Misaligned guide vanes (MGVs), runner hub modification and J-grooves in draft tubes have been proposed and numerically evaluated. It was inferred that all the cases changed the flow configuration of the draft tube without significant loss in efficiency at a given operating point.
Introduction
- Prelude
- Small hydro powers and hydro turbines
- Energy conversion in hydro turbine
- Components of Francis hydro turbine
- Runner
- Casing, guide vanes and draft tube
- Cavitation in Francis turbine
A runner's specific speed is closely related to the runner's form. The main attribute of x-blade designs is related to the uniform flow distribution in the runner [9].
Design and performance analysis of 70kW runner
Design
For maximum efficiency, a free swirl power is applied at the runner outlet with no swirl for best performance ie. the eddy component of the velocity at the outlet is zero. The hydraulic efficiency of the turbine is expressed as,. where 'U' is the peripheral speed of the impellers and 'Cu. Using the empirical relation to calculate the reduced peripheral velocity of the blade at the inlet.
From calculation iteration and experience, take u1 =0.8 5. Now the diameter of the runner at the inlet is 60. And the height of the runner blade equal to the width of the guide shoes is determined by solving the continuity equation. The runner was modeled in Unigraphics NX6 using the preceding calculated parameters with legitimate improvement in dimension considering the thickness of the blades (Fig. 2.4).
The ratio between the diameter of the runner and the diameter of the circle passing through the center of the guide vanes, Dg.
Numerical analysis
The commercial 3D Navier-Stokes CFD solver ANSYS-CFX was used to analyze the performance of the turbine. Considering a steady state incompressible uniform flow of fluid in the circumferential direction of the turbine, a steady state single phase (water only) analysis of the full turbine model was performed at full load and part load conditions maintaining head and rpm (Fig .2.7) ). Effective pressure in terms of pressure is mentioned at the inlet of spiral case and mass flow rate is specified at the outlet of the draft tube as boundary conditions.
The input boundary condition consists of the radial velocity or transport velocity and the eddy component of the velocity for the given operating conditions. The complete turbine model is designed by combining components with a frozen rotor interface, each between the casing and the runner and the runner and the draft tube using the Generic Grid Interface (GGI) method for the grid connection.
Results and dissusions
The pressure distribution in the middle plane of the house at 4 different operating conditions is shown. Fig. The symmetrical pressure distribution in the strut vane area near the BEP indicated uniform flow distribution in the housing. A symmetrical info in the runner at full load operating point could be observed in fig.
However, as the flux decreased, there was disruption of the streamlines due to the asymmetric current distribution in the spiral case. The distribution of pressure in the runner depends on the inflow of water, as the water exerts a different pressure in different modes of operation. As the flow rate decreased, the pressure on the vane surface on the pressure side, especially at the leading edge (LE), also decreased.
The additional inertial force produced by the flow moving past the draft tube elbow makes the flow in the diffuser unbalanced.
Partial load performance of Francis turbine
Flow theory
The commercial 3D Reynolds Averaged Navier-Stokes CFD solver Ansys-CFX v13.0 was used to investigate the time varying unsteady flow through a vertical 70 kW Francis turbine in its stationary and transient passages at 100% load (0.5 m3/s). 72% load (0.36 m3/s) where the transient flow fields in the casing, runner and draft tube are simulated. Due to the non-uniform inlet from the spiral case and the uneven lift of the guide vanes and runner, the computational grid of the entire turbine with all runner and cascade channels together was considered, without applying any periodicity to any component. For the transient analysis, a 2° runner rotation time step was taken for 10 full rotations of the runner.
The governing equations are discretized by the finite volume method in the spatial direction and by the finite difference method in the time direction.
Flow analysis
However, the pressure distribution in the spiral case is symmetrical enough to maintain uniform flow. The pressure distribution in the pressure side and suction side of the runner sheet was uniform. Similarly, the magnitude of velocity in the pressure side was small while the flow on the suction side of the blade was accelerated.
From the flow feature in the draft tube at full load, as shown in Figure 3.6, the flow in the volute casing accelerated as it slid through the vanes and entered the runner. A large part of the turbine performance depends on the flow behavior in the draft tube and the flow condition in the draft tube depends on the operating point. The swirl velocity that appears behind the runner at part load has a major influence on the flow condition in the draft tube.
The vibration level in the runner blade is relatively higher than that in the housing and draft tube.
Vortex shedding in draft tube
Vortex rope in part flow
The phenomenon of flow associated with the undulation of the drag tube is called vortex shedding, and its movement is called vortex precession. The resulting hydraulic loss in the draw pipe is strongly influenced by the intensity of the eddy current at the exit from the guide [37]. A cavitation vortex occurs when the pressure in the draft tube is low enough and the corresponding pressure in the vortex core falls below the vapor pressure of the liquid.
