Experimental investigation of heating – cooling boiling hysteresis on CNT – Cu microporous surfaces: effects of CNT composition and sintering temperature

Boiling hysteresis behavior of CNT – Cu microporous surfaces was experimentally investigated during heating and cooling processes. Microporous coatings with different CNT compositions (0–0.5 wt.%) were fabricated by sintering at 700, 750, and 800°C. Boiling hysteresis was evaluated using heat flux – wall superheat curves obtained under increasing and decreasing heat flux conditions. While previous studies primarily focused on boiling heat transfer enhancement and critical heat flux performance, the present study quantitatively examines the path-dependent boiling behavior associated with nucleation cavity activation and deactivation during heating and cooling. To characterize the hysteresis behavior, average hysteresis intensity, hysteresis loop area, and hysteresis index were introduced as quantitative descriptors. The results revealed distinct differences between the heating and cooling branches for all test surfaces. The hysteresis parameters generally increased with increasing sintering temperature, whereas their dependence on CNT composition was non-monotonic. The maximum hysteresis index of 3.87 was obtained for the 0.2CNTs–99.8Cu-coated surface sintered at 800°C. For the surfaces sintered at 800°C, surface roughness decreased with increasing CNT content; however, the hysteresis behavior did not show a direct correlation with arithmetic roughness. These results suggest that boiling hysteresis is governed by vapor entrapment and activation/deactivation of nucleation cavities within the CNT – Cu microporous structure rather than by surface roughness alone. The proposed hysteresis metrics provide a practical framework for evaluating thermal-history-dependent boiling behavior on microporous boiling surfaces.