Onset of flow instability and critical heat flux in thin horizontal annuli

The onset of flow instability (OFI) and critical heat flux (CHF) in heated thin horizontal annular flow passages cooled by subcooled water were investigated. For OFI, six different test sections, with inner diameter of 6.4 mm, annular gap widths of 0.724-1.001 mm, and heated lengths of 174-197 mm were used. The test parameter ranges were: 85-1428 kg/m(2) s mass flux, 0.344-1.034 MPa exit pressure, 50-150degreesC inlet temperature, 0.124-1.0 MW/m(2) surface heat flux and 0-infinity inner-to-outer surface heat flux ratio. In addition, the effect of dissolved non-condensables on OR was examined by performing similar experiments with degassed water and water saturated with air at test section inlet temperature and exit pressure. A total of 138 OFI test were run addressing important parametric trends. A theoretical model based on the solution of one-dimensional fluid conservation equations, which assumes no voidage upstream the onset of significant void (OSV) point, and accounts for thermodynamic non-equilibrium beyond the OSV point using an empirical quality profile fit, was shown to predict the conditions leading to OFI reasonably well. For CHF, the test section was an annulus with 6.45 and 7.77 min inner and outer diameters, respectively (0.66 min gap width), and an 18.5-cm long heated section. The experimental parameters investigated covered the following ranges: test section exit pressure: 0.344-1.034 MPa; coolant (water) mass flux: 100-380 kg/m(2) s; wall heat flux: 0.231-1.068 MW/m(2); water inlet temperature: 30-65degreesC. The measured CHF values were considerably lower than the expected CHF values for vertical test section configuration. In all the tests CHF occurred at relatively high equilibrium qualities, and was preceded by flow stratification which caused dry-out of the upper surface of the flow channel. The data were correlated in two ways: by introducing empirical correction multipliers into three widely used correlations for vertical channels; and based on the compensated distortions method. (C) 2002 Published by Elsevier Science Inc.