Low-flow-onset of flow instability in heated microchannels

Onset of flow instability (OFI) in uniformly heated microchannels cooled wi th subcooled water at very low flow rates was experimentally investigated. Four different microchannels, all of which were 22 cm long with a 16-cm-lon g heated section, were used. Two were circular with 1.17- and 1.45-mm diame ters. The other two represented flow channels in a microrod bundle with tri angular array and had a hydraulic diameter of 1.13 mm; one was uniformly he ated over its entire surface, and the other heated only over the surfaces o f the surrounding rods. The test parameter ranges were as follows: 220 to 7 90 kg/m(2).s mass flux, 240- to 933-kPa channel exit pressure, 30 to 74 deg rees C inlet temperature, and 0.1 to 0.5 MW/m(2) heat flux. In addition, th e effect of dissolved noncondensables on OFI was examined by performing sim ilar experiments with degassed water and water saturated with air with resp ect to the test section inlet temperature and exit pressure. Conditions leading to OFI were different from those reported for larger cha nnels and for microchannels subject to higher coolant mass flow rates. In a ll the experiments, OFI occurred when equilibrium quality at channel exit w as close to zero or positive, indicating the possibility of insignificant s ubcooled voidage in the channel and indicating that the widely used models- and correlations that are based on the OFI phenomenology representing large r channels may not apply to microchannels at low-flow rates. The channel to tal pressure drops were significantly greater in tests with air-saturated w ater as compared with similar tests with degassed water The impact of the d issolved noncondensable an the conditions leading to OFI was relatively sma ll, however. With all parameters including heat flux unchanged. the presenc e of dissolved air changed the mass fluxes that led to OFI typically by onl y a few percent.