## 2 Methodologies
### Experimental Validation
#### 2.3.3 Experimental Result  
Specimen-Wise Analysis  
Temperature sensors were pasted onto the surface of the steel tubes according to the experimental design. Then, silicone-heating film was coated onto the model's surface to facilitate the heat flux entry. The temperature distribution of Specim 1 is shown in Figure 12. During the heating stage, the specimen started to warm up rapidly from room temperature (approximately 14 \({}^{\circ}\)C). When the temperature reached 26 \({}^{\circ}\)C, it stabilized and slowly rose to the maximum temperature of 27 \({}^{\circ}\)C. After the heating stopped at 55 min, the specimen cooled quickly and dropped to below 22 \({}^{\circ}\)C at 68 min. The temperature distribution of Specim 2 is shown in Figure 13. points 1, 2 and 3 were located in the void area, whereas points 4 and 5 were located in the solid area. All points showed similar temperature trends. Affected by the height of the void, the temperature of point 1 was the highest and the temperature increase rate was the fastest. When the temperature reached 26.8 \({}^{\circ}\)C, the temperature increase rate decreased and slowly rose to the highest temperature of 28.1 \({}^{\circ}\)C. The temperature dropped \begin{table}
\begin{tabular}{c c c} \hline \hline
**Experiment Number** & **Specimen Number** & **Heating Direction** \\ \hline 1 & 1 & upper semicircle \\ 2 & 2 & upper semicircle \\ 3 & 3 & upper semicircle \\ 4 & 2 & lateral heating \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental procedure. Figure 12: Temperature of each point in Experiment 1.  
to approximately 23.3 \({}^{\circ}\)C in 15 min during the cooling stage. The temperature distribution of points 2 and 3 were approximately equal due to symmetry. Their temperature increase rates were slightly lower than Point 1 and started to slow down at the temperature of 26 \({}^{\circ}\)C. The maximum temperature was 27.4 \({}^{\circ}\)C. It then dropped to 22.7 \({}^{\circ}\)C in 15 min after heating. The temperature increase rates of points 4 and 5 were the lowest, and slowly rose to 26 \({}^{\circ}\)C after reaching the temperature of 24.5 \({}^{\circ}\)C. When heating was stopped, the temperature dropped to 21.6 \({}^{\circ}\)C.  
This figure shows that there was a significant difference in temperature between the solid and the void areas. The mean temperature difference was 1.5 \({}^{\circ}\)C. Because of the existence of the void, the heat absorbed by the upper surface transferred slowly to the void area. The temperature distribution of Specim 3 is shown in Figure 14. At the maximum height of the void, the temperature of point 1 was the highest. It rapidly rose up to 28 \({}^{\circ}\)C through heating and then slowly increased to 30 \({}^{\circ}\)C. Because of the void's wide range, the specimen accumulated a large amount of heat and Point 1 still maintained a high temperature for 6-7 min during the cooling stage. After 15 min, the temperature dropped to 26 \({}^{\circ}\)C. The temperature of points 6 and 7 were slightly lower than point 1, and the highest temperature was 29.2 \({}^{\circ}\)C. The position of minimum temperature was found in points 8 and 9. Heating up to 25.6 \({}^{\circ}\)C, the rate began to slow down and the maximum temperature could only reach 27.1 \({}^{\circ}\)C. The difference in temperature between the solid and the void areas was evident from this figure. The average temperature variation was more than 2 \({}^{\circ}\)C.  
Changing the heating direction, the temperature distribution of Specim 2 is shown in Figure 15. The temperature of point 1 was still the highest with the fastest rate of increase. The maximum temperature was 28 \({}^{\circ}\)C. The temperature dropped to 23 \({}^{\circ}\)C in 15 min after cooling. The temperature of points 2 and 4 located at the edge of the heating surface rose gently. The maximum temperature was 18.5 \({}^{\circ}\)C and 17.2 \({}^{\circ}\)C, respectively. Because of their low temperature, the points showed no significant cooling trend.