Development of Design Tool for Low-Head Francis Turbine. * Corresponding author

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1 Proceedings of the International Symposium on Current Research in Hydraulic Turbines CRHT VI March 14, 2016, Turbine Testing Lab, Kathmandu University, Dhulikhel, Nepal Paper no. CRHT Development of Design Tool for Low-Head Francis Turbine Lars Frøyd 1*, Kristine Gjøsæter 2 and Ole G. Dahlhaug 3 1 Turbine Testing Lab, Kathmandu University, Dhulikhel, Nepal 2 Multiconsult ASA, Drammen, Norway 3 Waterpower Laboratory, Norwegian University of Science and Technology, Trondheim, Norway * Corresponding author (lars.froyd@4subsea.com) Abstract This article describes an ongoing development of a numerical design methodology for low-head Francis turbines and the implementation of this methodology in an existing parametric design tool. Design of low-head Francis turbines is in some ways similar to high-head Francis design, but is more complicated from a numerical design point of view. The parametric design software Khoj was initially developed for design of high-head Francis turbines, but has been modified and expanded to allow modelling of certain geometrical features typical for low-head Francis turbines. An example design of a low-head Francis turbine was developed based on the characteristics of a large scale power plant. Furthermore, the applicability of the low-head designs are verified numerically by CFD analyses. The analysis of the example design shows that the overall results match the expected results well, which indicates that the resulting turbine and blade geometry resulting from Khoj is of good quality also for the low-head design. Keywords: Low-head Francis Turbine, Hydraulic turbine design, Integrated design tool, CFD, Hydropower 1. Introduction Nepal is blessed with access to large water resources from regular monsoon rain and glacier run-off from the Himalayas, giving a great potential for hydropower. The estimated potential is MW, out of which approximately MW is technically feasible. Despite of this, only about 2 % of the feasible capacity has been exploited [1]. Hydropower development in the Himalayan region is challenging, due to large amounts of sediments that are carried by the rivers. The sediment transport in the rivers reaches its peak during the monsoon period. Most of Nepal s hydropower plants are run-of-river plants, which are especially vulnerable to sediment content due to the lack of sediment settling facilities that a dam provides. To reduce the sediment transport through the turbines large and costly sediment settling basins are often constructed, but many hydropower plants still experience significant sediment induced turbine wear problems, causing loss in energy generation and frequent maintenance stops. In order to facilitate turbine designs that are more resilient towards sediment erosion, the Turbine Testing Laboratory (TTL) at Kathmandu University (KU) and the Waterpower Laboratory at the Norwegian University of Science and Technology (NTNU), have jointly developed a parametric design software Khoj, implemented in MATLAB [2]. The software is based on a simple and efficient methodology for

hydraulic design of high-head Francis turbines developed over many years at NTNU, and includes facilities for automated model export to industry-standard CFD tools and CAD systems for detailed

2 hydraulic design of high-head Francis turbines developed over many years at NTNU, and includes facilities for automated model export to industry-standard CFD tools and CAD systems for detailed numerical analyses of runner flow characteristics and structural properties. The Khoj software gives the designer extensive control of the runner blade curvature and the velocity of the flow relative to the blade. Optimizing these characteristics are believed to have the potential to significantly reduce the blade erosion rate for a given sediment load of the turbine. This ability together with the automated export facilities provided by Khoj has induced a range of research activities related to Francis turbine design and sediment erosion at both KU and NTNU, e.g. [2,3,4]. However, only a limited amount of Nepal s hydropower match the high head and moderate flow characteristics typically associated with the application of high-head Francis turbines. The site characteristics of the larger rivers and lower hill areas and flatlands may typically suggest low-head Francis turbines, or in some cases other lower head turbine types. These streams, although often more slowly running, may still have significant sediment transport, and similar hydraulic optimization techniques as applied for high-head Francis turbines could have a positive effect on sediment erosion in the turbine runners of low-head Francis turbines as well. However, a similar design methodology for lowhead Francis turbines has not been available, and this deficiency in efficient design capabilities has led to the current efforts to expand and further develop Khoj to also include low-head Francis turbines. 2. Design Considerations for Low-Head Francis Turbines Although the designs are conceptually similar, the runner geometries of high-head Francis turbines and low-head Francis turbines are in many ways different. Typical cross-section views of high-head Francis turbines and low-head Francis turbine are shown in Figure 1. Figure 1. Typical cross-section views of high-head Francis turbines (left) and low-head Francis turbine (right). The most important differences in a low-head design compared to a high-head design from a numerical design perspective is: 1. The curved shape of the blade leading edge, which results in different inlet diameters at the top and the bottom of the turbine inlet. 2. The small difference between inlet and outlet diameters, giving a high shroud curvature. 3. The straight vertical or sometimes even outward sloping shape of the shroud at the turbine outlet. In the design method, the streamlines in the cross-section views are calculated from a defined shroud curve incrementally inwards towards the hub, and finally defining the hub curve. With this approach items 2 and 3 listed above lead to numerical difficulties. With minor tuning of the numerical incremental design approach, the high curvature issue (item 2) has mainly been overcome, whereas an outward sloping outlet (item 3) is still not possible with the current design approach, and may require a complete revision of the numerical design approach. However, it is still possible to investigate designs with a

