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978-3-8439-4841-8, Reihe Mikrosystemtechnik
Local Sensing in Microchannel Boiling
196 Seiten, Dissertation Albert-Ludwigs-Universität Freiburg im Breisgau (2021), Softcover, B5
The present work focuses on a novel method of studying microchannel boiling that is based on an electrical capacitive sensing concept. Studying local process parameters in microchannel boiling has been under investigation as the main motivator for this study. For this purpose, a test rig and a microreactor were developed and built that provided the opportunity to visualize the boiling in a specific location of the microchannel while impedance measurements were being recorded at the same point. Indium tin oxide electrodes were patterned on a glass lid as transparent capacitive sensing elements. These recordings are done meanwhile temperature measurements along the channel plate and heater block were performed, line pressure in the inlet as well as the pressure drop along the channel were recorded and room temperature in the experiments was measured. A novel measurement technique was introduced with its advantages, possible applications and limitations explained. The experimental results in this thesis are analyzed all in the steady-state condition.
The measurements for impedance were performed with a bench-top LCR-meter and impedance analyzer. In addition, a cheaper alternative for performing such measurements is presented based on an integrated AD5933 capacitive sensor chip and the application of this system was tested by experimentation on slug flow. A local flow regime detection method using impedance amplitude measurements that is verified by simultaneous videography as ground truth is presented. The confidence interval ranges of the impedance peaks in heat flow boiling regimes are exported for different flow regimes and shown in results. The importance of knowing the flow regime is highlighted when the correlation between heat transfer coefficient prediction models and the flow regime in each location are shown. The possibilities of using impedimetric sensing results as a feed-in into heat transfer models specially for slug flow is presented.