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Micro-grooved surfaces to enhance flow boiling in a macro-channel

Vlachou Maria C., Efstathiou Charikleia, Antoniadis Aristomenis, Karapantsios Thodoris D.

Πλήρης Εγγραφή

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Περίληψη

The influence of the boiling surface morphology on subcooled flow boiling heat transfer is investigated. Flow boiling experiments are conducted in a macro-channel with water entering at 30 °C. The channel has an orthogonal cross-section (10x40 mm) with a short (length: 120 mm) one-sided heated wall. Experiments are performed at two flow directions, horizontal and vertical upward. The examined mass and heat fluxes range between 330–830 kg/m2s and 200–1000 kW/m2, respectively. Two copper boiling surfaces are manufactured by laser etching: one with micro-grooves parallel to the flow direction (surface #1) and one with micro-grooves perpendicular to the flow direction (surface #2). The grooves have the same width (420 μm) and depth (290 μm) but their length varies: 100 mm along the channel's length (surface #1) and 30 mm across the channel's width (surface #2). The presence of grooves yields ∼ 8% increase of heat exchange area in both surfaces. A smooth plain copper surface is employed as reference. Micro-grooves lead to boiling inception at lower wall superheats (−70% for horizontal and −30% for vertical channel inclination) and also enhance heat transfer coefficients (10–15% for horizontal and 5–7% for vertical channel inclination) compared to the smooth surface; this is for two reasons: (a) laser etching creates micro-scale-roughness inside the grooves, which provide more active bubble nucleation sites, and (b) the bottom of the grooves is hotter than the rest of the surface. As a result, many bubbles are generated inside the grooves, where they grow and coalescence with other bubbles at a greater extent than the rest of the boiling surface. The beneficial effect of the grooved surfaces is beyond the gain offered by the rise in surface area and it is seen mainly in the horizontal inclination, whereas it is less evident in the vertical inclination. This is comparable with the discrepancy observed between inclinations for the smooth boiling surface.

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