Aerodynamic Investigation of Steam Turbine Exhaust System Through CFD Modelling: Design Performance Optimization and Off-Design Assessment

Steam turbines play a key role in the world energy scenario since they are widely used, as thermal engines, in fossil-fueled, nuclear, and concentrated solar power plants. The most recent trends in steam turbine design practice are, therefore, strongly related to the development of the energy market, which is even more oriented towards a fast renewable energy expansion. As a direct consequence, due to intrinsic variability of the green-energy resources, the steam turbines address the need to increase their flexibility to ensure the stable functioning of the power grid. Increasing the flexibility of conventional power plants usually involves retrofitting certain components, for instance, new high-performing low- pressure blades are coupled with a standard exhaust hood. The latter has a great influence on the overall turbine performance since it converts the residual kinetic energy of the flow which leaves the last stage into static pressure increasing the last-stage power output. In most steam turbines, the exhaust hood has a radial shape to reduce the overall length of the component, however, this solution leads to a complex aerodynamic behavior since the flow turns by 90 deg in a very short distance and this generates highly rotational flow structures which are the main source of losses within this component. The retrofitting of the new blades with an “old” exhaust hood might drastically affect the pressure recovery performance due to the strong coupling between these components, reason why it should be associated with a re-design of the exhaust hood to ensure high performance. In this context, this work deals with the development of a design approach for the steam turbine exhaust hood aimed at maximizing he pressure recovery performance, for a defined last stage geometry, through CFD modeling. Due to the already mentioned strong coupling between the last stage and the exhaust hood, it is necessary to consider the presence both in the fluid domain. However, modeling the unsteady full 3D turbine stage coupled to the exhaust system results in a remarkable number of grid elements with a significant computational effort and it cannot be applied to an optimization strategy that requires a high amount of numerical simulations, it has been used only in the final part of this work to investigate the off-design conditions where the flow unsteadiness must not be neglected. For the optimization strategy different simplified interfaces have been tested, among these, the mixing plane interface has been selected as the best trade-off between accuracy and computational effort. The presented numerical approach has been applied to an exhaust hood manufactured by Baker Hughes. As baseline geometry, a standard exhaust system has been designed however it shows low-pressure recovery performance confirming the risks of retrofitting. By performing detailed aerodynamic post-processing of the baseline geometry results, the main issues of such geometry have been detected and consequently, the re-design procedure has been undertaken. Since the strong fluid-dynamic coupling existing between exhaust hood casing and diffuser (which are the main components of the exhaust hood), a parametric model has been developed including geometrical parameters of both the components, which represent the input data for the optimization procedure. Due to the significant number of parameters taken into account, a simplification of the fluid model is necessary to perform the optimization analysis in a feasible time. For this purpose, a simplified fluid domain has been developed based on the idea to consider the flow in the diffuser as symmetric, allowing to keep the number of grid elements low by considering as fluid domain a single stator and rotor passage coupled to a periodic slice of the diffuser, with the mixing plane as coupling interface. This model neglects the effect related to the asymmetry of the exhaust hood casing and consequently a verification with a 3D model is required as the final step of the procedure is required. A response surface has been achieved as a function of the key geometrical parameters, therefore an optimization method has allowed identifying the best performing configuration. A 3D model of the optimized periodic geometry has been then generated to assess the effectiveness of the procedure here presented. The comparison between the periodic and the 3D model has highlighted a good agreement in both the averaged pressure recovery factor prediction and flow field resolution within the diffuser, with a speedup of the computational time by about one order of magnitude. The optimized geometry identified presents a pressure recovery factor, calculated in the condenser section, 28% greater than the baseline one. The improvement of the performance is achieved even with a reduction of the exhaust system volume equal to 4%. Once optimized the design condition performance and therefore defined the geometry of both the last stage and the exhaust hood, the focus is moved on the assessment of the off-design conditions. In the already presented energy scenario, due to intrinsic variability of the green-energy resources, the steam turbines address the need to increase the operation at a low load. Such off-design conditions are extremely critical for the last stage bucket (LSB) of the low-pressure turbine since they might experience non-synchronous aerodynamic excitations triggered by the onset of unsteady fluid behavior characterized by the presence of rotating instabilities similar to the ones widely investigated in the compressors. Due to unsteadiness and strong asymmetry of the flow field in these conditions, the presented simplified numerical setup is not suitable for detailed investigation of this phenomenon but it can offer just a preliminary screening of the most dangerous conditions to be tested with a more accurate numerical setup. The flow field has been indeed studied by performing 3D unsteady CFD simulations (URANS) of the low-pressure turbine the last stage coupled with the exhaust hood, with structural struts included. The full annulus mesh of both the last stage and diffuser is considered with the transient stator-rotor interface to properly account for unsteady interaction effects. The Influence of the operating conditions on the fluid dynamic behavior is assessed by considering six different operating conditions. Starting from the design condition and gradually decreasing the mass flow rate. The presence of rotating instabilities is demonstrated by monitoring the fluid dynamic variables during the simulation and by using advanced post-processing techniques, such as Proper Orthogonal Decomposition (POD). In the light of the results of this investigation, the operation limits of the turbine have been updated to exclude the most dangerous conditions from the machine operability range. The design solutions aimed at countering the characteristic frequencies of the rotating instabilities are to be excluded since these phenomena act on several frequencies which are unstable in time and strongly related to operating conditions.