# Enhancement of Subcooled Flow Boiling Heat Transfer with High Porosity Sintered Fiber Metal
## 3 Results and Discussion
### Critical Heat Flux  
Due to the critical heat flux involving a phase change process, the analysis of this phenomenon is very complex and continues to be under study. However, researchers concluded that the CHF is directly related to the bubble formation on the solid surface, thus the importance of the surface modifications to enhance the CHF. [PERSON] (2018) reported that the CHF enhancement is directly related to the water-supply ability through porous media. On the bare surface, bubbles are generated, coalesce, and finally, cover the heating area interrupting the water supply by promoting an abrupt increase in the wall temperature. On the other hand, in the case of the porous media, the distribution of the small fibers promotes a continuous water flow through the heating surface area, and the flow turbulence on the porous media promotes bubble formation and departure rate; thus, the porous media keeps the heating surface at lower temperatures compared with the bare surface. The heating surface is higher for the porous surface than for the bare surface, and, due to this fact, the heat rate increases at the same wall temperature, and the film boiling presents a delay. The surface tension and the wickability effects keep the heating surface at lower temperatures for the porous surface. The effect mentioned above enhances the CHF in the porous surface. In the particular case of the subcooled temperature of \(\Delta T_{\text{sub}}\), 30 K and a mass flux of 600 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), the critical heat flux was reached on the bare surface, around \(\Delta T_{\text{sat}}\) 50 K, the wall superheat increased rapidly to \(\Delta T_{\text{sat}}\) 63 K, and film boiling was observed during the boiling phenomenon, as shown in Figure 12a.  
In addition, the wall temperature variations concerning the time are shown in Figure 12b. The variation on the bare surface wall temperature degree was between \(-0.4\) and \(0.4\) K. However, for the porous surface case, the variation was barely \(0.1\) K. The bare surface reached the CHF, and the film boiling was observed on the heated wall, as shown in Figure 13a. Contrarily, using the high-porosity sintered porous metal, the CHF was not reached and the wall superheat stayed below the \(\Delta T_{\text{sat}}\) 40 K. The flow patterns are shown in Figure 13b,c for the 0.5 and 1 mm porous surfaces, respectively. On the porous surface, the burn out did not occur due to the constant water supply prompted by the bubble formation and the bubble departure rate was higher on the porous surfaces.  
Figure 11: Enlarged view of the boiling curves for each surface at the subcooled temperature \(\Delta T_{\text{sub}}=30\) K, (**a**) Enlarged view of the boiling curve for each surface at the subcooled temperature \(\Delta T_{\text{sub}}=30\) K, mass flux \(G=150\) kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). (**b**) Enlarged view of the boiling curve for each surface at the subcooled temperature \(\Delta T_{\text{sub}}=30\) K, mass flux \(G=300\) kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\).  
Figure 12: The critical heat flux (CHF) or maximum heat flux. (**a**) Boiling curve at the subcooled temperature \(\Delta T_{\rm sub}\) = 30 K and mass flux \(G\) = 600 kg\(\cdot\)m\({}^{-2}\).s\({}^{-1}\). (**b**) Variation of each surface at the heat flux q = 2500 kW\(\cdot\)m\({}^{-2}\).  
Figure 13: Flow pattern of the different surfaces at the heat flux q = 2500 kW\(\cdot\)m\({}^{-2}\). (**a**) Visualization of the flow configuration on a bare surface at heat flux q = 2500 kW\(\cdot\)m\({}^{-2}\). (**b**) Visualization of the flow configuration on a 0.5 mm porous surface at heat flux q = 2500 kW\(\cdot\)m\({}^{-2}\). (**c**) Visualization of the flow configuration on a 1.0 mm porous surface at heat flux q = 2500 kW\(\cdot\)m\({}^{-2}\).