International Symposium on Current Research in Hydraulic Turbines IV. Performance Analysis of 70 kw Micro-class Francis Hydro Turbine by CFD.

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2 International Symposium on Current Research in Hydraulic Turbines IV Performance Analysis of 70 kw Micro-class Francis Hydro Turbine by CFD Anup KC Flow Informatics Lab, Korea Maritime and Ocean University, Busan, South Korea Current Affiliation Department of Mechanical Engineering, Kathmandu University, Nepal 4/7/ Supervisor Prof. LEE, Young Ho, Ph.D Flow Informatics Lab Department of Mechanical and Energy System Engineering School of Engineering Korea Maritime and Ocean University Busan, South Korea 4/7/2014 2

3 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/ Francis Turbine for Micro hydropower Pelton High head, low discharge Impulse Type Turgo Hydro Turbine Cross Flow Francis Reaction Type Propeller/Kaplan Bulb Low head, high discharge 4/7/2014 4

4 Design Parameters Hydraulic Head (H n ): 18m Discharge(Q): 0.5m 3 /s Chosen Rotational Speed (n): 900 rpm Specific speed (Ns): Pt QhH n 88kW Theoretical power of the turbine Considering 80% efficiency of the turbine, Pr QhH n 70kW n N s P 5 4 H 4/7/ Turbine Components Stay vanes Spiral casing Guide vanes Runner CFD Domains 1. Spiral casing 16 guide vanes, 35 8 stay vanes Draft tube cone 2. Runner 13 runner vanes 3. Draft tube Single channel draft tube Draft Tube 4/7/2014 6

5 Grid Discretization 4/7/ Meshing CW from top: Spiral case, Runner, Draft Tube and Inlet Pipe Tetra elements with prism layers, Hexa for draft tube Total no. of nodes: 5.4Millions 4/7/2014 8

6 Numerical Analysis: Boundary Conditions Input Parameters: Working Fluid: Water vapor at 25C Physical Timescale: (1/ɷ) Turbulence Model: k-ɷ SST Reference Pressure: 1 atm. Simulation Type: Steady state Solver: Ansys CFX v.13 inlet Inlet: 18m Pressure head (H n ) Outlet: mass flow rate (Q) Rotational Speed: 900 rpm (f n =15Hz) outlet 4/7/ Performance Evaluation Power (kw) Flowrate (m 3 /s) BEP Power (kw) Efficiency (%) Efficiency (%) Flowrate (m 3 /s) 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 tube Cases Discharge (Q, m 3 /s) Net head (H n,m) Shaft Power (P r, kw) Efficiency (%) (100%) (86%) (72%) (58%) Head (m) /7/

7 Flow in Runner Cp cms cms cms cms R/Rt Pressure distribution in runner and Cp graph at different flow rates 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/ Flow in Draft Tube 0.5m 3 /s 0.43m 3 /s 0.36m 3 /s 0.29m 3 /s 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/

8 Transient Analysis 4/7/ Numerical Methods: Numerical Code Ansys CFX v.13 Simulation Type Transient Turbulence Model kɷ-sst for full load analysis RNG k-ϵ for vortex shedding Timestep [s] Total time [s] Spiral case Runner inlet 2 nd order, Upwind Scheme 0.5m 3 /s flow rate for time dependent analysis 0.36m 3 /s flow rate (72% load) for vortex shedding Steady state result with kɷ-sst model used as initial value for transient simulation Approx X 3 (coefficient loop) iterations, 7 days of CPU time to complete the simulation Draft tube outlet 4/7/

9 Flow features in mid span of spiral case Velocity vector Velocity contour Pressure contour 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 4/7/ Flow feature in tandem cascade Pressure contour and velocity distribution in runner and spiral case 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/

10 Flow feature in runner blade Pressure in runner blade Velocity streamlines on 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/ Torque distribution in runner blades T/Tavg [Nm] 16 peaks/runner rotation rotation avg-b7 avg-b11 avg torque on all 11 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 4/7/

11 Pressure distribution in runner Pressure fluctuation in runner blade Avg. pressure fluctuation in draft tube 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/ Draft Tube Surge 4/7/

12 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 During design, C 2 =c m2 (c u2 =0 for no swirl) c u2 (Swirl component) Velocity diagram at runner outlet 4/7/ 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/

13 Vortex breakdown in draft tube Flow(Q) 0.36m 3 /s (72% load) Net Head(H) 9.54[m] Shaft Power (P) [kW] Hydraulic Efficiency(ὴ) 58.48[%] Shape of vortex rope and time averaged Pressure map in central plane of draft tube cone A non-cavitating rope in low pressure zone at the centre of draft tube Rotates with 20-30% of runner s rotation speed 4/7/ Vortex rope in draft tube Growing vortex rope at different instances of runner rotation 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/

14 Flow features in draft tube: Flow vector at mid section of draft tube Flow vector at A-A section of draft tube cone 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 4/7/ Amplitude spectra at runner blade blade pressure side, bp2 Amplitude/ Pa Amplitude/Pa st harmonics 2nd harmonics st harmonics 2nd harmonics Frequency (Hz) Frequency (Hz) blade suction side, bs2 Fourier transformed pressure signals in blade suction side and pressure side Dominating frequency- 0.81% of f n 12.15Hz for pressure side, 12.18Hz on suction side Amplitudes on the suction side for both 1 st and 2 nd harmonics were larger than those on the pressure side 4/7/

15 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/ /7/