straight vertical or slightly inwards sloping outlet, which appears to be more commonly used than outward sloping outlet, except for certain extreme low-head designs.

In the vaneless space between the guide vane outlet and the runner blade leading edge, the flow is in theory undisturbed and thus follows the curve of a free vortex.

3 straight vertical or slightly inwards sloping outlet, which appears to be more commonly used than outward sloping outlet, except for certain extreme low-head designs. The curved blade leading edge (item 1 above) means that the blade leading edge at the top of the inlet is at a smaller diameter than the outlet. In the vaneless space between the guide vane outlet and the runner blade leading edge, the flow is in theory undisturbed and thus follows the curve of a free vortex. Thus, the inlet angle of the blade must be corrected accordingly. With these changes it is possible to use Khoj for design of low head Francis turbines. There are also other less important design aspects related to the empirical parameters used in the software such as the estimated submergence to avoid cavitation. However, these are not critical for the ability to design and analyze a low head design, and they can be corrected based on empirical data, where available, or through detailed CFD simulations. 3. Low-Head Francis Design Example An example of a low-head Francis turbine has been developed in Khoj, based on the site characteristics for the El Cajon Hydroelectric Power Plant in Honduras. The power plant consists of four Francis turbines delivered by Hydro Vevey SA, Switzerland. Each turbine has a nominal power of 75 MW and main characteristics as listed in Table 1. Table 1. Main characteristics of El Cajon hydroelectric power plant Design parameter Nominal value (pr. unit) Head H (m) Flow Q (m 3 /s) 53.6 Power P (MW) 75.0 Rotor speed n (rpm) 300 Additional characteristics of the runner geometry such as inlet and outlet diameter and inlet height, have been available for the design, but these are not presented here as the data is not publically available in general. Although based on the El Cajon geometry and design characteristics, the example design has been slightly modified to exaggerate the characteristic differences between low-head and high-head Francis turbines as discussed in the previous section. As shown in Figure 2, the example design features an even higher shroud curvature and smaller difference between inlet and outlet diameter, and an even larger slope of the blade leading edge. This leads to a noticeable difference in the blade angle at the bottom and top of the inlet as seen in the 3D turbine view in Figure 2 (right). Figure 2. Cross-section of El Cajon turbine (left) and modelled example low-head turbine (right).

4 4. Design Example Verification by CFD Analysis The example low-head design has been analyzed using CFD simulations. The purpose of the CFD analyses was merely to demonstrate the new features of Khoj, and not to perform a very detailed analysis. Due to this a fairly course mesh was used. The CFD domain was meshed using Ansys Turbogrid with characteristics as listed in Table 2. The mesh for one blade passage is illustrated in Figure 3. Table 2. Mesh characteristics, one blade passage Parameter Mesh type Meshing Value hexahedral ATM optimized Nodes Elements Figure 3. Illustration of mesh for one blade passage, taken from blade half-span One of the main features of Khoj is to export the hub, shroud, and blade spanwise curves that allow the blade to be easily meshed using the ATM optimized method of Ansys Turbogrid. This was possible also for the example low-head design, despite the exaggerated design features. This feature allows efficient transfer of designs from the Khoj software to analysis software and thus enables an automated workflow. Simulations were performed with Ansys CFX 15. Only nominal conditions were considered. Inlet condition was absolute pressure, based on nominal head, and the outlet condition was nominal flow. Turbine surfaces were modelled as smooth surfaces, and the SST turbulence model with automatic wall function definition was applied. The overall simulation results are shown in Table 3. Table 3. Characteristics of El Cajon hydroelectric power plant compared with CFD simulation Design parameter Nominal value (pr. unit) Value from CFD Head H (m) Flow Q (m 3 /s) Power P (MW) Hydraulic efficiency η (%) N/A 95.4

