This paper presents the computational methodology, and experimental investigations accomplished to enhance the efficiency of a turbine stage by applying non-asymmetric profiling on the rotor hub wall. The experimental setup was a two-stage axial turbine which was tested at “LISA” test facility at ETH Zurich. The 1^st^ stage was considered to create the flow history for the 2^nd^ stage, which was the target of the optimization. The hub cavity of the 2^nd^ stage was designed with large dimensions as a requirement of a steam turbine. The goal was to optimize the interaction of the cavity leakage flow with the rotor passage flow to reduce the losses and increase efficiency.

The computational optimization was completed using a Genetic Algorithm coupled with an Artificial Neural Network on the 2^nd^ stage of the test turbine. Unsteady time-accurate simulations were performed, using in-house developed “MULTI3” solver. Besides implementing all geometrical details (such as hub and tip cavity structures) from the experimental setup into the computational model, it was found to be crucial to apply unsteady boundary condition at the inlet of the 2^nd^ stage during the optimization. The importance of this implementation was tested by performing a steady simulation on the optimized geometry. The predicted 0.27% efficiency gain from unsteady simulations were only 0.06% if steady simulations were performed.  Excluding the cavity geometry was also assessed in a different simulation setup showing 3.9% over-prediction in the efficiency level.

Comprehensive steady and unsteady measurements were performed utilizing pneumatic, Fast Response Aerodynamic (FRAP), and Fast Response Entropy (FENT) probes, on the baseline and profiled test cases. More than 50% of the unsteadiness measured at the 2^nd^ stage rotor inlet was imposed from the upstream rotor. This finding shows the necessity of considering the flow history from the upstream stage in the optimization procedure. The end-wall profiling was found to be successful in weakening the strength of the hub passage vortex by 19% reduction in the under-over turning. As a result, the blockage is reduced near the hub region leading to more uniform mass flow distribution along the span. The flow angle deviations at the higher span position were also corrected due to better control on the flow angles. Furthermore, the improvements were confirmed by the reduction of 3.5%, 8.2% and 3.1% in the entropy, Secondary Kinetic Energy and pressure unsteadiness, respectively. Finally, an efficiency gain of 0.26% was measured from the experimental setup.

 


Unsteady Steam Turbine Optimization Using High Fidelity CFD

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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