## 3 Results and Discussion
### Heat Flux Regimes and Dominant Mechanisms  
To understand the boiling phenomenon, particularly where heat transfer surfaces recorded higher heat fluxes without reaching the traditional CHF, we discuss the boiling heat transfer performance of the tested surfaces by dividing the data into three distinct heat flux regions: the low heat flux region (\(\leq\)750 kW/m\({}^{2}\)), the medium heat flux region (>750 kW/m\({}^{2}\) and \(\leq\)2000 kW/m\({}^{2}\)), and the super-high heat flux region (>2000 kW/m\({}^{2}\)). These regions represent different boiling regimes characterized by evolving bubble dynamics and dominant heat transfer mechanisms. Two key factors influencing performance across these regions are surface wettability (affected by surface roughness and capillary forces) and bubble formation and departure dynamics. Essentially, the heat flux regions correspond to increasing vapor generation rates. * Low heat flux region (\(q\leq 750\) kW/m\({}^{2}\))  
In this region, discrete bubble formation and departure play a critical role. On both plain and porous surfaces, bubble formation occurs at isolated nucleation sites where individual bubbles grow before departing. For enhanced surfaces, the key factor is not intrinsic surface wettability but rather the promotion of frequent bubble departure and effective rewetting. The structured porous surfaces of the Large Lattice, Small Lattice, and Staggered promoted earlier bubble incipience and enhanced nucleation, leading to more frequent and improved bubble departure with minimum resistance and allowing for better surface rewetting. Here, the primary heat transfer mechanisms are evaporation at the bases of isolated bubbles and natural convection. The porous structures enhance heat transfer by increasing the density of active nucleation sites and maintaining good surface wettability through improved liquid access to the bubble base. At the onset of nucleate boiling, the Sintered Plain surface exhibited higher wall superheat than the Large Lattice, Small Lattice, and Staggered. For instance, the onset of nucleate boiling on the Large Lattice and Small Lattice occurred at very low wall superheats, indicating highly effective nucleation sites provided by the porous structures. This contrasts with the Sintered Plain surface, which required a higher wall superheat to initiate and sustain bubble growth. The porous structures served as vapor traps and acted as effective nucleation sites for the Large Lattice, Small Lattice, and Staggered. The Large Lattice and Small Lattice surfaces exhibited superior boiling heat transfer compared to the Sintered Plain and generally performed slightly better than the Staggered surface in the very low heat flux range. As the heat flux increased within this region, the Staggered surface achieved higher heat fluxes at lower wall superheat values than the other surfaces;  
Medium-to-high heat flux region (750 < \(q\)\(\leq\) 2000 kW/m\({}^{2}\))  
From approximately 750 kW/m\({}^{2}\) to 2000 kW/m\({}^{2}\), the boiling behavior transitions from isolated bubbles to more complex vapor structures, including vapor columns and mushroom-like formations, accompanied by increased bubble frequency and coalescence. Bubble growth accelerates significantly, making surface wettability and effective liquid replenishment critical to prevent dry spots. The interconnected porous structures of the Large Lattice, Small Lattice, and Staggered surfaces were particularly effective in promoting capillary-driven liquid replenishment to the heat transfer surface. This capillary action facilitated liquid flow through separate vapor-liquid pathways within the structure to maintain wetted areas. While the Large Lattice and Small Lattice structures benefit from their 3D spatial connectivity, the Staggered surface demonstrates the best performance from around 1000 kW/m\({}^{2}\).