discovered_01_2016

WWW.HZDR.DE discovered 01.16 RESEARCH Research with imaging measurement methods The young scientist’s work is directed by his supervisor Uwe Hampel. He is the head of Experimental Thermal Fluid Dynamics at HZDR and holds the AREVA Endowed Chair of Imaging Techniques in Energy and Process Engineering at TU Dresden. Such fundamental research would not be possible without the kind of state-of-the-art measuring technologies available at HZDR. To study heat transfer processes, the researchers use ultrafast, high-resolution X-ray tomography, high-speed and infrared cameras, 3-D-scans and other measuring methods. Their findings will help improve nuclear safety and increase the efficiency of industrial processes. Debasish Sarker and Thomas Geißler, who are also Uwe Hampel’s doctoral students, are pursuing the same goal. Since mid-2014, both of them have been working on evaporation processes. More specifically, they are studying the formation of steam bubbles and their behavior in a flow. When the boiling crisis becomes a problem Boiling and evaporation are powerful cooling mechanisms. In a nuclear power plant, they are used to conduct heat that is released at the fuel elements. The steam drives a turbine; the attached generator converts the motion energy into electrical energy. The fuel rods inside the fuel elements are separated by a grid. To harvest the largest amount of energy, the distance from this grid must be minimal, while still allowing the cooling water to flow freely. Under no circumstances may the fuel rods heat to the point of a boiling crisis. When a liquid begins to boil, it produces bubbles. The liquid evaporates. If a critical level of heat flux density is surpassed at the heated surface, a film of steam will form, isolating the liquid from the surface and blocking heat transfer. This is called a boiling crisis. Everyone has seen water droplets dancing on a hot stove. The droplet will hover or glide on a cushion of steam, and it will take quite a while before it evaporates. A boiling crisis can occur very fast and cannot be reversed in an operating nuclear reactor. The isolating steam layer keeps the fuel elements from being cooled properly, the fuel rods heat up. In the worst case, entire fuel elements can be damaged. A look inside The boiling crisis, this sudden transition from regular boiling to what is called film boiling, is the subject of Thomas Geißler’s dissertation. At what point does the steam layer form and how does it relate to flow? How can the boiling crisis be delayed? A lot remains to be discovered. To find answers, Geißler, who is a chemical engineer, set up an experiment. Beneath a protective aluminium cover is a thin titanium rod about 40 centimeters long. Due to its lower boiling point, it is filled with a refrigerant rather than water. It is surrounded by several gold mirrors. When the rod heats up, the young scientist can watch the liquid evaporate. He uses ultrafast X-ray tomography and an infrared camera for his observations. While the tomograph looks deep into the interior of the rod, the camera captures the temperature fields on the exterior via various mirrors. "With this data, I am able to visualize the formation of steam bubbles and their behavior," he explains. "Some travel along the wall with the flow, others fuse into bigger bubbles." He then derives new theoretical models from his experimental findings. Such models exist already, but the processes are so complex that none of the models describes them comprehensively. The model must be expressed BOILING: How does a copper cylinder respond to increasing surface temperature? (a) convective heat transfer without bubbles, (b) nucleate boiling with just a few bubbles, (c) eruptive boiling, (d) film boiling above the Leidenfrost Point: the cylinder is completely encased in vapor.

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