Heat transfer correlation of smooth inverted annular film boiling in a circular tube

Experiments on steady film boiling heat transfer are conducted in a circular tube with test conditions covering the range of 10-50 K inlet subcooling, 0.2-2.5 MPa pressure, and 100-1000 kg/m2s mass flux. Through analysis of experimental data and comparison with corresponding predictions by the existing correlations, they are demonstrated to be insufficient for accurately capturing the varying trends of the experimental data within inverted annular flow boiling (IAFB) regime. It is discovered that IAFB is divided into three regions, namely, smooth IAFB, rippled IAFB and turbulent IAFB, by the distribution of wall temperature and its rate of change along axial direction, which is consistent with relevant observations from previous study. Only the vapor film in the smooth IAFB can be attributed to laminar vapor film flow, which leads to the discrepancy in the overall trend of existing correlations and experimental data. As fluid flows downstream, interface interaction due to velocity difference between vapor film and liquid core accounts for the fluctuations. The assumption that transition between smooth and rippled IAFB is result from Kelvin-Helmholtz (K-H) instability accounts for the flow characteristics of vapor film in smooth IAFB. Heat transfer at interface is obtained based on vapor-liquid velocity. Additionally, a new heat transfer correlation for smooth IAFB is proposed. The MAPE of the new correlation is 16.4%, and the RMSPE is 18.5%. The new correlation could more accurately describe the heat transfer characteristic in smooth IAFB.