An investigation of horizontal and vertical flow boiling in a single channel with a confinement number beyond the threshold of micro-scale flow.

The gravitational force effect is of concern when designing flow boiling systems deployed on Earth and in space. A study of the R-134a flow boiling inside the horizontal and vertical channels was experimentally performed to explore the flow orientations' effect on flow pattern and heat transfer characteristics. The test section was a 0.5 mm diameter tube from which the corner effect was excluded. The single channel was used to avoid disturbances that flow maldistribution could possibly cause in the multiple channels. The experimental two-phase flow corresponded to a 1.75 confinement number, which was above the threshold for micro-scale flow. The results indicated the flow regime map, which was less affected by the channel orientation except for that slug flow pattern that was unable to survive during the vertical downflow and vertical upflow. Also, based on the present micro-scale flow, the heat transfer results were mostly independent of the gravitational force effect, and the nucleate boiling and convective mechanisms tended to govern them. [ABSTRACT FROM AUTHOR]