Heat-transfer characteristics of climbing film evaporation in a vertical tube

Heat-transfer characteristics of climbing film evaporation were experimentally investigated on a vertical climbing film evaporator heated by tube-outside hot water. The experimental setup was designed for determining the effect of the height of feed water inside a vertical tube and the range of temperature difference on local heat transfer coefficient inside a vertical tube (h(i)). In this setup, the height of feed water was successfully controlled and the polypropylene shell effectively impedes the heat loss to the ground. The results indicated that a reduction in the height of feed water contributed to a significant increase in h(i) if no dry patches around the wall of the heated tube appeared inside the tube. The height ratio of feed water R-h = 0.3 was proposed as the optimal one as dry patches destroyed the continuous climbing film when R-h is under 0.3. It was found that the minimum temperature difference driving climbing film evaporation is suggested as 5 degrees C due to a sharp reduction in h(i) for temperature difference below 5 degrees C. The experiment also showed that h(i) increased with an increase in temperature difference, which proved the superiority of climbing film evaporation in utilizing low-grade surplus heating source due to its wide range of driving temperature difference. The experimental results were compared with the previous literature and demonstrated a satisfactory agreement. (C) 2010 Elsevier Inc. All rights reserved.