Computational Flow Analysis on 45degrees inclined, Real-scale Archimedes Screw Turbine

摘要

Archimedes Screw Turbine (AST) is a clean and renewable hydro source of energy. It is identified as the most sustainable hydropower generation method since it offer many economic, social, and environmental advantages. As an ecofriendly power generation method, AST has a minimum impact on wildlife; especially fishes. Further, ASTs are recording relatively high-efficiency having low installation and maintenance costs over other small scale hydropower generation methods. Furthermore, AST is considered as a solution to generate electricity in rural areas; where exists a hydro energy potential but hard to access to the national grid distribution. Due to these advantages, many researchers are interested in AST developments. Currently, there are hundreds of AST installations almost entirely in Europe. But AST technology is considered as relatively new hydro technology since there is no perfect theory or rule for the optimal hydraulic design and already installed AST power plants are highly dependent on the experience of the design engineer. Therefore, the development of AST technology is essential to utilize hydro energy all around the world. Now a day, many researchers are focusing on the theoretical design procedure for AST considering various losses. Almost many of them are lab testing scaled models claiming 80% average efficiency for low inclined angles. In the case of a real site having a small inclination, the length of the screw is getting larger enough to affect by bending forces and bearing limitation.Therefore, this research conducted to analyze the computational flow field in real scaled AST having the maximum possible inclination of 45 degrees. Initially, AST was supposed to install in a fish farm, operating under drainage water while discharging to sea. Further, the design was adopted without the upper and lower reservoir, since it planned as a run-of-river system. Since the lack of English guidance for the optimum design procedure of AST and after considering many kinds of research recommendations, the current screw turbine was designed according to Chris Rorris, “The optimum design for an Archimedes Screw,” which optimized for the screw pump. To come up with a reliable computational fluid dynamic (CFD) setup in Ansys CFX, several simulations were conducted to validate the already experimented small scale AST. The CFD result showed a good agreement with the reference study. Therefore, the designed AST simulated using the same CFD setup.Simulated, three-blade real scale AST resulted in 80% maximum efficiency for 5m hydraulic head. The turbine’s outer diameter was calculated as 1.2m, which is suitable for 0.23m^{3/s and 45 degrees’ inclination. When examining the CFD result, large overflow observed at the first few screw turns because of the incoming water velocity. It can recognize as a power loss. But, relatively higher force distribution observed on the same screw turns because the water strike on the screw blades generates additional force. After the water passes the first few screw-turns, it follows a steady flow field having almost the same water volume per bucket while generating the same pressure difference on each blade as a major driven force. But there can notice a small over-flowing at every screw turn because of the high inclination of the screw. At the same time, water leakage could be observed through the 5mm gap between the screw tip and the trough wall all alone the screw length. At the end of the screw, the highest pressure difference generates on the screw blade because of no backpressure built at the outlet since it is only open to atmosphere. Further, the Rorris’ optimized ratios are identified as compatible ratios for the screw turbine even they are optimized for the screw pump. Since the main driven force in AST is the pressure difference generated by the weight of water, Rorris’ optimized method of maximizing the water volume per bucket is an almost suitable design procedure even for AST design.Many researchers claim about relatively low efficiency for AST in higher inclinations. Finally, this CFD study concluded with claiming that even for 45degree inclined real-scale AST can achieve about 80% hydraulic efficiency in run-of-river system by maintaining the optimum bucket water level with small leakage gap and optimum screw pitch length.}