Optimization of blade stacking in the last stage of Low Pressure (LP) steam turbines constitutes one of the most delicate and time consuming parts of the design process. This is the first of two papers focusing on the stacking strategies applied to the last stage guide vane (G0). Following a comprehensive review of the main features that characterize the LP last stage aerodynamics, the 3D CFD code used for the investigation and options related to modeling of wet steam are described. Aerodynamic problems related to the LP last stage and the principles of 3D stacking are reviewed in detail. In this first paper the results of a systematic study on an isolated LP stator row are used to elucidate the effects of stacking schemes such as lean, twist, sweep and hub profiling. These results show that stator twist has the most powerful influence on the reaction variation but it also produces undesirable spanwise variations in angular momentum at stator exit. These may be compensated by introducing a positive stagnation pressure gradient at entry to the last stage.
During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade (LSB). Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the LSB increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of LSBs continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effects from geometry, mesh density, turbulence model, and time dependency. The CFD results were verified against steam turbine data from a scaled test facility. The proposed solutions include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.
A fast method is presented for evaluating the impact of blade manufacturing variations on the aerodynamic performance of turbomachines. The method consists of two parts. Firstly, an adjoint method is developed to evaluate the aerodynamic sensitivities for multistage turbomachines. This sensitivity information may then be used to perform fast direct Monte Carlo simulations to obtain the statistical distribution of the variations of aerodynamic performances resulting from any given set of manufacturing variations. Secondly, a method is developed to construct reduced-order models for the three-dimensional blades manufacturing variations using the Principal Component Analysis (PCA) method. Monte-Carlo simulations with the adjoint sensitivities can then be applied to the full and individual modes of the blade manufacturing deviations. The proposed method is applied to the last two stages of a low-pressure steam turbine. A total of 29 sets of measured manufacturing deviations of the last-stage rotor blades are used to construct a reduced-order model of the manufacturing variations. The manufacturing variation reduced-order model helps identify origins of the manufacturing deviations connected to the machining processes of the blades. Relations of the statistics of the aerodynamic performance variations such as mean, standard deviation, etc. to the different modes of manufacturing deviations are studied and analyzed.
On the basis of their inherent favourable aerodynamic properties coupled with past progress, 50 per cent reaction stages already achieve a high efficiency level. Developments aimed at further performance enhancement entail employment of advanced design features that require a deep understanding of the flow phenomena involved and their interactions. In addition, substantial on-going efforts are needed to improve the quality of the design tools. This paper focuses on the key design issues, including advanced quasi-three-dimensional and three-dimensional design aspects. It further describes developments by the authors' company during the last decade for the design of modern reaction blading and establishment of state-of-the-art design tools.
During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade. Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the last stage moving blade increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of last stage moving blades continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effect of geometry, mesh density, turbulence model and time dependency. The CFD results were verified against steam turbine data from a test facility. The proposed complimentary solutions to limit the temperature increase include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.
Optimization of blade stacking in low-pressure (LP) steam turbine development constitutes one of the most delicate and time-consuming parts of the design process. This is the second part of two papers focusing on stacking strategies applied to the last stage guide vane and represents an attempt to discern the aerodynamic targets that can be achieved by each of the well-known and most often used basic stacking schemes. The effects of lean and twist have been investigated through an iterative process, involving comprehensive 3D computational fluid dynamics (CFD) modeling of the last two stages of a standard LP, where the basic lean and twist stacking schemes were applied on the last stage guide vanes while keeping the throat area (TA) unchanged. It has been found that it is possible to achieve the same target value and pattern of stage reaction by applying either tangential lean or an equivalent value of twist. Moreover, the significance of axial sweep on hub reaction has been found to become pronounced when the blade sweep is carried out at constant TA. The importance of hub-profiling has also been demonstrated and assessed. Detailed analysis of the flow fields has provided an overall picture, revealing the differences in the main flow parameters as produced by each of the alternative basic stacking schemes.
Optimization of blade stacking in the last stage of low-pressure (LP) steam turbines constitutes one of the most delicate and time-consuming parts of the design process. This is the first of two papers focusing on the stacking strategies applied to the last stage guide vane (G0). Following a comprehensive review of the main features that characterize the LP last stage aerodynamics, the three-dimensional (3D) computational fluid dynamics (CFD) code used for the investigation and options related to the modeling of wet steam are described. Aerodynamic problems related to the LP last stage and the principles of 3D stacking are reviewed in detail. In this first paper, the results of a systematic study on an isolated LP stator row are used to elucidate the effects of stacking schemes, such as lean, twist, sweep, and hub profiling. These results show that stator twist not only has the most powerful influence on the reaction variation but it also produces undesirable spanwise variations in angular momentum at stator exit. These may be compensated by introducing a positive stagnation pressure gradient at entry to the last stage.
Optimization of blade stacking in Low Pressure (LP) steam turbine development constitutes one of the most delicate and time consuming parts of the design process. This is the second part of two papers focusing on stacking strategies applied to the last stage guide vane and represents an attempt to discern the aerodynamic targets that can be achieved by each of the well-known and most often used basic stacking schemes. The effects of lean and twist have been investigated through an iterative process, involving comprehensive 3D CFD modelling of the last two stages of a standard LP, where the basic lean and twist stacking schemes were applied on the last stage guide vanes whilst keeping the throat area unchanged. It has been found that it is possible to achieve the same target value and pattern of stage reaction by applying either tangential lean or an equivalent value of twist. Moreover, the significance of axial sweep on hub reaction has been found to become pronounced when the blade sweep is carried out at constant throat area. The importance of hub-profiling has also been demonstrated and assessed. Detailed analysis of the flow fields has provided an overall picture, revealing the differences in the main flow parameters as produced by each of the alternative basic stacking schemes.
In steam turbine power plants, the appropriate design of the last stage blades is critical in determining the plant efficiency and reliability and competitiveness. A high level of technical expertise combined with many years of operating experience are required for the improvement of last stage designs that increases performance, without sacrificing mechanical reliability. This paper focuses on three main development areas that are key for the development of last stage blades, namely the aerodynamic design, the mechanical design and the validation process. The three different lengths of last stage blade (LSB) were developed of 41in, 45in and 49in (and a number of scaled variants). The aerodynamic design process involves 3D CFD and flow path analysis, considerations such as last stage blade flutter and water droplet erosion, and last stage guide design. The mechanical design includes finite element stress and dynamic analysis, appropriate selection of the blade material, the coupling of the LSB with the rotor and the design of the LSB snubber and shroud. Experimental measurements form a key part of the product validation, from both the mechanical reliability and performance points of view.
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