An investigation was conducted to understand flow boiling of de-ionized water in locally heated parallel microchannels. High speed visualization technology was employed to visually observe the transient phase change process in an individual microchannel. The process at locally heated condition was different from those at entirely heated condition where vapor column(s) stayed quasi-stable for a long time without venting out. Intensive interface oscillation occurred during phase change. Thin film evaporation appeared to be the main evaporation mode at the upstream vapor column cap, whereas nucleation happened occasionally at favorable flow, heat transfer and local wall conditions. Continuous condensation at the downstream vapor column cap led the flow back to single liquid phase regime out of the microchannel. A signal analysis method was introduced to study the interfacial oscillation behavior at the evaporating cap, which showed marked impact of the evaporation mode on the characteristic frequency.

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