The vortex rope profile changes as the ratio of the two components changes. The pressure distribution in the middle section of the cone of the jet pipe in different cases of rotation of the runner is shown in Fig. Time-varying pressures were recorded at 5 locations of the flow tube (Fig. 3.12) and case to capture the pressure pulsation.
The pressure amplitudes in the wall of the draft tube cone were found to be higher than in the center because these areas were in close contact with the vortex rope (Fig. 4.5 b, c, d).
Remedial attempts
Other typical devices are the fins attached to the draft tube wall, concentric cylinders in the draft tube which are supposed to suppress vortex breakdown in partial discharges. Extension of runner cone in the form of a snorkel, either attached to the runner or fixed in the draft tube, sometimes in tandem with air injection, can also change the reverse flow region to reduce flow instabilities.
Evaluation of vortex control techniques
Influence of runner hub profile
Among the different profiles evaluated, two profiles, one with an asymmetrically cut bottom (belonging to the frustum of a cone) and the other with a symmetrically cut center with a central cut helped to improve the flow in the pipe. current and minimized the rotation speed (Fig. 5.1). Between the two profiles, case 2 performed better in minimizing the vortex rope size and reducing the swirl intensity, as seen in Figs. a) Average circular velocity in 4 layers of the current tube with case1. b) Average circular velocity in 4 layers of the current tube with case2. From the pressure contours checked in 4 different layers of the jet tube cone, the low pressure area and the rotation intensity have relatively.
The strength of vorticity at the core of the draft tube flow was also lowered in both cases by modified hub that improved the flow in the axial direction (Fig. 5.3). The circumferential speed at the upper part of the draft tube cone along planes #1, #2 and #3 in the inner section of the flow has been reduced admirably with new hub box2. As the flow moved past the cone region, the average velocity was almost the same magnitude as with the old hub in the lower part of the draft tube.
The flow velocity beyond 0.8 r/R and near the draft tube wall remains constant along the length of the draft tube cone as studied in its 4 sections.
Influence of misaligned guide vanes (MGVs)
5.6, the circumferential position of the first MGV is kept above 45° from the vertical axis to minimize the influence of the outer layer inlet boundary, and the location of the second MGVs is symmetrical to avoid oscillations caused by resonant coupling. Time-dependent numerical analysis was performed followed by steady-state simulation with the same time step setting of 2° for 10 complete revolutions of the runner. The pressure distribution in the central vertical plane of the jet pipe as in Fig.
The average circumferential velocity taken at 4 different levels of the draft tube cone in Fig. Rather a very small amount of increment in swirl velocity could be observed at the core flow region of the draft tube with 40° MGV. The vortex intensities in the outer part towards the draft tube wall were similar in both cases.
From the results obtained from the transient analysis, the size of the vortex rope and its intensity were both larger with MGV, at both opening angles.
Application of J-grooves in the draft tube
A comparative pressure map in fig. 5.13 shows the pressure distribution in the center plane of the draft tube with the basic model and 4 aforementioned cases with J-grooves. The pressure map of the base model draft tube has a low-pressure zone at the central flow field, which is the result of the swirling flow and thus contributes to the formation of vortex ropes. The average circumferential velocity in the J-groove draft tube is compared with that in the base model draft tube in fig.
The magnitude of the velocity decreased more prominently along the wall of the draft tube (r/R>0.8) due to the presence of J-grooves. The swirl speed has been reduced noticeably in all the 4 different layers of the draft tube cone along the vertical locations near the J-groove area. Correspondingly, the axial velocity in the central flow area of the grooved draft tube has been significantly improved.
The eddy and turbulent kinetic energy distributions of the draft tube flow were examined for 13G-10 mm and 16G-10 mm relative to those of the baseline draft tube model.
Conclusion
Thus, the techniques discussed here can be extended for further design optimization and evaluation to best suit the flow characteristics of the specific turbine under consideration. In this study, only one operating point at partial load, 72% load, is considered to investigate the flow features and vortex rope genesis. The nature (size, shape, direction of rotation, dominant frequencies) of the vortex rope can be controlled for overloads as well as for other regimes of part-load operations.
IAHR International Working Group Meeting on Cavitation and Dynamic Problems in Hydraulic Machines and Systems, Czech Republic. 3D Numerical flow analysis in a Francis turbine runner with medium specific speed at off-design operating conditions. CFD Simulation of Pressure and Discharge Surges in Francis Turbine at Non-Design Conditions, 26.
Experimental and numerical analysis of part-load cavitating vortex dynamics of low-head hydraulic turbines.
