Flow pattern transition instability during flow boiling in a single microchannel

We studied the unique characteristics of flow boiling in a single microchannel, including the periodic pressure drop, mass flow rate, and temperature fluctuations, in terms of a long time period. Experiments were conducted using a single horizontal microchannel and deionized water to study boiling instabilities at very small mass and heat flow rate conditions. A Polydimethylsiloxane (PDMS) rectangular single microchannel had a hydraulic diameter of 103.5 μm and a length of 40 mm. A series of piecewise serpentine platinum microheaters were fabricated on the inner bottom wall of the rectangular microchannel to supply thermal energy to the test fluid. Real-time flow visualizations of the flow pattern inside the microchannel were performed simultaneously with measurements of the experimental parameters. Tests were performed for mass fluxes of 170 and 360 kg/m² s and heat fluxes of 200–530 kW/m². The test results showed that the heated wall temperature, pressure drop, and mass flux all fluctuated with a long period and large amplitude. These periodic fluctuations exactly matched the transition of two alternating flow patterns inside the microchannel: a bubbly/slug flow and an elongated slug/semi-annular flow. Therefore, the flow pattern transition instability in the single microchannel caused a cyclic behavior of the wall temperature, pressure drop, and mass flux, and this behavior had a very long period (100–200 s) and large amplitude.