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 degrees 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 degrees C. For the surfaces sintered at 800 degrees 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.
This study investigates thermo-hydraulic and second-law performance of ethylene glycol - water (EG/W) mixtures in a heat exchanger tube with conical wire inserts. Experiments were performed under turbulent conditions for Re = 3300- 21,000 using three EG/W ratios and five insert spacings. Nusselt number, friction factor, THPF, entropy generation number, Bejan number, and second-law efficiency were evaluated. Increasing EG content enhanced Nusselt number but increased frictional irreversibility. Maximum entropy generation (0.9221) occurred for CW-0 with 40:60 EG/W at Re = 21,000, whereas maximum second-law efficiency (0.428) was obtained for CW-0 with water at Re = 3300.
