Suppressed Dry-out in Two-Phase Microchannels via Surface Structures

摘要

We demonstrated suppressed dry-out on structured surfaces during flow boiling in microchannels. We designed and fabricated microchannels with (Figure 1) well-defined silicon micropillar arrays (heights of ～25 μm, diameters of 10 μm and pitches of 40 μm) coated with silicon dioxide on the bottom heated channel wall. We visualized the flow fields inside a smooth and structured surface microchannel during the annular flow boiling regime (heat flux q" = 430 W/cm~2 at a microchannel mass flux G = 300 kg/m~2s) with a high speed camera at a frame rate of 2000 fps. Time-lapse images (Figure 2) revealed two distinct dry-out dynamics for the two types of surfaces. For the smooth surface, the thin liquid film broke-up into smaller liquid drops/islands and the surface stayed in a dry state after the drops evaporated. The micro structured surface, on the other hand, preserved the thin liquid film initially due to capillary wicking. Dry patches eventually formed at the center of the microchannel which indicated wicking in the transverse direction (from the sidewalls inward) in addition to wicking in the flow direction. Overall, the structured surface showed less instances of dry-out both spatially and temporally. These visualizations aid in the understanding of the stability of the thin liquid film in the annular flow boiling regime and provide insight into heat transfer enhancement mechanisms by leveraging surface structure design in microchannels.