Heat transfer characteristics of downward saturated boiling flow in vertical round pipes

• Characteristics of downward flow heat transfer were summarized. • A downward flow heat transfer experimental database was created. • Prediction performance for downward flow heat transfer was evaluated. • Factors required to predict downward heat transfer were extracted. This study reviewed the saturated boiling heat transfer research in downward flows. A database of downward flow heat transfer experiments was created using experimental studies. Saturated boiling heat transfer correlations in internal flows were collected, and no downward flow-specific heat transfer correlations were identified. The applicability of heat transfer correlations to downward flow heat transfer experiments was evaluated, and no correlation could predict the heat transfer coefficients accurately for all experimental databases. However, correlations that could predict heat transfer coefficients reasonably well were determined for each channel size. Cooper's correlation [Int. Chem. Eng. Symp. Ser. 86 (1984) 785–792] had a mean absolute percentage error ( MAPE ) of 11.7 % for mini-channels and Kim and Mudawar's correlation [Int. J. Heat Mass Transf. 64 (2013) 1239–1256] had an MAPE of 66.5 % for macro-channels. Furthermore, because the advection direction between the liquid-phase and the generated bubbles differed depending on the liquid-phase velocity in downward flows, we evaluated the prediction performance of the heat transfer coefficient for the liquid-phase velocity. For some experimental data, the prediction performance of the existing correlation for downward flow heat transfer worsened as the advection velocity of the bubbles decreased. This result is one of the issues to be addressed in the future development of heat transfer correlations.