ESTIMATION OF PERFORMANCE CHARACTERISTICS OF HYDRAULIC TURBINE USING CFD

Abstract

The flow through a hydraulic turbine is extremely complex, since it is generally turbulent, unsteady and highly three-dimensional 3D in nature due to strong effects of rotation and curvature. The traditional design process is based on experiments, measurements and model tests, which requires large money and time investments. These methods were quite tedious and time consuming. These machines are high power consuming or power producing machines. Therefore small improvements in the geometry of rotating elements of a turbine or compressor can have a large effect from the point of view of operation and maintenance costs. During the last 20 years the numerical simulation of the flow, called Computational Fluid Mechanics (CFD), has been adopted as an additional element in the design process allowing shortening the development times and saving money. CFD helps to simulate the flow field which helps in hydraulic design and performance prediction of the machine. In the present work the performance of the Francis turbine is evaluated at different operating points using CFX. Complete turbine is considered and the structured hexahedral elements are generated in the whole domain. The numerical values of head, power and efficiency over the operating range of flow rates of the turbine have been found to be in reasonable agreement with the experimental results provided. The flow inside the different components i.e. spiral casing, stay & guide vanes, runner and the draft tube is closely observed using the interactive features of the CFX post software. It is observed the generation of the vortices in the runner and the draft tube is the reason for the lower efficiency of the operating points other than the rated point. Also the pressure fluctuation occurring at the interface of the rotor and stator is analyzed which is purely unsteady phenomenon. The observed trend is very much similar with the simulated and the experimented behavior observed by the other researchers.