Onset of flow instability (OFI) in uniformly heated microchannels cooled with subcooled water at very low flow rates was experimentally investigated. Four different microchannels, all of which were 22 cm long with a 16-cm-long heated section, were used. Two were circular with 1.17- and 1.45-mm diameters. The other two represented flow channels in a microrod bundle with triangular array and had a hydraulic diameter of 1.13 mm; one was uniformly heated over its entire surface, and the other heated only over the surfaces of the surrounding rods. The test parameter ranges were as follows: 220 to 790 kg/m2s mass flux, 240- to 933-kPa channel exit pressure, 30 to 74°C inlet temperature, and 0.1 to 0.5 MW/m2 heat flux. In addition, the effect of dissolved noncondensables on OFI was examined by performing similar experiments with degassed water and water saturated with air with respect to the test section inlet temperature and exit pressure.

Conditions leading to OFI were different from those reported for larger channels and for microchannels subject to higher coolant mass flow rates. In all the experiments, OFI occurred when equilibrium quality at channel exit was close to zero or positive, indicating the possibility of insignificant subcooled voidage in the channel and indicating that the widely used models and correlations that are based on the OFI phenomenology representing larger channels may not apply to microchannels at low-flow rates. The channel total pressure drops were significantly greater in tests with air-saturated water as compared with similar tests with degassed water. The impact of the dissolved noncondensable on the conditions leading to OFI was relatively small, however. With all parameters including heat flux unchanged, the presence of dissolved air changed the mass fluxes that led to OFI typically by only a few percent.