Examination of phase transition in microstructures reveals some phenomena which occur to a minor extent or are not present in macro scale devices. This paper presents results of research activities in phase transition and related phenomena as well as multiphase flow obtained in microchannels. Different phase transition phenomena will be presented, like vapour plugging and vapour slugging. Evaporation of water in arrays of parallel microchannels has been examined in a special electrically heated device with an integrated window which allows the optical investigation of fluid flow and phase transition in single-microchannels and microchannel arrays. By understanding and controlling these phenomena, it is possible to create a stable and controllable phase transition inside microchannel arrays and to produce vapour flow with constant vapour quality, or overheated steam. All investigations deal with evaporation in horizontally arranged microchannel arrays with rectangular channel cross-sections and were performed by digital high-speed videography with recording frequencies of up to 200 000 frames per second.
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