Influence of hydrophobic coatings on fouling mechanism of combined fouling in enhanced tubes

The rate and degree of fouling in internally helical enhanced tubes increase significantly with the enhancement of heat transfer. Surface modification technology with lower cost and simpler operation to mitigate the rapid deterioration of heat transfer performance due to fouling has aroused wide attention of researchers. However, surface modification technology has not been well integrated with fouling growth mechanism in existing research. In this study, three types of coatings were coated on the inner surface of enhanced tubes with the same structure to obtain different surface characteristics. Compared with the original surface, the hydrophobicity of coated surfaces was improved, and their surface energy decreased by 18.48 %–45.32 %. Moreover, a combined fouling test was performed at 0.9 m/s for more than 1200 h. The results of fouling thermal resistance showed that the anti-fouling effect of Coating B was the most significant, while Coating C promoted the fouling growth. The correlations between surface characteristics and two important parameters (including sticking probability and deposit bond strength) determining the fouling process were studied using the Kern-Seaton model. As indicated by the data, the sticking probability in the combined fouling ranged from 3.9841 × 10^-2 to 9.1679 × 10^-2, and the deposit bond strength was between 6.0720 × 10⁶N·s/m² and 7.1364 × 10⁶N·s/m². Furthermore, the sticking probability was positively correlated with the surface energy, while the deposit bond strength was found to be opposite. The above result suggested that for the enhanced tubes with lower surface energy, the foulants in the fluid will be more difficult to adhere to the heat transfer surface after coming into contact with the surface; but once adhering to the heat transfer surface, they will not be easily removed. Besides, with the increase in the surface energy, the asymptotic fouling resistance of the coated enhanced tubes also increased.