Rehabilitation of old francis turbines using reverse engineering and computational fluid dynamics simulations

In the last decades, the refurbishment and upgrading of hydro-turbines have been increasingly demanded due to the performance loss, reliability reduction, and frequent maintenance need. Before, starting the rehabilitation process, a key point is presenting the state of the power plant. Without understanding the current problems, which cause a reduction in efficiency, generated power and reliability, starting the rehabilitation process can cause unnecessary expenses and modification to the units with no substantial increase in the performance. The objective of this research is to undertake Computational Fluid Dynamics analyses of selected hydroelectric power plants for the purpose of evaluating upgrade potential as well as identification of operational characteristics. For this purpose, turbine parts were scanned in the field then a hybrid reverse engineering methodology were developed which combines the traditional reverse engineering steps with the flow dynamics of hydraulic turbines. Steady state analyses are performed to determine the efficiency and power output of the turbine. A hill chart is generated to investigate the behavior of the turbine at different head and flow rate values. Obtained CFD results are compared with the site measurements. The CFD results showed that at the full-load condition the turbine has 93.8% efficiency with 51.56 MW power output which are consistent with the guaranteed values from manufacturer. In the sixth part of the study, the problems that decrease the turbine efficiency are detected by performing transient analysis. According to the CFD results, cavitation zones are observed on the runner blades and these zones also matches with the site photos. Transient analysis and visualization techniques are used for diagnostics of turbine which enabled identification of different cavitation mechanisms. Results depicted that blades suffers from cavitation close to the trailing edge. In addition to this, especially at the part load conditions, powerful draft tube swirl is observed, and it is reason of the severe vibrations in the powerhouse. It is recommended that modifying the conventional runner blade to the X-blade will increase the performance and prevent the cavitation. With the additional fins on the draft tube cone or with the pressurized water injections system draft tube performance can be increase and this will also cause that turbine operate more stable and efficient. Keywords: Rehabilitation, CFD, Reverse Engineering, Francis Turbine, Renewable Energy, Transient Analysis