Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0 ± 0.1 °C, the superheating of 4.0 ± 0.1 °C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500 W/m2 K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500 W/m2 K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).