Development of sediment friendly crossflow hydro turbine

Abstract

Micro-hydropower plants (MHP) provides the best solution to the power needs of rural and small communities which serve as a decentralized power source to meet the local population requirement. Energy requirements for lighting, cooking, heating, drying, agro-processing and other small scale industrial activities can be met through these MHPs most reliably in the rural areas of the country like Nepal. Crossflow turbines are used widely in such MHPs due to their simple design, easier maintenance, low initial investment and modest efficiency. Also, because of their suitability under the low head, their efficient operation under a wide range of flow variations and ease of fabrication, crossflow turbines (CFT) have been extensively employed. In the context of Nepal, MHP alone has effectively generated about 28,000 kW of electricity with a total of 2900 Microhydro powerplants of different sizes and capacities by mid of July 2012. Thus, supplying electricity to well above 350,000 households in remote areas of Nepal. However, there is not much traceable knowledge about CFT design, and Nepalese manufacturers follow the design of foreign institutions. In addition, due to a lack of knowledge of material technology, the use of materials is limited to mild steel for the production of critical components such as runners and guide vanes. In addition, the design of most researchers worldwide focus only on optimizing turbine efficiency and does not take into account the impact of sediment erosion on the CFT, which is a local operational problem. As a result, erosion can occur on the runner and guide vane and thereby reduce service life and efficiency. This research work intends to focus on developing sediment resistive CFT. It includes design, numerical simulation and testing of the turbine to address the local sediment problem. The simulation works consist of locating erosion on turbine blades, modifying the blade profiles and efficiency measurement of modified CFT. Numerical analysis has been performed in a full CFT turbine through a computational fluid dynamics (CFD) Analysis simulation tool ANSYS CFX. Five different runner designs, with different inlet blade angles, were analyzed numerically. For each design, ten numbers of guide opening from the full opening are named 0°,2°, 4°,6°,8°, 10°,12°,14°,16°and18°.effect of sediment erosion rate density hill diagram, as well as efficiency hill diagram for the runner, are considered for each design to determine the best-optimized design. In the first stage, all five designs have a similar velocity distribution at the same flow rate and rotational speed. In the second stage, however, among all the designs, the velocity for the 16° inlet blade angle is way less than all other designs. Further, it is observed that the velocity for all the designs is higher on the suction side of the blade in the second stage. After the close study of the velocity distribution of all the design, a design that will have less sediment erosion. Moreover, it is observed that among these design efficiency for the 16° is higher which may be due to a better stagnation angle in the first stage. Therefore, an inlet blade angle of 16° is found to be the best design with higher efficiency and low sediment erosion rate density.Two types of erosion testers are designed and developed to observe the effect of sediment on blade specimens and the efficiency of the optimized turbine. At the initial stage of this research work, rotating disc apparatus (RDA) was manufactured and experiments were done to investigate the wear pattern and estimate the erosion resistive behavior through the weight loss method on different materials with and without coating tungsten carbide. The high erosion resistance of the material is found to be the dependence of erosion rate on the hardness value, ductility, and tensile stress. The tungsten carbide coatings on the different materials have enhanced the erosion resistance properties. Another erosion tester, Non- Re-circulating (NRC) type was developed and the experiment was conducted in a closed loop at Turbine Testing Laboratory, Kathmandu University. This setup is one of its kind for its ability to conduct sediment erosion tests on model turbines by simulating the operating conditions that exist in actual micro hydropower plants. CFT with 30 blades and an inlet blade angle of 160 was selected to perform the tests. The four sections of the Optimized model runner with detachable blades was 3D printed with the provision of four slots for blades insertion. The sediment was injected into the tester to observe wear patterns and amount of weight loss in the test specimens. Test specimens were dried thoroughly to remove the water to avoid chances of error in weight measurement. For the performance measurement, calibrated instruments Electro-magnetic flow meter, pressure transducer and pressure transmitter are used to measure the flow, torque and pressure respectively. The turbine is coupled with an induction motor through a torque transducer, which is driven by a VFD to stabilize the RPM of the rig and dumping of generated power. Data logging is carried out using LabVIEW software. Wear pattern and performance curves of the optimized runner at different RPM are obtained and compared with the CFD results. The regions with maximum efficiency obtained through CFD is very similar to the results obtained from the experimental analysis. The locations of the erosion from the experiment results are similar when compared with the CFD simulation results and with the eroded runner of Daram Khola which is one of the representative Micro Hydro Project which was operated where water is loaded with sediment.