Transient CFD-analysis of a high head Francis turbine

Abstract

The purpose of the present work has been to investigate how well transformation methods work as a method to get realistic inlet boundary conditions to a draft tube without using excessive amounts of computational resources. The mesh supplied for the Francis-99 workshops was cut and modified to prepare it for the use with ANSYS CFX transformation methods. Transient simulations at BEP were conducted on the modified mesh containing one guide vane, one runner blade + splitter and the draft tube using the profile transformation method. The results were compared to results from a simple draft tube simulation, a TRS simulation, LDV measurements and experimental pressure measurements. The profile transformation method simulation was done for one and seven runner rotations. It was found that the profile transformation simulation used far less computational resourced compared to the TRS simulation but general and detailed flow properties in the draft tube were not accurate. It was concluded that the transformation method approach has potential and that several sources of error must be further investigated to be able to conclude how well the transformation method works for this purpose. It was also concluded that simulating one runner rotation was not enough for fully converged results.