Three-dimensional mixed boiling: A strategy for critical heat flux enhancement

To improve the critical heat flux (CHF) in boiling heat transfer, an idea of triggering three-dimensional mixed boiling was proposed, and the corresponding surface with an extended thin film (ETFS) was designed by adding a film structure on the plain surface (PS). With ethanol as working fluid, experiments were conducted to explore mixed boiling heat transfer and CHF enhancement mechanisms on ETFS. The results demonstrate that ETFS induces spatially uneven temperatures on the boiling surface, which triggered mixing boiling in the spatial dimension—the coexistence of nucleate and film boiling—leading to a significant CHF enhancement. Nucleate boiling in the falling liquid along the extended surface is the key to stabilizing the surface temperature during the formation of dry spots on the base surface. Compared with PS and the traditional pin–fin surface, ETFS increased CHF by approximately 110% and 45%, respectively. Analysis indicated that mutual interference between nucleate boiling and film boiling effectively inhibits heat transfer deterioration caused by the deactivation of nucleating points. Through decoupling analysis of the mutual interference process in mixed boiling, it is shown that at heights of 2.0, 4.0, 6.0, and 8.0 mm, the contributions of extended film to total heat transfer are 12%, 64%, 37%, and 24%, respectively, indicating an optimal extended film height of 4.0 mm for maximum boiling heat transfer performance. This novel boiling surface with extended film structure not only improves CHF, but also reduces design costs, offering valuable guidance for radiator design purposes.