Full steam ahead: CFD software aids development of efficient turbine
Computer simulation played an important role in the development of what is purported to be the world’s most efficient steam turbine, built by Siemens Power Generation (PG) for the VEAG power plant in Boxberg, Germany. This turbine has demonstrated an internal turbine efficiency of 94.2% for the high pressure turbine and 96.1% for the intermediate pressure turbine, resulting in a world-record 48.5% gross efficiency.
Siemens’ Muelheim an der Ruhr, Germany, engineering center is responsible for architecting design rules and tools used by the company’s worldwide engineering network to design turbines to customer specifications. In the early 1990s, the company moved from traditional 2D parallelsided blades to 3D airfoil designs as part of a continuing effort to improve turbine efficiency. The change required major modifications to both design and manufacturing processes. The first generation of 3D blade designs were based on conventional quasi-3D blade design concepts addressing the issue that flow conditions vary along the height of the blade, along with the rotational speed. However, amid the development process, engineers recognized that fully optimizing the geometry of the blade would lever further efficiency improvements leading to a novel fully 3D blade design concept. “The use of a model turbine is only partially suitable of exploring these issues,” Deckers explains. “One disadvantage is that it is very expensive to build and to instrument. Another issue is that the test results do not provide much information on the detailed flow fields inside the turbine, which are critical in understanding the underlying physics. We needed to get a better understanding of the flow and pressure fields inside the turbine, so we began looking at the various CFD tools in an effort to determine which one would give us the most help.”
To achieve high efficiencies at Boxberg, Siemens engineers performed a comprehensive series of parametric simulations using CFX computational fluid dynamics (CFD) software from ANSYS Inc., Canonsburg, PA, in order to derive design rules that are used subsequently to generate optimized blade-path designs based on the pressure, temperature and mass flow rate defined by the customer. They varied the blade airfoils over the height of the blade to better suit local flow conditions, reducing losses associated with secondary flows. The company has also used CFD to reduce losses in the intake and exhaust flow channels. “CFX software played an important role in the development process by allowing us to predict flows and pressures inside the turbine with a minimum amount of model turbine tests”, says Mathias Deckers, manager of Steam Turbine Blading Technology for Siemens, Muelheim an der Ruhr. “The CFD software also provides accurate trends in efficiency predictions, making it possible to try a large number of alternatives and pick the one that works best.”
“We selected CFX-TASCflow primarily because it is capable of analyzing a much broader range of turbine design features than the other packages that we looked at,” says Deckers. “While we were comparing different codes, CFX-TASCflow seemed to be the only one capable of modeling all different turbine types and over the years it has proven its ability to model most of our designs. We are particularly impressed with its ability to quickly define grid interfaces between neighboring computational domains. We have also found that the staff provides very competent technical support.” Part of CFX’s integrated system of CFD products for turbomachinery, CFX-TASCflow provides a generalized grid interface to connect completely dissimilar, non-matching grids, regardless of how they were created; multiple frames of reference to efficiently model flows with rotor/ stator interactions; features particularly suited for subsonic, transonic, or supersonic flows; special fluids analyses for wet steam and refrigerants; and efficient parallel processing on any combination of single- or multiple-CPU or networked UNIX workstations and NT PC machines, including mixed UNIX/PC cluster.
“When we began to use the software, our first question was how accurately it was able to simulate conditions in side a turbine,” says Deckers. He started with geometry that was generated in ASCII format from the company’s design team. He read the initial airfoil geometry into CFX-Turbogrid, a mesh generation tool that works with other CFX tools, and generated a mesh for each stage or row of blades of the turbine. He used the generalized grid interface feature within CFX-TASCflow to specify the interface between the different stages. The multiple frame of reference capability of the CFD software was then used to model the flows within the turbine. Siemens engineers compared the results of their early simulations to a linear cascade physical model. After some adjustments to the grid size, cell geometry and turbulence model, they were able to obtain a very close correlation between the physical and virtual prototype.
Improving blade efficiency
“The ability to simulate flows inside the turbine enabled us to gain a much better understanding of the issues involved in improving efficiency,” recalls Deckers. “We were able to visualize the flow fields inside the turbine, and determine the optimum inlet and outlet angles at each section of the blade. We systematically varied design parameters in order to increase our understanding of how they affect the blade shape and flow field. Our original goal was to make the flow as uniform as possible. We also became aware of the importance of secondary flows that are generated at the root and shroud platform as the steam is guided through the blade passages of each successive stage while the boundary layers are turned. This insight spurred the development of our 3DS blade, in which the airfoil section changes along the length of the blade to match the complex 3D flow field. This design corrects for these secondary flows and provides a more uniform flow field in the turbine. This innovation by itself can increase the stage efficiency by as much as 2% compared with traditional blade designs. An improvement of this magnitude can generate millions of dollars in incremental revenues for a single large turbine. The automated tools in CFX helped to simplify what has become a far more complicated design process, with many different airfoil sections to consider rather than just one for each blade.”
The CFX postprocessor computes the exit angle, total pressure and velocity components as a function of radius, to facilitate performance analysis of each individual section of the blade. Then it calculates the entropy losses in each stage and finally provides an estimate of the efficiency of the turbine. “While the accuracy of these numbers is not perfect, they are good enough to determine easily whether one design alternative is better or worse than another,” Deckers comments. “But while CFD is getting taster all the time, it still is too time-consuming and complicated to use on a daily basis for every turbine design. So we distilled the understanding of the design sensitivity that we learned from running these simulations into a series of design rules, and embedded them into a novel design system based upon 1D and 2D in-house codes that we have provided to our local design centers.
Addressing other aspects of turbine design
More recently, Siemens has increased the modeling accuracy by including the leakage flow caused by the clearance left between the outside diameter of the turbine and the casing that surrounds it. Besides, CFD results have been used to improve other aspects of the turbine design. For example, the intermediate-pressure turbine exhaust flow diffuser has been changed to improve flow guidance and to reduce vortex formation in the exhaust steam section. The high-pressure turbine inlet section has been optimized to provide a more homogenous flow distribution in the entire inlet area to the stationary blade rings. CFD analysis of the exhaust section of the low-pressure turbine was used to develop baffle plates that further minimized the flow losses.
This approach has been used to design nearly all of the company’s recent high-performance steam turbines, and the result has been substantial increases in efficiency such as those provided by the Boxberg turbine. The Power Generation unit supplied VEAG with a five-cylinder turbine together with the associated condensers, generator and other accessories three years ago. The turbine is about 180 ft long and stands on a special spring-supported reinforced concrete foundation that is about 50 ft tall. “The 907-megawatt lignite-fired power plant represents a milestone in the production of energy-efficient, environmentally-friendly electrical power,” Deckers says. “The key to its success is a highly efficient and flexible design process based upon a suite of modern design tools such as CFX and a number of in-house codes. The consistent exploitation of this design environment has been established as a continuous process to minimize the aerodynamic losses in a turbine. We are now in the process of commissioning an even larger turbine at Niederaussem in the Cologne area that we expect will set another efficiency record. In the mean time, we are working to achieve even greater improvements by developing more streamlined interfaces and processes in order to facilitate the use of CFD in the day-to-day design process.”
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