Extremely precise and fast – steam flow calculation
A newly developed method becomes an international standard
The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), together with the Zittau/Görlitz University of Applied Sciences and Dresden University of Technology, has developed a method that accurately simulates the flow of steam in turbomachines up to 10 times faster than has previously been possible. This enables scientists to make much more precise predictions about turbine processes and also means that manufacturers have access to solid data, which can be used to further develop their facilities. Space researchers are also able to use the method to understand and simulate the processes in comets, moons and exoplanets. The method has been declared a new international standard by the International Association of the Properties of Water and Steam (IAPWS).
The properties of water can vary greatly depending on the temperature and pressure. Predicting processes in the three-dimensional environment of a turbine numerically requires calculating the behaviour of millions of dots in space at each point in time. For the past six years, scientists from the DLR Institute of Propulsion Technology have worked together with the Zittau/Görlitz University of Applied Sciences and Dresden University of Technology to develop a highly accurate method for calculations, which is 300 times quicker than previous methods. For the first time, it is possible to simulate the properties of the water vapour in complex processes.
"We were able to develop these new, highly accurate and also very quick algorithms, by taking the IAPWS's equations as a starting point, and using efficient interpolation techniques, as well as special variable transformations. This process revealed interpolation tables, which, in turn, accurately illustrated the properties of water vapour," said Hans-Joachim Kretzschmar from the Zittau/Görlitz University of Applied Sciences, who designed the new interpolation method (Spline-Based Table Look-Up) together with Matthias Kunick. Such accurate simulations are an important basis for the further development of turbines. This method enables manufacturers to test the properties and behaviour of prototypes with computer simulations; this not only reduces development time, but also significantly reduces the costs associated with development.
Computational Fluid Dynamics
A new feature of this method is the Computational Fluid Dynamics (CFD) database. This database limits the possible conditions of water or steam, such as pressure and temperature parameters, prior to calculation. It is not necessary for all possible aggregate conditions to be calculated; only the relevant ones are needed. "Water, or rather water vapour (steam), is very versatile. Simulations in a complex, three-dimensional environment of a turbine are, therefore, extremely difficult and intricate," stated Project Manager Francesca di Mare from the DLR Institute of Propulsion Technology. Highly accurate and realistic information on the three-dimensional and highly unsteady state of processes in a turbine can be obtained using computational fluid dynamics.
The method has already been used successfully in the Siemens 'Power and Gas' division. During this application, dynamic and steady state simulations of power plant processes were greatly accelerated without compromising the quality of the calculations obtained. "In some cases, we were able to more than double the speed of calculations in steady state simulations," said Ingo Weber, who is head of tool development for steady state power plant simulations in the field of 'Energy Solutions' at Siemens 'Power and Gas'.
Calculation methods in space research
Industry is not the only field that requires highly accurate methods to calculate the behaviour of steam. Water, in the form of vapour, liquid or ice, is one of the most important substances on Earth and in the Solar System. This calculation method can be used by planetary scientists to better understand, for example, the processes at play on Titan, or the icy moon Europa, or even in the vapour atmospheres of hot planets such as Venus and certain exoplanets. "Water exists in the Solar System in a wide range of forms. If we are able to model the processes, then it is simply not enough just to accept steam as an ideal gas. What is important here is codes, which are capable of quickly and accurately calculating those processes," explains Jens Biele, Deputy Project Manager of the Philae comet lander.
From idea to international standard
Thinking back, di Mare states, "we first came up with idea of linking highly accurate algorithms to a CFD database six years ago." Due to the complexity of water, this proved to be a long and difficult challenge for experts. Today, the method is so accurate and reliable that the International Association for the Properties of Water and Steam (IAPWS) has declared it the new international standard for calculating the properties of steam and water in Computational Fluid Dynamics (CFD) and complex steady state process simulations. The IAPWS is an international association made up of 12 national organisations that research the properties of water in all possible aggregate conditions. "Through the development of this innovative method, DLR has further established itself as a leader in virtual product technology," said Reinhard Mönig, Head of the Institute of Propulsion Technology. "With this, DLR is helping industry to solve general technical problems and is laying the foundations for efficient product development."
The new simulation method is the product of successful collaboration between scientists from three institutes. The highly accurate, thermodynamic algorithms were developed by Kretzschmar and Kunick from the field of technical thermodynamics at the Zittau/Görlitz University of Applied Sciences, in collaboration with Uwe Gampe from Dresden University of Technology. The algorithms were optimised alongside a group headed by di Mare from DLR, who used the algorithms in the TRACE software so they could be applied to the highly complex, three-dimensional numerical simulations of turbomachines.