Experimental Study of Tube Row Effects on Evaporation Heat Transfer Using a Micro-Scale, Porous-Layer Coating on a Horizontal-Tube, Falling-Film Heat Exchanger

An experimental study was conducted to investigate the effects of tube row and a micro-scale porous-layer coating on solution fluid wetting and heat transfer of a horizontal-tube, falling-film heat exchanger using an inline tube arrangement. A uniform layer of micro-scale copper particles was directly bonded onto plain copper tubes by sintering to create a 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 was dripped onto horizontal tubes from a solution dispenser, the conventional plain tubes were always partially wetted while the porous-layer coated tubes were completely wetted due to capillary action, even at 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 70% overall improvement at low solution flow rates) due to the complete solution wetting and thin solution liquid film on the evaporator tubes. It was also observed that the heat transfer and surface wetting of the horizontal-tube, falling-film heat exchanger are greatly affected by both the flow mode of the solution fluid between the tubes and tube wall superheats. The effect of the tube row of the falling-film heat exchanger on the solution wetting and heat transfer was significant.