Novel microstructured evaporation device

Microfluidic devices have become more and more important in the field of thermal or chemical process engineering within the last years (by Schubert et al. Microscale Thermophys Eng 5:17–39, 2001). Cooling is one point of research where microchannels and other microstructured geometries provide significant advantages compared to conventional devices as they offer much higher possible surface to volume ratios and short characteristic distances. Therefore, an intense amount of heat can be transferred by these devices, which can be significantly increased by phase transition. Thus, evaporation of a fluid flow in microchannels allows very compact, fast, and powerful cooling devices. In this research study a novel microstructured evaporator geometry consisting of curved microchannels was evaluated on its evaporation properties compared to previous studies dealing with evaporation of R134a (Tetrafluoroethane) in straight microchannels (by Wibel et al. Chem Eng J 167:705–712, 2011). This novel evaporator design takes advantage of the strong centrifugal forces acting on the (evaporating) two phase flow in the curved microchannels. Due to the feasibility of very small radii of curved microchannels, strong centrifugal forces can be obtained for the fluid flow inside the microstructures. Additionally, those forces are boosted as flow velocities within the channels become higher due to the volume increase induced by evaporation. Therefore, a phase separation could take place inside the microstructure with a higher liquid fraction of evaporating coolant near one side of the curved channels during the transition to vapor. A high liquid fraction inside the evaporator is aimed by an intended removal of the evaporated gas phase from the microstructure. Experimental results of the evaporation of water and R134a as coolants demonstrate the potential of this curved geometry in comparison to evaporation in straight channels. Optical investigations of the new micro evaporator concept by high-speed videography (by Maikowske et al. Appl Therm Eng 30:1872–1876, 2010) are carried out for further improvements of the design. Various bubble formations and movements of the evaporating fluid flow were studied for various vapor fractions by using different fluids. These investigations show how the separation and extraction of the vapor fraction of the novel microstructure concept could be improved.