Flow patterns during boiling instability of deionized water across silicon-based microchannels with inner pin-fin arrays have been studied experimentally. Three types of microchannels with different pin-fin structures and a hydraulic diameter of 210 μm were used. During the unstable flow boiling, two types of instability modes of temperature and pressure oscillations occurred: long-period/large-amplitude oscillation mode and short-period/small-amplitude oscillation mode. There were increasing and decreasing stages of the temperature measurement during a period of long-period/large-amplitude oscillation mode. According to visualization observation, in the increasing stage of temperature oscillation for the in-line pin-fin microchannel, four two-phase flow patterns, including bubbly flow, vapor-slug flow, stratified flow, and stream flow, occurred sequentially with time; for the staggered pin-fin microchannels, the four two-phase flow patterns, together with single liquid-phase flow and single vapor-phase flow occurred sequentially with time. The flow pattern transitions were inverse between the increasing and decreasing stages of temperature measurement. Under the short-period/small-amplitude oscillation mode, only the stream flow occurred. With the increase of heat flux, the stream flow and the single vapor-phase flow occupied more and more time ratio during an oscillation period in the in-line and staggered pin-fin microchannels, respectively.
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