A novel device for the optical investigation of phase transition in micro channel array evaporators

Evaporation is one of the most important unit operations in thermal and chemical process engineering. Microstructure devices are especially well-suited to act as evaporators. Due to the extraordinary heat exchange performance, electrically powered micro heat exchangers are predestinated to act as evaporators in order to provide sufficient thermal energy for evaporation. Phase transition in micro channels, especially in micro channel arrays, is accompanied by phenomena like vapor clogging, vapor slugging or pressure drop, mass flow and temperature fluctuations. These arrays consist of a multitude of micro channels, which are connected to a collective fluid inlet and outlet. The main objective of this research is to investigate phase transition in micro channel arrays through identify the dependence of phase transition on microstructure geometry. In this publication, a novel electrically heated device which enables the optical investigation of processes inside micro channels is presented and compared to a previous design. The device generates a nearly homogeneous temperature distribution in micro channel arrays and additionally provides various degrees of freedom concerning the microstructure geometry. Methods of investigation as well as equipment required for characterization are presented. The optical investigation of multiphase flow composed of water and steam is performed by an optical microscope and a CCD-based digital high-speed camera with recording frequencies in the range of several 1000 up to a few 100,000 pictures per second.