Boiling and condensation two-phase flow heat transfer on three-dimensional macroscale surfaces with microscale structures

Enhancing the heat transfer performance of two-phase flow in heat transfer tubes is of paramount importance. In pursuit of this goal, the research group fabricated heat transfer tubes featuring both microscopic and macroscopic surfaces containing staggered arrays of three-dimensional dimples (depth of 2 mm, diameter of 3 mm, and spaced at intervals of 10 mm), petal-shaped protrusions (depth of 850 μm, diameter of 980 μm, and spaced at intervals of 50 μm), and sandblast structure (diameter ranging from 5 to 10 μm and depth of approximately 1 μm). Experiments were carried out to examine boiling and condensation two-phase flow heat transfer in annulus outside the tubes, including smooth tube, sandblast tube (E1), dimple tube (E2), and sandblast/dimple composite surface tube (E3), all constructed from stainless steel with a tube diameter of 19.05 mm. Both heat transfer coefficient (HTC) and frictional pressure drop exhibited a positive correlation with mass fluxes. In condensation, HTC increased as vapor quality increased. Sandblasting of the dimple surface had a detrimental effect on HTC, with the HTC of the E3 tube falling between that of the E2 and E1 tubes. In flow boiling, sandblasting the dimple structure's surface further enhanced the vaporization core, resulting in the E3 tube displaying the highest HTC, benefiting from advantages of both sandblast and dimple structures. The effect of vapor quality on HTC was found to be minor at low mass fluxes, while at high mass fluxes, HTC increased initially and then decreased as vapor quality increased.