Capillary-assisted evaporation enhancement on finned tubes via hydrophilic ATH dip-coating for adsorption chillers

Capillary evaporators eliminate the need for pumps and spray devices in conventional falling-film systems by uniformly distributing refrigerant across heat exchanger surfaces through capillary action. This paper investigates the performance of a capillary evaporator in a condensation–evaporation cycle using a circular finned-tube heat exchanger. The capillary effect was induced by a hydrophilic aluminum trihydroxide (ATH) coating deposited via dip-coating . A coating solution of ATH particles was prepared on a 600 mm length tube with 93 circular fins. The effect of powder concentration on coating morphology and durability was studied, revealing that higher concentrations enhanced hydrophilicity but also induced surface cracking and particle detachment. An optimal concentration of 31 wt% was observed, balancing wettability and mechanical stability of coated surface under vacuum pressure and operational vibration. Experimental results demonstrated that, compared to bare finned tubes, coated heat exchangers achieved up to 1.45 times higher overall heat transfer coefficients under temperature variation and up to 2.13 times higher under flow rate variation, with external heat transfer coefficients enhanced by 5.29 and 4.34 times, respectively. A VOF–Lee CFD model, validated against the experimental results, was employed to reproduce capillary-driven evaporation on the coated finned tube. These findings indicate that ATH-based surface coatings offer a promising strategy to enhance the efficiency and compactness of adsorption refrigeration systems while reducing energy demand.