Subcooled flow boiling heat transfer in a partially-heated rectangular channel at different orientations in Earth gravity

Abstract

This study explores subcooled flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 mm height, 2.5 mm width (heated), and 114.6 mm heated length. This Flow Boiling Module (FBM) is part of the Flow Boiling and Condensation Experiment (FBCE), which is a long-term collaborative effort to study the effects of gravity on flow boiling and condensation for their implementation in future space missions besides other applications. Datasets obtained from subcooled-inlet experiments performed using the FBM prior to its launch to the International Space Station (ISS) are examined and local subcooled flow boiling datapoints compiled to form a consolidated database. The consolidated 2589 datapoints cover broad ranges of operating conditions (mass velocity: 172.79 – 3200.00 kg/m2s, pressure: 102.16 – 238.44 kPa, subcooling: 0.13 – 34.93°C, quality: -0.560 – -0.000, heat flux: 1.80 – 49.99 W/cm2), flow orientations (vertical upflow, vertical downflow, and horizontal flow in Earth gravity), and partial heating configurations (single- and double-sided heating). Prior seminal correlations for subcooled flow boiling heat transfer are assessed for their predictive performance. Each local flow boiling curve is analyzed and manually demarcated into three regimes: partially developed boiling (PDB), fully developed boiling (FDB), and nucleate boiling degradation (NBD). A simple, yet very effective correlation based on dimensionless groups and having a fully explicit functional form, is developed. The new correlation well predicts both the subcooled flow PDB and FDB regimes with overall mean absolute errors (MAEs) of 9.59% and 6.91%, respectively. Although predictions for the NBD regime are much higher than observed with an overall MAE of 41.71%, they can be treated as upper limits to heat transfer here. The correlation is independent of both flow orientation and heating configuration. Overall, the new correlation clearly outperforms all prior seminal correlations for the entire consolidated database, with excellent predictions for both partially and fully developed subcooled flow boiling, i.e., for heat fluxes ranging from onset of nucleate boiling to onset of nucleate boiling degradation.