A simultaneous visualization and measurement study has been carried out to investigate flow boiling instabilities of water in parallel microchannels at various heat fluxes and mass fluxes. Eight parallel silicon microchannels, with an identical trapezoidal cross-section having a hydraulic diameter of 186 μm and a length of 30 mm, were used in this experiment. It was found that the flow boiling pattern depended on the heat to mass flux ratio q/G. Stable flow boiling regimes existed for q/G < 0.96 kJ/kg and q/G > 2.14 kJ/kg whereas unstable flow boiling existed in the range of 0.96 kJ/kg < q/G < 2.14 kJ/kg. At a given heat flux in the unstable flow boiling regime, the mass flux decreases continuously to a constant value after a sufficiently long time, and eventually reaches a stable flow boiling mode. It was found that the physical mechanism for the unstable boiling flow is owing to bubble expansion in both upstream and downstream direction. In particular, the expansion of bubble upstream will increase the upstream wall temperature, and the temperature of entire microchannel will eventually reach a steady state, i.e., a stable flow boiling regime.

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