The low pressure (LP) exhaust system presents a promising avenue for improving the performance of large steam turbines. For this reason, LP exhaust systems have attracted the attention of the research community for decades. Nevertheless, we still lack understanding of the flow physics and loss mechanisms in the exhaust system, especially at part-load conditions. It is also unclear how the exhaust system should be designed when its required operating range widens. This thesis provides solutions to these aerodynamic issues through experimental and numerical investigations, and provides tools that could contribute to better designs of LP exhaust systems. Firstly, the Computational Fluid Dynamics (CFD) solver ANSYS CFX was validated against experiments performed on a scaled test rig under representative part-load flow conditions. This validation exposed the weakness of Reynolds-averaged Navier-Stokes (RANS) CFD when there is a highly swirling flow and large separation regions in the exhaust diffuser. To facilitate the numerical studies, a series of tools were also developed. A design suite, ExhaustGen, was used to automate the pre- and post-processing of CFD calculations. The exhaust diffuser was parametrised using "Minimum Energy Curves", which reduce the dimension of parameter space. Further, a suitable stage-hood interface treatment (Multiple Mixing Planes) was chosen to predict the circumferentially non-uniform flow in the exhaust hood at low computational cost. Numerical investigation of the baseline geometry provided insights into the key flow features and loss mechanisms in the exhaust system, over a wide range of operating conditions. In particular, the bearing cone separation was identified as a key source of loss at part-load conditions. The effect of stage-hood interaction on the performance and design of the exhaust system was studied by varying the rotor blade design, which can positively influence system performance. Finally, a global sensitivity study was performed to identify the most influential design parameters of the exhaust hood. These findings allow, for the first time, LP exhaust hood performance maps to be constructed, so that the benefits of choosing a suitable hood geometry and blade design can be revealed. The thesis also offers contribution towards formulating LP exhaust system design guidance for a wide operating range.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:767883 |
| Date | January 2019 |
| Creators | Ding, Bowen |
| Contributors | Xu, Liping |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/290137 |
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