Head in guide vane, runner

and draft tubeCases Discharge (Q, m3/s) Net head (Hn,m) Shaft Power (Pr, kW) Efficiency (%)

1 0.5 (100%) 16.15 69.5 88.01

2 0.43 (86%) 12.4 42.37 81.32

3 0.36 (72%) 10.47 22.35 60.62

4 0.29 (58%) 8.6 11.06 45.4

Flowrate (m3/s)

0.25 0.30 0.35 0.40 0.45 0.50 0.55

Pow

er (k

W)

0

20

40

60

80

100

Effi

cien

cy (%

)

0

20

40

60

80

100

Power (kW)Efficiency (%)

Flowrate (m3/s)

0.25 0.30 0.35 0.40 0.45 0.50 0.55

Hea

d (m

)

0

5

10

15

20

4/7/2014 10

BEP

Flow in Runner

• Pressure difference between crown and band decreases at part load and lowers the magnitude of torque produced

• Cp graph depicts the blade pressure distribution at different flow rates• At part flow rates, smaller pressure difference in blade surfaces, smaller torque

transfer

4/7/2014 11

Pressure distribution in runner and Cp graph at different flow rates

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cp

R/Rt

0.5 cms

0.43 cms

0.36 cms

0.29 cms

Flow in Draft Tube

• Streamlined flow at full load, free of recirculation zone and flow separation• Flow recirculation, dead water region at central region• swirl at part loadsdue to swirling flow• Swirl flow responsible for vortex breakdown and fluctuations in torque/pressure• Hinders efficiency and performance

4/7/2014 12

0.43m3/s 0.36m3/s 0.29m3/s0.5m3/s

4/7/2014 13

Transient Analysis

Numerical Methods:

• 2nd order, Upwind Scheme• 0.5m3/s flow rate for time dependent analysis• 0.36m3/s flow rate (72% load) for vortex shedding

• Steady state result with kɷ-SST model used as initial value for transient simulation• Approx. 1800 X 3 (coefficient loop) iterations, 7 days of CPU time to complete the

simulation

Spiral case

Runner

Draft tube

Numerical Code Ansys CFX v.13

Simulation Type Transient

Turbulence Model kɷ-SST for full load analysisRNG k-ϵ for vortex shedding

Timestep 0.00037037[s]

Total time 0.66667 [s]

4/7/2014 14

inlet

outlet

Flow features in mid span of spiral case

• Velocity profile at the inlet is uniform and evenly distributed• Equivalent pressure contour is coherent with uniform flow velocity• No collision at inlet and flow separation at outlet, at full load operation

Velocity vector Velocity contour Pressure contour

4/7/2014 15

Flow feature in tandem cascade

• Flow velocity increased as it moves in the runner, corresponding pressure decreased

• Pressure distribution at stay vanes higher, lower at the runner inlet – flow transformation from radial to axial

• Strongly accelerated flow toward the runner indicate relative reduction in pressure

4/7/2014 16

Pressure contour and velocity distribution in runner and spiral case

Flow feature in runner blade

• decrease in pressure from inlet of the blade to outlet, on both pressure and suction side

• Velocity magnitude in pressure side is smaller than that in suction side

4/7/2014 17

Pressure in runner blade Velocity streamlines on runner blade

Torque distribution in runner blades

• Torque distribution profile in 2 runner blades for 5 runner rotation• 7% fluctuation in Torque in each blade• Periodic pattern of torque spectra• Average torque fluctuation (of all 13 blades) steady• Peak to peak amplitude indicates guide vane influence

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

6 6 6 6 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 10

T/Ta

vg [N

m]

rotation

avg-b7

avg-b11

avg torque on all 11 blades

16 peaks/runner rotation

4/7/2014 18

Pressure distribution in runner

• Runner blades– Average pressure fluctuation of 5%– Periodic throughout the simulation

• Draft tube– Variation of static pressure at full load is uniform in draft tube

4/7/2014 19

Pressure fluctuation in runner blade Avg. pressure fluctuation in draft tube

4/7/2014 20

Draft Tube Surge

Vortex shedding:• Draft tube surge occurs in Francis turbine when operated in part load regime• flow at the exit of the runner is non-uniform and unstable• Flow instability due to swirl component of the velocity downstream the runner• Pressure fluctuation due to unsteady vortex behavior in draft tube• Causes vibration, variation in power output and component damages

Velocity diagram at runner outletcu2 (Swirl component)

4/7/2014 21

During design, C2=cm2 (cu2=0 for no swirl)

Effect of Turbulence Models• Turbulence models have remarkable influence on CFD results• Appropriate turbulence model required to accurately obtain the shape of vortex

rope and flow pattern

kɷ-SST SAS-SST RNG-kϵ

4/7/2014 22

Vortex breakdown in draft tube

• A non-cavitating rope in low pressure zone at the centre of draft tube• Rotates with 20-30% of runner’s rotation speed

Flow(Q) 0.36m3/s (72% load)

Net Head(H) 9.54[m]

Shaft Power (P) 19.644[kW]

Hydraulic Efficiency(ὴ) 58.48[%]

Shape of vortex rope and time averaged Pressure map in central plane of draft tube cone

4/7/2014 23

Vortex rope in draft tube

• Vortex rope has shape of a rotating cork screw• A non-cavitating rope in low pressure zone at the centre of draft tube• 10 full runner rotations to achieve fully developed vortex rope• Rotates with 20-30% of runner’s rotation speed

4/7/2014 24

Growing vortex rope at different instances of runner rotationggGrowGrowininng ng vovorte

Flow features in draft tube:

• Vorticity on vortex rope has tangential component (left) and axial component (right)

• Intensity of rope (twist) depend on flow rate and g.v. opening angle• The flow is reversed in the mid section of the draft tube due to tangential vortex

Flow vector at mid section of draft tube Flow vector at A-A section of draft tube cone

4/7/2014 25

Amplitude spectra at runner blade

• Fourier transformed pressure signals in blade suction side and pressure side

• Dominating frequency- 0.81% of fn– 12.15Hz for pressure side, 12.18Hz

on suction side

• Amplitudes on the suction side for both 1st and 2nd harmonics were larger than those on the pressure side

4/7/2014 26

Frequency (Hz)

0 20 40 60 80 100

Am

plitu

de/ P

a

0

2000

4000

6000

8000

10000

blade pressure side, bp2

1st harmonics

2nd harmonics

Frequency (Hz)

0 20 40 60 80 100

Ampl

itude

/Pa

0

2000

4000

6000

8000

10000blade suction side, bs2

1st harmonics

2nd harmonics

Conclusion• Design of micro scale Francis Hydro turbine• BEP: 18m head, 0.5m3/s, 35 guide vane angle• 69.7kW at 88% efficiency

• Time dependent analysis to analyze rotor stator interaction• Fluctuation of pressure and torque due to the interaction between guide vanes

and runner

• At part load operation, flow in draft tube suffers instability- swirling flow• Results in vortex shedding, induces pressure fluctuation/vibration• Rotated approx. with 19% the rotational frequency of runner

• Different vortex control techniques are tried as further study

4/7/2014 27

4/7/2014 28

anupkc@ku.edu.np