Effects of Tube Row on Heat Transfer and Surface Wetting of Microscale Porous-Layer Coated, Horizontal-Tube, Falling-Film Evaporator

An experimental study was conducted to investigate the effects of tube row on surface wetting and heat transfer of a horizontal-tube, falling-film evaporator. Two types of the evaporator tubes were used for comparison: plain copper and copper coated with a microscale porous-layer. Distilled water was used as solution and heating fluids. A visual observation experiment performed in ambient with no heat input showed that when solution fluid was dripped onto the evaporator tubes from a solution dispenser, the plain tubes were partially wetted, while the porous-layer coated tubes were completely wetted due to capillary liquid spreading, even at low solution flow rates. It was found from the heat transfer experiment performed in a closed chamber under saturated conditions that the porous-layer coated tubes exhibited superior evaporation heat transfer (up to 100% overall improvement over the plain tubes at low solution flow rates) due to complete solution wetting and thin-film evaporation. It was also observed that the surface wetting and heat transfer are greatly influenced by both intertube flow mode of solution fluid and tube wall superheat. The effects of the tube row on the solution wetting and heat transfer were significant, especially for downstream tubes.