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The constant growth of renewable energy sources has changed the working condition of the modern power plants, where a greater flexibility of the conventional units is required. Large steam turbines, for instance, have to be capable of partial load operation as well as quick starts in order to ensure a stable functioning of the power grid. This reflects on the design of the steam turbine and in particular on exhaust hoods which have a significant impact on the overall turbine performance, recovering the energy left and therefore affecting the power output. Exhaust hoods have to convert kinetic energy at the turbine last stage exit into static pressure at the condenser by decreasing the flow speed. In axial-radial hoods the flow must turn by 90 degrees in a very short distance, therefore, if such components are not well designed, this may lead to flow separation phenomena which reduce the hood recovery performance. Due to the nowadays operational requirements, it is mandatory to carefully design the steam turbine exhaust system for a wide range of operating conditions, in order to ensure high efficiencies also at partial loads. This work concerns a numerical investigation on a low-pressure steam turbine exhaust hood through computational fluid dynamic simulations, assessing several numerical methods to couple the exhaust casing with the turbine rear stage. The numerical outcomes were compared in terms of flow field distribution and pressure recovery coefficient, by allowing to define a numerical setup which is a good trade-off between results accuracy and computational effort. [ABSTRACT FROM AUTHOR]

Nowadays, the growing need for advanced solutions to limit the environmental impact in thermal power generation has given rise to significant changes in steam turbines design and operability. In this context, improvement in efficiency and more flexibility in different operating conditions are the major challenges. An increased flexibility is related to rapid load changes, fast start-up and frequent shutdown that could lead to high material thermal stresses in the unit. Hence, in order to achieve the rise in efficiency and to preserve the lifetime of the turbine, a detailed knowledge of the thermal behavior of the turbine is required. This work is focused on the investigation of flow and heat transfer phenomena in the steam chambers between stator blade carriers. Correlative 1D models are usually used in the industrial practice to estimate heat transfer coefficients to be used as boundary conditions in detailed FEM simulation of the Whole Engine Modelling (WEM). However, the flow pattern in this chamber is complex and this approach could represent an oversimplification, therefore computational fluid dynamic (CFD) analyses have been performed with the aim to obtain a more accurate estimation. All the analyses have been carried out with ANSYS CFX with a quasi-2D axis-symmetric approach. This methodology, despite the need for some simplifications and assumptions to reduce the problem from 3D to quasi-2D, ensures less expensive simulations in terms of computational resources when compared to a fully 3D. After a numerical setup validation in terms of mesh sensitivity, a parametric CAD has been generated and tested. The goal is to analyze how geometrical parameters (such as the axial distance between the stator blade carriers or the height of the chamber) of the chamber will influence heat transfer coefficients, considering different operating conditions. A design of experiment (DOE) with ANSYS Workbench platform has been carried out in order to perform automatically a set of simulations. Finally, the results obtained have been post-processed to extract a sufficiently accurate correlation for the average Nusselt number applicable for almost all the steam chambers in the turbine even in different operating conditions. [ABSTRACT FROM AUTHOR]