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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE // What happens when water and steam meet in a pipeline system? What impact does this have on the safety of nuclear power plants or on the efficiency of chemical production processes? Researchers at the HZDR are using an experiment that is closely linked to industry to investigate when exactly the two phases block each other and are developing physical models based on their insights. _TEXT . Christine Bohnet Translation . Sarah Gwillym-Margianto Water versus steam – comparing unequal forces in industrial plants The chemical industry is one of the largest branches of German industry. It is a branch with a particularly large consumption of resources, resulting partly from a high energy consumption and partly from the use of crude petroleum as a raw material. According to a study on behalf of the German Federal Office of Statistics, the chemical industry has in the past been responsible for using some 21.6 million tons of organic raw materials, of which about 75 percent were converted into the basic chemical substances: ethylene, propylene and aromatic compounds. This conversion takes place inside large chemical apparatuses, where steam is applied to split the crude petroleum (a process known as steam cracking). In order to maintain pure products, the substances then have to be elaborately broken down into separating columns. Scientists at the Helmholtz-Zentrum Dresden-Rossendorf want to use the most modern measuring technology to glean insights into the processes used in such complex industrial apparatuses. Their new goal is to optimize the established production process of chemical elements in such a way that considerable amounts of costly energy and valuable organic raw materials can be saved. In this way they can also contribute to the energy turnaround as well as the international competitiveness of the chemical industry, which is among the world’s large chemical producers. Combinations of phases such as liquid-steam or gas-liquid- solid mixtures characterize the numerous steps in the process from crude petroleum to pure substances for the chemical industry, but they are also important for the safe operation of nuclear and solar power plants or for oil and gas extraction. Especially in nuclear engineering a reliable forecast of such flows is of great significance – they influence the safety of the plants. In a pressurized water reactor in the primary circuit only water flows under high pressure and at temperatures just below 300 degrees Celsius. The energy transported by this hot water is used in the steam generator unit to supply the turbines with steam in the secondary circuit. Leakages cannot be excluded surely in such large technical plants, but even then a safe cooling of the reactor core needs to be guaranteed. This has to be demonstrated by comprehensive safety analyses, whereby the computer models that are implemented must be able to predict flows with a high degree of reliability. When the pressure falls due to a leakage, these flows are often two-phase flows, meaning that water and steam occur simultaneously in the primary circuit. In the past the results from such safety analyses could not be transferred to other plants with a different geometry or size. Experts at the HZDR have a lot of collective experience investigating multi-phase flows of steam and water in the primary circuit of a pressurized water reactor in the event of a potential failure – not only in experiments but also in simulations. Both research methods have to go hand in hand when dealing with such highly complex problems. One example from daily life illustrates this: if you turn on a tap in your home, the water flows in a steady laminar flow into the sink. However, if you turn the tap as far as it will go, the water gushes out of the tap in a turbulent flow, even dragging air along with it. Considering that flows of two or more phases are considerably more complex than those with only a single phase, it is understandable that a wide research field is open for turbulent multi-phase flows that will continue to preoccupy the next generation of researchers. A simulation tool for every flow pattern The type of flow determines how individual substances are either mixed or separated. Single-phase flows can already be calculated very well in simulations, whereby many expensive experiments (e.g. for new car and aircraft types in the flume) are no longer necessary. Instead, parameters like for example the friction of the air for the object of investigation depending on the geometry of the object but also on other variables such as temperature, velocity and gas flow rate can be determined both quickly and precisely using special computer programs. These CFD-programs (Computational Fluid Dynamics) are still very much in their infancy for multi-phase flows. When steam and water flow through a pipe, they do not only come into contact with the pipe, but also the surfaces and the interfacial layers of both phases come into contact with each