# Enhancement of Subcooled Flow Boiling Heat Transfer with High Porosity Sintered Fiber Metal
## 3 Results and Discussion
### Effect of High-Porosity Sintered Fibrous Metal  
#### 3.2.1 Effect of Porous Thickness and Boiling Curve  
Figure 8 shows the boiling curve of the bare and porous surface at a subcooled temperature of 50 K and mass flux of 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). We found that was no significant difference between porous and bare surfaces in the region where forced convection heat transfer was dominant; however, around \(\Delta T_{\rm sat}\) = 20 K, where the bubble formation became intense and nucleate boiling heat transfer became dominant, the heat flux of the porous surface was improved in comparison to the bare surface at the same wall superheat degree. In addition, Figure 8 shows the heat flux at the same wall superheat, and the heat flux increased at higher porous thickness. The heat flux on porous surfaces with a thickness of 1 and 0.5 mm increased by 60% and 40%, respectively, compared to the bare surface at \(\Delta T_{\rm sat}\) = 40 K. For the same heat flux at \(q\) = 1000 kW\(\cdot\)m\({}^{-2}\), the wall superheat was reduced to around 5 and 10 K for the 0.5 and 1 mm porous thickness, respectively, in comparison with the bare surface.  
Figure 8: Boiling curve on all heating surfaces at the subcooled temperature \(\Delta T_{\rm sub}\) = 50 K and mass flux \(G\) = 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\).  
Figure 7: Comparison between the experimental and [PERSON]’s correlation.  
In the forced convection and transition regions, the effect of the porous body was minimal on the heat flux performance; however, in the nucleated boiling region, the porous body effect was evidently higher. The enhancement is mainly attributed to three effects: (I) the bubble dynamics, the bubble formation and departure rates increased, as shown in Figure 9a-c; (II) the cavitation sites increased due to the micro-pores formed by the random distribution of the copper fibers; (III) wettability promoted an easier water supply between the surface area and the bubbles.  
Figure 9 shows the subcooled flow boiling pattern of each heat transfer surface at a wall superheat of 40 K. As the thickness of the porous body increased, the amount and volume of the bubbles also increased. The bubbles were generated only on the surface of the heat transfer surface and quickly condensed on the case of bare surface 9a; however, the size of the bubbles increased and reached the middle of the channel at the 0.5 mm  
Figure 9: Visualization of the heating surfaces at the subcooled temperature \(\Delta T_{\text{sub}}\) = 50 K, mass flux \(G\) = 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) and \(\Delta T_{\text{sat}}\) = 40 K. (**a**) Visualization of the flow pattern on a bare surface at the subcooled temperature \(\Delta T_{\text{sub}}\) = 50 K, mass flux \(G\) = 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) and \(\Delta T_{\text{sat}}\) = 40 K; (**b**) visualization of the flow pattern on a 0.5 mm porous surface at the subcooled temperature \(\Delta T_{\text{sub}}\) = 50 K, mass flux \(G\) = 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) and \(\Delta T_{\text{sat}}\) = 40 K; (**c**) visualization of the flow pattern on a 1.0 mm porous surface at the subcooled temperature \(\Delta T_{\text{sub}}\) = 50 K, mass flux \(G\) = 300 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) and \(\Delta T_{\text{sat}}\) = 40 K.  
porous surface. In addition, bubbles reached the top of the channel on the 1 mm porous surface. This may be due to the increase in the number of cavities in the porous fibers, which results in nucleation boiling in more locations. We considered that the porous heat transfer surface showed high heat flux at the same wall heating degree, and the increase in the wall superheat was suppressed at the same heat flux.