Effects of Mass Flux, Flow Quality, Thermal and Surface Properties of Materials on Rewet of Dispersed Flow Film Boiling

Abstract

The effects of mass flux, flow quality, material thermal properties, surface roughness, and surface oxidation on the rewet-wall superheat for dispersed vertical flow were experimentally investigated. The mass fluxes tested were 40.7, 81.4, 169.5 and 271.3 kg/s-m2. Flow qualities varied from 10–90 percent. The test materials were copper, aluminum, and inconel-600. Overall dimensions of the test pieces were 10.16 mm i.d., 25.4 mm o.d., and 25.4 mm long. A smooth inside surface was prepared for each material, with roughness amplitudes of approximately 0.5 μm. Two inconel test pieces had roughnesses of 15–20 μm and 66 μm, respectively. Another inconel piece was oxidized to a thin oxide layer of approximately 1.3 μm. The test fluid was liquid nitrogen. The results indicated that the rewet wall superheat increased with mass flux. The rewet wall superheat decreased with increasing flow quality, with the rate of decrease being more rapid at higher mass fluxes and higher qualities. Increases in wall roughnesses, and the presence of surface oxidation, increased the rewet superheat. The effect of an oxide layer increased with increasing mass flux and decreasing quality, and was thought to result from the decrease in contact angle between the liquid and the oxidized surface. Differences in wall thermal properties were not very significant up to a mass velocity of 81.4 kg/s-m2. Above this mass flux, the copper data did not clearly show the increasing Tmin with increasing mass flux, while the inconel data did. A comparison of Tmin for several materials with different thermal properties therefore was not made above mass flux of 81.4 kg/s-m2.