Experimental Study of Capillary-Assisted Solution Wetting And Heat Transfer Using A Micro-Scale, Porous-Layer Coating On Horizontal-Tube, Falling-Film Heat Exchanger
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The horizontal-tube falling-film heat exchangers have many different applications ranging from large refrigeration systems to desalination due to their high heat transfer coefficients with low liquid inventory. However, the problem of partial solution wetting severely deteriorates the heat transfer capability of the falling-film evaporators. Therefore, an experimental study was conducted to investigate the effects of a micro-scale porous-layer coating and tube row on solution fluid wetting and evaporation heat transfer of a horizontal-tube, falling-film heat exchanger. Micro-scale copper particles were directly bonded onto plain copper tubes by sintering to create a uniform, porous-layer coating on the tubes. Distilled water was used as solution and heating fluids. The visual observation performed in open ambient condition revealed that when the solution fluid was dripped onto the horizontal tubes from a solution dispenser, conventional plain tubes were partially wetted while the porous-layer coated tubes were completely wetted due to capillary action, even at very low solution flow rates. It was shown from the comparison of the evaporation heat transfer results of the plain and porous-layer coated tubes tested in a closed chamber under saturated conditions that the porous-layer coated tubes exhibited a superior evaporation heat transfer rate (around 100% overall improvement at low solution flow rates) due to the complete solution wetting and thin film formation. It was also observed that the evaporation heat transfer and surface wetting are greatly affected by both the flow mode of the solution fluid between the tubes and tube wall superheat. The effect of the tube row on the solution wetting and heat transfer was significant, especially for downstream tubes.