discovered_01_2016

WWW.HZDR.DE 28 29 RESEARCH // THE HZDR RESEARCH MAGAZINE This is where Sebastian Unger’s hedgehog comes in. In his quest to optimize the interplay of flow and heat transfer, the Ph.D. candidate is starting with the cooling tubes. The hedgehog is nothing other than an example for an unconventional heat transfer structure. The enlarged exterior surface and a special coating inside the tube are designed to optimize heat transfer efficiency. Promising coating In a passive cooling system, circulation is generated solely by the difference in density of the cooling medium, which is usually water. Hot steam flows into the cooler, condenses along the walls and runs back as water. The smaller the difference in temperature, the smaller the forces propelling the circulation. To make sure that emergency cooling works efficiently nevertheless, it is important to minimize heat resistances and improve condensation. Sebastian Unger has found a promising approach in a thin, hydrophobic coating on the interior of the cooling tube. It will ensure that condensed water droplets will run off quickly rather than forming a film that impedes heat transfer from the inflowing steam to the wall. In his search for a suitable coating technique, Sebastian Unger turned to a specialist at HZDR, electrochemist Ulrich Harm. He is an expert on wet-chemical processes – a method that lends itself very well to coating the inside of a tube. It harnesses a special property of metals, the fact that they form a thin oxide layer at the surface. If a metal tube is dipped into the coating bath, a single layer of molecules, called a monolayer, will attach to this oxide layer. This kind of coating favours the formation of water droplets, which means that the surface will not get very wet. "It is like the lotus effect," Sebastian Unger explains. "Since the monolayer is between a few ångströms and two nanometers thick, it improves heat transfer during condensation without causing any additional thermal resistance." To find the right coating, various chemicals and surface structures were tested to achieve the desired wettability and thermal resistance. Previous studies relate to coated copper or gold, which is used at a smaller scale, mainly in electronics. Power plants, however, mainly use steel or aluminium. HZDR researchers are therefore breaking new ground with this hydrophobic coating of steel. Unger is hoping to publish initial results as early as this year. The printed hedgehog In a passive cooling system, the only option is often to release heat into the surrounding air. In order to improve the transfer of heat and air circulation, Sebastian Unger developed a special surface design. He coated the outside of the tube with a host of thin spikes, thereby increasing the heat exchange surface. To create his spiky heat-transfer structure, the young researcher chose a novel additive manufacturing method generally known as 3D printing. Since this technology also works for metals, it offers plenty of creative leeway for designing special components at relatively low cost. He was therefore able to calculate the optimum shape and have his little model made at the TU Dresden Institute for Materials Science. Soon, a foot-long (35 cm) piece of the ‘hedgehog rod’ will be tested in the flow channel. "I will heat it from the inside and measure heat conductivity," Sebastian Unger says. "I will then be able to compare the results with traditional cooling fins." If the experiments are successful, the cooling design will have to be calculated and adapted to the large scale. BASICS: A droplet of water on a coated stainless steel sample is used to measure the wettability between sample and fluid. The aim is to optimize evaporation and condensation processes. Photo: Oliver Killig COATING: Good wettability – on simple steel the droplet "lies flat" on the surface. In the image on the right, by contrast, the surface repels the droplet.

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