A novel stepped microchannel for performance enhancement in flow boiling

Abstract

Flow boiling behavior in rectangular cross-section microchannel is influenced by its size and aspect ratio. For channels with same depth, small width microchannels show better flow stability owing to their early transition to annular flow at the cost of more pressure drop as compared to wider channels. In the present work, an effort has been made to integrate the effects of narrow and wider straight microchannel into a novel stepped converging microchannel. A novel design of stepped converging microchannels (SCMC) has been proposed and its performance in flow boiling has been compared with rectangular cross-section microchannel (RMC). The stepped converging microchannel is a combination of V-shape channel at the bottom and converging wider step channel at the top. Both the channels (SCMC and RCM) have been fabricated on a copper block with footprint area of 10 × 10 mm2. Flow boiling experiments have been performed with deionized water in nine numbers of parallel microchannels of hydraulic diameter Dh = 350 µm. Nucleating bubbles growing in the narrow V channels departs into the wider top flow passage providing additional space for their further growth. Moreover, converging step imparts additional inertial force to overcome the backward evaporative momentum of the vapor slug. SCMC configuration reported a maximum of 98% increase in heat transfer coefficient, whereas a maximum of 77% reduction in overall pressure drop compared to RMC. The stepped configuration also mitigates wall temperature and pressure drop fluctuations thus reducing flow boiling instability.