Low-Flow Critical Heat Flux in Heated Microchannels

TLDR

The study experimentally investigates critical heat flux of subcooled water flowing in heated microchannels. Four 16‑cm microchannels—two circular (1.17 mm and 1.45 mm diameters) and two microrod bundle channels (1.131 mm hydraulic diameter, one uniformly heated, one heated only on rod surfaces)—were tested over mass fluxes 250–1000 kg m⁻² s⁻¹, pressures 344–1043 kPa, and inlet temperatures 49–72.5 °C, with additional runs using degassed and air‑saturated water to study noncondensable effects. CHF occurs at high flow qualities (≥0.36) indicating dryout, increases monotonically with mass flux or pressure, rises with channel diameter at low mass flux, is slightly enhanced by dissolved air in the smaller circular channel but reduced in others, and the Bowring‑1972 correlation predicts the data reasonably well.

Abstract

AbstractCritical heat flux (CHF) associated with the flow of subcooled water in heated microchannels is experimentally investigated. Four different channels, all 16 cm in length, are used: two are circular and uniformly heated and have 1.17- and 1.45-mm diameters, and the other two represent flow channels in a microrod bundle with a triangular array and 1.131-mm hydraulic diameter, with one uniformly heated over its entire surface and the other heated only over the surfaces of the surrounding rods. The test section parameter ranges are as follows: 250 to 1000 kg/m2·s mass flux, 344- to 1043-kPa exit pressure, 407- to 1204-kPa inlet pressure, and 49 to 72.5°C inlet temperature. The effect of noncondensables (air) on CHF is also examined by repeating some of the experiments with degassed water and with water saturated with air at test section inlet pressure and temperature.Critical heat flux occurs at very high flow qualities (0.36 and higher) in all the tests and indicates the occurrence of dryout. Furthermore, the CHF appears to monotonically increase with increasing mass flux or pressure. The CHF depends on channel cross-section geometry, and unlike high mass flux data, it increases with increasing channel diameter. The dissolved air slightly increases the CHF for the smaller circular channel and reduces the CHF for the other test sections. The experimental data are compared with the predictions of three widely used empirical correlations. The Bowring-1972 correlation could predict the data with reasonable accuracy.