# Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber
## 3 Results  
### Boiling Curves under Different Flow Rates and Inlet Subcooling Temperature  
Figure 4a-c, show the boiling curve under an inlet subcooling temperature \(\Delta T_{sub}=30\) K, for all the five different porous thicknesses, at 200, 400, and 600 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), respectively. As expected, the onset nucleate boiling depends on the mass flux and subcooling inlet temperature. For the particular case of the 0 mm at 400 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), the critical heat flux was reached as the increment on the wettability hinders the active nucleation sites and diminishes the vapor trapping ability. In addition, the tendency of the porous effect was visible. The 0.2, 0.5, and 1 mm porous thickness presented an enhancement concerning the 0 mm surface, however, the 2 mm porous thickness performance was lower than the 0 mm. This effect was attributed to the increment in the number of vapor scene channels. The vapor tends to combine and form a big vapor bubble and it hinders the water supply to the heating surface. In Figure 5, convective boiling curves for \(\Delta T_{sub}=50\) K, and 200, 400, and 600 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)are shown. A decrease in the wall superheat was appreciated in comparison with the 0 mm, and the critical heat flux was not reached at low mass fluxes, whereas for the 600 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)the critical heat flux on the 2 mm porous thickness was reached at a wall superheat around \(\Delta T_{sat}=70\) K. For all the three mass fluxes, the 0.5 mm presented the highest heat flux at the same wall temperature. The reduction in the wall superheat was attributed to the increment in the nucleation sites, the number of vapor scene channels, and the wettability. The 2 mm tendency was the same as inlet subcooling temperature \(\Delta T_{sub}=30\) K, with the difference that a higher inlet subcooling temperature and higher mass flux, the critical heat flux was reached on the 2 mm porous body. Finally, the heat fluxes at inlet subcooling temperature \(\Delta T_{sub}=70\) K were presented in Figure 6. At high inlet subcooling temperature, the critical heat flux was easily reached on the high thickness body porosity. For the case of the 2 mm thickness, and independently to the mass flux, the critical heat flux was reached. The wall superheat temperature at the critical heat flux was lower at higher mass flux. In Figure 6c, an abrupt increment on the wall superheat was appreciated on the higher porous thickness due to the formation of a vapor blanket inside the porous body.  
In general terms, the porous body also increased the heating surface area, the turbulence due to the irregularities on the body, and the thermal resistance was also affected. These three factors need to be considered in the flow boiling process. In addition, the higher thickness porous body increment the number of vapor scene channels, and this effect diminished the water supply to the heating surface. In addition, the bubbles nucleated, departed, and coalesced inside the porous body creating a vapor blanket, thus, the critical heat flux and the vapor film regimen were reached. A lower thickness porous body increased the nucleation sites. Thus the bubble rate formation and departure also increased, meanwhile the working fluid kept the heating surface at a lower temperature.  
At low mass fluxes and before the ONB (\(\Delta T_{sat}<20\) K), the effect of the porous body on the heat transfer was almost marginal. On the other hand, a higher inlet sub-cooling temperature affects the onset nucleate boiling, and it was shifted to a lower wall temperature.  
Figure 4: Maximum heat flux for all the porous thickness, inlet subcooling temperature \(\Delta T_{sub}=30\) K: (a) \(G=200\) kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), (b) \(G=400\) kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), (c) \(G=600\) kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\).