## 4 Discussion  
### Heating Process to Dryout-Replenishment  
Figure 4 shows that the temperature distribution in the bronze powder porous media is very different from the temperature distribution in sand porous media. Moreover, the influence of the nature of the saturating liquid in sand porous media is very important.  
To clarify the causes of such differences, the value of the Rayleigh number and the Bond number were calculated for the temperature difference between the ends of the porous medium when the heating plate reaches the temperature of 100 \({}^{\circ}\)C. (Table 4): \(\Delta T\) = 70 \({}^{\circ}\)C for B+DS and B+W, and \(\Delta T\) = 35 \({}^{\circ}\)C in the S+DS and S+W). The uncertainty values of Rayleigh and Bond numbers shown in Table 4, as in the rest of tables, were obtained by propagation of error analysis according to [PERSON] and [PERSON] [30].  
The values obtained from the Rayleigh number for the porous media B+W and B+DS, are much lower than the limit for instability to appear, set at 27.10 according to [PERSON] and [PERSON] [31]. Nevertheless, the values for the sand-based porous media significantly exceed the limit of instability; consequently, the process of heat transfer in this medium is conduction driven by convection/advection, while in the former it is practically by conduction (Fourier). On the other hand, the value of the Bond number is less than 1 in all porous media; therefore, the flow is dominated by capillarity.  
A more detailed analysis allows us to elucidate that the process of heating the porous media consists of several stages with notable differences between them. For this purpose, the temperature ratios of the thermocouples T5/T6 and T4/T6 are depicted in Figure 6.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Porous Medium** & \(\Delta T\) (\({}^{\circ}\)C) & **Ra** & **Bo** \\ \hline B+W & 70 \(\pm\) 3 & 0.60 \(\pm\) 0.03 & \(1.67\times 10^{-5}\pm 3\times 10^{-7}\) \\ B+DS & 70 \(\pm\) 3 & 1.9 \(\pm\) 0.1 & \(3.06\times 10^{-5}\pm 6\times 10^{-7}\) \\ S+W & 35 \(\pm\) 2 & 115 \(\pm\) 6 & \(1.78\times 10^{-4}\pm 3\times 10^{-6}\) \\ S+DS & 35 \(\pm\) 2 & 826 \(\pm\) 46 & \(6.25\times 10^{-4}\pm 1\times 10^{-5}\) \\ \hline \end{tabular}
\end{table}
Table 4: Rayleigh and Bond number of each porous medium.  
Figure 5: Volume of liquid accumulated in the tray.  
By observing Figure 6, the process of heating to dryout-replenishment is characterized by several thermic stages. The peculiarities of each stage are as follows:  
* Stage A-B. Heating of the porous medium essentially by non-stationary conduction (B+DS), or by conduction/convection/non-stationary advection (S+DS) until the surface temperature of the heating plate reaches 100 \({}^{\circ}\)C.
* Stage B-C. Progressive increase of the temperature in the porous medium until boiling occurs at the plate-porous medium interface, keeping the plate temperature constant at approximately 102 \({}^{\circ}\)C.
* Stage C-D. Boiling extends throughout the porous medium, which is maintained at a constant temperature slightly higher than 100 \({}^{\circ}\)C until the beginning of the dryout. Subsequently, continuous rising of the temperature of the plate above 100 \({}^{\circ}\)C occurs while the porous medium keeps its temperature slightly higher than 100 \({}^{\circ}\)C.
* Stage D-E-F. For the sand-based porous media only, the relative evolution of the temperature porous medium-plate when the dryout-replenishment occurs is seen. From the beginning of the process to the end of the CD stage, displacement of liquid from the porous medium to the tray takes place. Liquid displacement results in the formation of a two-phase zone in a porous medium.