Influence of dissolved gas concentration on subcooled flow boiling critical heat flux in short vertical tube

The objectives of the present study is threefold. First, it is to measure the CHF (critical heat flux) on the rough and smooth finished inner surfaces of short vertical tubes with 3 mm inner diameter and 66.5 mm heated length for a wide range of dissolved gas concentrations (ranging from saturated condition to near 0 ppm) and flow velocity. Secondly, it is to confirm the applicability of CHF correlations against outlet and inlet subcooling for low dissolved gas concentration region. Thirdly,it is to discuss the mechanisms of subcooled flow boiling heat flux in a short vertical tube. It appears that first, the CHF data under the saturated vapor pressurization by using the pressurizer are in good agreement with the values calculated by the CHF correlations against outlet and inlets subcooling, based on the experimental data under the N2 gas pressure previously obtained in the whole experimental range. Secondly, the CHF for the rough finished inner surface test tube is kept almost constant for dissolved oxygen concentrations ranging from 6.59 to 0.0265 ppm and for flow velocities ranging from 4.0 to 13.3 m/s. Thirdly, the CHF for smooth inner surface and for higher flow velocities are about 10% or more lower than that for the rough surface. And fourthly, no direct transition to film boiling was observed on the smooth surface used here even at the lowest dissolved oxygen concentration