Wind tunnel tests with AIRBUS
This DLIP process was developed in a concerted effort between Fraunhofer IWS and TU Dresden in order to find the optimized DLIP surface structure. Finally, the IWS experts developed the patterning process to transfer the optimized structure onto final demon-strator: a complex three-dimensional NACA airfoil which served as a miniaturized but realistic wing pendant. The NACA airfoil was then tested by AIRBUS experts in the wind tunnel. The performance tests were carried out with a structured NACA airfoil and an unstructured NACA airfoil serving as a reference under realistic conditions at wind speeds ranging from 65 to 120 m/s, with air temperatures below minus ten degrees Celsius and at various humidity levels.
The partners from AIRBUS were able to demonstrate that ice growth on the functionalized surface is self-limiting. In fact, the ice falls off after a certain amount of time without requiring added surface heating. Additional experiments also showed that it took 70 seconds for the ice on an unstructured airfoil to melt at 60 watts of applied heat. The ice on the structured airfoil receded completely after just five seconds at the same amount of applied heat. The DLIP technology accelerated the process by more than 90 percent. It took 75 watts, or 25 percent more heating power compared to the DLIP surface, to remove the ice on the unstructured demonstrator. “In this wonderful collaboration with AIRBUS, we demonstrated for the first time and in a realistic way the great anti-icing potential that can be tapped with large-scale laser surface patterning. With our DLIP approach, we realized biomimetic surface structures on a complex component like the NACA airfoil, and demonstrated its distinct advantages over other laser processes", says Dr. Tim Kunze, Team Leader Surface Functionalization at Fraunhofer IWS. His colleague Sabri Alamri adds, “The application of micro- and nanostructures on metal prevents water droplets from adhering. Inspired from nature, this is widely known as the lotus effect. With our new DLIP process, we can create a fragmented surface to significantly reduce the number of adhesion points for ice. We will soon publish a paper on the results.” Project partner Elmar Bonaccurso, Research Engineer for Surface Technology / Advanced Materials at AIRBUS, adds, “Ice formation is particularly dangerous during landing. Water on the surface freezes within milliseconds when the aircraft flies through the clouds at sub-zero temperatures. This can disrupt the functions of control elements such as landing flaps and slats, which impairs the aerodynamics. Today, we use hot air sourced from the engines to heat wing surfaces. The water-repellent structure, which we developed with our partner Fraunhofer IWS in the EU project Laser4Fun, is an attempt to replace conventional technologies with ecofriendly, more cost-effective alternatives.” The partners’ next step will be to optimize the method and adapt it to various air zones. They will take into account the results obtained in real-world flight tests currently underway with an A350 aircraft whose surfaces have been treated with DLIP.
A key technology
The research team has established a key technology by using short and ultrashort pulse lasers for Direct Laser Interference Patterning. It can serve many applications, for example, to structure functional surfaces on wind turbines or other components that can ice over in cold regions. This technology can be also applied to very different fields such as product protection, biocompatible implants and improved contacts for electrical connectors. “We can apply functional microstructures over large areas and at high process speeds, thereby achieving benefits for a number of applications that, until now, had been inconceivable,” says Tim Kunze.
A scientific publication on this development was issued on 24 February 2020 under the title "Design Rules for Laser-Treated Icephobic Metallic Surfaces for Aeronautic Applications": https://onlinelibrary.wiley.com/doi/10.1002/adfm.201910268.