Effects of dissolved gases on subcooled flow boiling from small heated regions with and without streamwise concave curvature

Coolants such as fluorocarbon liquids usually contain high levels of dissolved gases. When heated, these gases are liberated from the liquid; if the liquid is boiling, these gases may influence the supply of liquid to the boiling surface. In this study, the effects of dissolved air in perfluorinated hydrocarbon, FC-72, on flow boiling heat transfer characteristics were experimentally investigated over a range of heat flux from the onset of nucleate boiling (ONB) to the critical heat flux (CHF). The experiments were conducted in both straight and curved channels. The boiling surface was a 3 mm×3 mm patch located on the channel wall. In the curved flow, it was located at the 90° position of a U-bend on the concave wall. Gas content was varied from 1.16×10^-5 to 2.55×10^-3 mole/mole, velocity ranged from 0.8 to 6.9 m/s, and pressure was controlled to either 1.41 or 1.92 atm. Subcooling, based on total pressure, was maintained at 27.5°C. The data show that reduction of incipience superheat at ONB due to dissolved gases under these forced-convection conditions is much less than with pool boiling or low-velocity flow boiling. Boiling curves for gassy and degassed cases differ at low heat flux levels but merge at higher heat fluxes. The merging may indicate that the near-wall liquid layer is being degassed at higher heat fluxes. Though the high-heat-flux portions of the boiling curves were apparently unaffected by the content of dissolved gas in the approaching liquid stream, the critical heat flux was decreased by as much as 10%. Explanations of this behavior are presented in terms of the two mechanisms for the liquid supply to the macrolayer