Optical Studies of Evaporation in Microchannel Arrays

Phase transition in microchannels has become an interesting filed of research within the last few years. Using arrays out of a multitude of parallel microchannels, it is possible to transfer a huge amount of thermal energy by taking advantage of the latent heat of evaporation. Another point of interest concerning this research field is the stable generation of steady vapor with homogeneous parameters such as vapor quality, mass flow, pressure or temperature. Phase transition and accompanying phenomena during evaporation of water in microchannel arrays as well as influences of microstructure geometry were observed during these research studies. Optical investigations have been done using a digital high-speed camera for visualization of transient processes, e.g. explosively and confined bubble growing behavior or phase transition fluctuation. Beside this, a novel test device for optical investigation is presented in this report. The test device enables to vary several variables like temperature, microstructure or pressure drop, to name but a few. Furthermore, results and influences of different microstructure geometries on phase transition as well as different shapes of phase transition fronts in microchannel arrays are presented. Additionally, the visualization of complete and stable phase transition in microchannel arrays with steam superheating is shown.