However, looking at the pressure distribution on the blade in Figure 4, a large low pressure zones is seen at the suction side of the blade towards the trailing edge and shifted towards the shroud

5 The simulated results and shows a good correspondence with the expected result, and a high but reasonable value of the hydraulic efficiency. This indicates that the resulting turbine and blade geometry from Khoj is of good quality also for the low-head design. However, looking at the pressure distribution on the blade in Figure 4, a large low pressure zones is seen at the suction side of the blade towards the trailing edge and shifted towards the shroud line. Little consideration was made with regards to the outlet of the turbine, no draft tube geometry was modelled, only a straight outlet. This leads to very limited pressure recovery and lower pressure at the blade trailing edge. It is well known that the geometry of the draft tube is of relatively much larger importance for a low-head Francis turbine compared to a high head Francis turbine, and this suggests that more careful modelling of the turbine outlet geometry (draft tube) may be required Figure 4. Pressure contours on pressure (left) and suction side (right) of the rotor blades. Although less visible, a large negative pressure is also found at the suction side of the leading edge, which indicates that the inlet flow angle is not perfectly matched with the blade. This can likely be corrected by more carefully matching the inflow angle relative to the blade. The low pressure zones and the large negative spikes at the inlet are also clearly visible in the blade loading charts in Figure 5. Figure 3. Streamwise blade loading charts at 20 %, 50 % and 80 % span (from hub to shroud).

6 5. Conclusion and further work The parametric design software Khoj initially developed for design of high-head Francis turbines has been modified and expanded to better allow modelling of low-head Francis turbines. One example design of a low-head Francis turbine has been developed based on the characteristics of the El Cajon power plant in Honduras, but some design features typical for low-head turbines have been exaggerated. CFD analysis of the example design shows that the overall results match the expected results well, but some details, e.g. related to the inlet geometry and the inflow angle may require further attention. A large negative pressure zones is observed on the suction side of the blade towards the trailing edge, and it is suggested that more careful modelling of the turbine outlet geometry (draft tube) may be required to achieve realistic and acceptable cavitation characteristics for low-head turbines. An outward sloping turbine outlet geometry, which is a geometric feature sometimes seen in low-head Francis turbines, is still not possible to model due to the numerical design approach involved in calculating the streamline distribution between the shroud and the hub in the meridional view. For most purposes, this limitation is of minor importance, but the aim is to complement the design method to also allow this special case, although this will likely require a major revision of the numerical approach. References [1] Aryal, R.S. and G. Rajkarnikar. Water resources of Nepal in the context of climate change [2] Gjøsæter, K. Hydraulic Design of Francis Turbine Exposed to Sediment Erosion, Master Thesis NTNU, 2011 [3] Thapa, B. S., Gjosater, K., Eltvik, M, Dahlhaug, O.G., and Thapa, B., "Effects of turbine design parameters on sediment erosion of Francis runner," Developments in Renewable Energy Technology (ICDRET), nd International Conference on the, Dhaka, 2012, pp [4] Koirala, R., Chitrakar, S., Panthee, A., Neopane, H.P. and Thapa, B., Implementation of Computer Aided Engineering for Francis Turbine Development in Nepal International Journal of Manufacturing Engineering, Volume 2015 (2015),

Spanwise re-stacking techniques in turbo-machinery blades and application in Francis runner

Spanwise re-stacking techniques in turbo-machinery blades and application in Francis runner Sailesh Chitrakar* 1, Binaya Baidar 1, Ravi Koirala 1 1 Turbine Testing Laboratory, Kathmandu University, Nepal