Enhancement of Solution Wetting and Heat Transfer on Horizontal-Tube, Falling-Film Evaporator Using Porous-Layer Coating

An experimental study using a porous-layer (wick) coating on a horizontal-tube, falling-film evaporator was conducted to investigate the solution wetting of evaporator tubes and its effects on the evaporator performance. The partial solution wetting and the local dry-out on the evaporator tubes are the commonly encountered problems for the horizontal-tube, falling-film evaporator. In this study, the porous-layer coating was used in an attempt to enhance the solution wetting on the tubes and thus the evaporator performance. An experimental setup was built to measure the solution wetting and the performance of the evaporators using micro-scale hatched tubes (baseline design) and porous-layer coated tubes. Distilled water was used as the solution and the heating fluids. The solution wetting and the falling-film flow modes were visually observed in the range of the solution Reynolds number from 6 to 61. The evaporator using the micro-scale hatched tubes had the partial solution wetting on the tubes in the range of the solution Reynolds number considered for the study. In contrast, the evaporator using the porous-layer coated tubes showed the complete solution wetting which was confirmed by a visual inspection. The evaporator performance was evaluated by a thermal resistance analysis for the ranges of the solution and the heating fluid Reynolds numbers from 18 to 154 and from 3070 to 7270 respectively. The evaporation thermal resistances for the micro-scale hatched and the porous-layer coated tubes were increased as the solution flow rates were increased. The evaporation heat transfer coefficients for the micro-scale hatched tubes were higher than those for the porous-layer coated tubes. However, the evaporation thermal resistances with the porous-layer coated tubes were lower than those with the micro-scale hatched tubes because the complete solution wetting was achieved using the porous-layer coating.