Numerical analysis of loss and performance optimization efforts for LP steam turbine exhaust hoods

Abstract

Most of the world’s power is produced by large steam turbines using fossil fuel, nuclear and geothermal energy. The LP exhaust hoods of these turbines are known to contribute significantly to the losses within the turbine, hence a minor improvement in their performance, which results in a lower back-pressure and hence higher enthalpy drop for the steam turbine, will give a considerable benefit in terms of fuel efficiency. This thesis is divided in two parts: In the first part, a detailed numerical analysis of sources of loss in LP exhaust hoods is carried out. The methodology used in doing this starts with a well known approach from literature where the diffuser inflow is divided into sectors and the streamlines originating from these sectors are used for flow field visualization. In the new approach, this existing procedure is developed further such that the flow properties of the various streamlines are analysed at predetermined evaluation surfaces within the flow domain. In so doing, this more advanced methodology shows clearly where most losses occur within the flow domain and makes it easier to make decisions for improving the exhaust hood performance. Using this approach, most losses are found to occur at the upper hood and are associated with the swirling flows resulting from the difficulty experienced by the flow in turning towards the condenser. At the lower side of the diffuser, the initial flow direction is more or less towards the condenser hence these flows contribute less to exhaust hood losses. The numerical results of the reference configuration based on a scaled axial-radial diffuser test rig operated by ITSM are thoroughly validated at both the design and overload operating conditions and at three tip jet Mach numbers (0, 0.4 and 1.2). The second part of the thesis focuses on possible modifications of LP exhaust hoods to achieve better performance. Having identified that most losses occur at the upper hood and the reason for it well understood, the influence of changing the hood height above the diffuser is extensively investigated at design load. It is found that the hood height has huge impact on performance and that an optimum hood exists for a given tip jet Mach number. Deflector configurations at the upper hood are also investigated. They are found to redirect the flow at the upper hood and minimise the intensity of the swirling flows hence leading to improvement in performance of LP steam turbine exhaust hoods. The best performing deflector configuration, the double wall deflector, is found to give a considerable improvement in performance amounting to 20% at design load and 40% at overload both at tip jet Mach number of 0.4 (corresponding to shrouded last stage blades).