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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE other. Many other parameters such as the different speeds of the phases, the viscosity or the surface tension of the water and steam and, last but not least, the pipe’s geometry all have to be taken into consideration to achieve a reliable computer simulation. If other factors such as obstacles and bends in the pipelines or the typically high temperatures and pressures for pressurized water reactors are also taken into account, then the list of parameters to be considered seems endless. Neither is the available computer technology fast enough to reflect the processes for all scales in simulations, nor are the data obtained from each experiment sufficient as a basis for simplifying computer models. For this reason it will still take several decades to establish a universal simulation tool for multi-phase flows and it will still take a lot more experiments to reliably determine all of the relevant parameters and to feed these into the respective CFD-programs. At the same time, every successful and prospectively designed experiment is an important step towards an industrial benchmark, the results of which can be transferred immediately into CFD- codes. Consequently, the efficiency of separating columns or the safety of nuclear reactors can be improved step by step. Penetrating intransparent walls Production processes with multi-phase flows take place in industry behind thick pipes and intransparent walls and are neither amenable nor observable. Further, there is the fact that separation columns and primary circuits in nuclear reactors are not exactly set up simplistically – quite the opposite. A separating column is a large cavity with several intermediate floors with partly leveled superstructural parts, where, depending on the height, liquid mixtures can be thermally separated. In this respect, the different properties of the individual substances are utilized to capacity. A typical example of this is multiple vaporization and condensation – like for example in oil refinement, where eventually the water vapor chemically converts the crude petroleum. This is how multi-level processes are used to produce the required basic substances for diverse branches, i.e. in machine building and automotive engineering or for cosmetics and the food industry. Whether or not the established production processes run quickly, efficiently and safely in the chemical industry has always been more or less a question of the skills and experience of the operators, and even planners of new plants have very little to go on in the way of insights gained from process operations. More reliable insights into the dynamics of material flows could however enable designs that would go hand in hand with considerable increases in efficiency. HZDR scientists therefore want to concentrate on this in the future. They aim to examine chemical plants from the inside using the most state-of-the-art measuring tools and to optimize flow. At the same time, they are conducting groundbreaking experiments on two-phase flows at the TOPFLOW test facility at the Helmholtz center, and with the results obtained they are improving the respective CFD-programs. One particular highlight that was extended from an experiment to a new simulation model was the successful test series on counter-current flow limitations. For his PhD work on this topic, the nuclear engineer Christophe Vallée received an HZDR PhD award in 2011. A counter-current flow limitation occurs when a water flow is limited by a steam flow from the opposite direction, which could occur for example in the case of interference in the primary circuit of a pressurized water reactor and must therefore be dealt with. A place of extremes In a pressurized water reactor, hot water circulates under high pressure in the primary circuit, which cannot vaporize due to the high pressure. In the case of leakage however this changes. In this case, the cooling water leaks out and the pressure in the reactor core can fall to around 70 bars. With the subsequent vaporization, the water level in the reactor is lowered. The steam rises along the piping system towards the steam generator, where it condenses to water and flows back into the reactor. This is desirable because the water contributes to cooling down the reactor core and thus plays an important role in the passive safety of a nuclear power plant. The water on the way back from the steam generator to the reactor vessel has to pass through a thick pipe with a diameter of almost one meter, where it has to demonstrate its force against the steam flowing in the opposite direction. As the boiling point of water is higher than under normal pressure, the density of the steam is greater while the density of the water is lower. In this way, the denser steam presses against the surface of the water and slows it down completely with a counter-current flow limitation. In order to be able to make sound statements about the process of leakage failure, and consequently about the safety of nuclear power plants, Christophe Vallée devoted his PhD thesis to this topic and came up with an outstanding experimental set-up. With a replication of the complex geometry of a reactor’s hot leg, he exposed a steam-water flow to high pressure and high temperatures. While water rushed in from one direction, steam was fed into the system from the other side. The steam flow rate was gradually increased until it completely blocked the water flow. For the first time ever the window of observation for this kind of experiment was not only a few square centimeters, but one entire square meter. Consequently, Christophe Vallée was able to observe the behavior of the converging phases in detail with a high-speed camera. Putting experiments and simulations into practice The fact that the experimental set-up with the window and the camera were able to endure a pressure of 50 bars and temperatures of around 275 degrees Celsius is due to the pressure vessel at the TOPFLOW plant. An ingenious set-up which guarantees pressure equalization allows thin-walled experimental set-ups as well as the use of special measuring techniques. This effort was necessary to obtain data for the phenomenon of the counter-current flow limitation, with