Measured local heat transfer data and visually observed two-phase flow behavior are reported for convective boiling of methanol and water in a partially heated horizontal channel with offset strip fins. In this study, a special test section was used that permitted direct visual observation of the two-phase flow while simultaneously measuring the convective boiling heat transfer coefficient at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for top heating and bottom heating of the horizontal channel. For the conditions examined here, the two-phase flow was in the stratified, wavy, or annular flow regime over almost the entire length of the channel. It was visually observed that virtually no nucleate boiling is present when the flow is in these regimes. A flow regime map is presented, which summarizes the observed two-phase flow characteristics. At the same quality and mass flux levels, the measured heat transfer coefficients for top heating and bottom heating were found to be virtually the same for the conditions examined here. In addition for the same quality and mass flux, the convective boiling heat transfer coefficients for top heating and bottom heating agree well with the results for comparable experiments with the channel oriented vertically. Hence, prior to dryout of the liquid film, the boiling heat transfer performance appears to be insensitive to channel orientation. Observed differences in the dryout conditions for top heating and bottom heating are also discussed.
Two-dimensional heat transfer coefficients with simultaneous flow visualisations during two-phase flow boiling in a PDMS microchannel