Flow-boiling of Perfluorohexane (FC-72) in horizontal micro/mini channels was investigated experimentally and numerically at different rotational orientations in terms of gravity.One-sided uniform channel heating was considered experimentally for rotational angles ranging from 0° (heating from below) to 180° (heating from above) in increments of 30°. The micro/mini channel had a high aspect ratio of 10 (5 mm x 0.5 mm) and a hydraulic diameter of 909 m. Inchannel flow visualisations were recorded and heat transfer coefficients were determined for mass fluxes of 10, 20 and 40 kg/m 2 s at a saturation temperature of 56 °C. Suitable heat fluxes were applied to span the onset of nucleate boiling to near dry-out conditions within the channel. It was found that the rotational angle had a significant influence on the heat transfer performance due to its influence on bubble detachment. Bottom-heated cases (0° orientation) resulted in local heat transfer coefficients that were up to 201% higher than for any other rotational orientation. Channel orientations of 60° (slanted heating surface) and 90° (heating from the side) generally produced the lowest local heat transfer coefficients. Insight into the influence of the gravitational orientation on single-bubble growth within the nucleation and detachment region was obtained via two-and three-dimensional numerical simulations. Bubble behaviour after detachment and its effect on heat transfer were also investigated transiently until detachment. The numerical simulations mirrored the experimental trends and it was found that the presence of growing bubbles interrupted the velocity streamlines and the thermal boundary layer downstream of the nucleation site.

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