## 2 Model Setup and Validation
### Model Validation  
Validation exercises were performed to ensure the reliability of the results from the CFD simulations. The temperature fields in a test chamber were investigated by experimental and numerical simulation. Figure 4 gives the detailed arrangement of the test system.  
The experiment was carried out in an environment chamber with dimensions of 3.6 \(\times\) 3.0 \(\times\) 2.6 m (L \(\times\) W \(\times\) H), which is placed in a large laboratory, as shown in Figure 4a. The walls of the chamber were made of 0.01 m thickness colored steel plates which were hollow and filled with 0.07 m thickness insulated rock wool. The heat transfer coefficient of the envelopes was about 0.9 W/(m\({}^{2.\circ}\)C). The dimension of door was 0.7 m \(\times\) 2 m. The door was open, and the area of which was covered with plastic film. The fresh air ratio was regulated to 10%, and the condition of slightly positive pressure was maintained in the chamber during the experiment. Two boxes with dimensions of 0.5 m \(\times\) 0.3 m \(\times\) 0.28 m (see Figure 4b), containing mixture of dry sand and steel reinforcing rods were placed at the floor near one internal wall. Moreover, the mass ratio of the dry sand to the steel reinforcing rods was about 0.91, and the thermal conductivity of the mixture was 0.7 (W/(m\({}^{.\circ}\)C)).  
The air conditioning system was used to warm the chamber. The supply velocity and temperature of the warm air inlet is 1.62 m/s and 36.9 \({}^{\circ}\)C, respectively. The air inlet was placed at the height of 0.15 m with the dimension of 0.3 m \(\times\) 0.1 m. The exhaust device was mounted at the center of the ceiling with the dimension of 0.3 m \(\times\) 0.2 m.  
The temperature was measured by Type-K thermocouples with an accuracy of \(\pm\)0.1 \({}^{\circ}\)C. The temperature of each wall was measured at the central point by thermocouples. Because the walls were made of colored steel plates with uniform temperature distribution, the value of the points can represent the corresponding surface temperature. In addition, four thermocouples were placed uniformly in each mixture, the turn of which from external to internal side is measuring point 1, 2, 3 and 4. The distribution of four measuring points in the mixture is shown in Figure 4c. All the thermocouples were covered with aluminum foil tape to minimize radiation effects.  
Before the tests, the air temperature of the chamber and the outside laboratory had been kept at 10 \({}^{\circ}\)C for two hours by adequately ventilating. Then the air conditioning system was operated, and the air temperature of the outside laboratory was kept at 10 \(\pm\) 1 \({}^{\circ}\)C during the experiment.  
Moreover, the RNG \(k\)-\(\varepsilon\) model was verified by experimental data to simulate the thermal performance of heating room and the dynamic thermal behavior of thermal mass. Figure 5 gives the comparisons of temperature distribution of the mixture 1 between CFD simulations and the experimental data.  
Figure 5a-d indicates that the numerical predictions of the temperature distribution in Mixture 1 are in good agreement with the measured results. The temperature difference of Point 2 is the maximum of the four points, while which is less than 0.3 \({}^{\circ}\)C. Moreover, the temperature difference of the other three points is less than 0.1 \({}^{\circ}\)C. Hence, it can be concluded that the numerical simulation used in the present study is appropriate for simulating the temperature fields in the heating room.  
Figure 4: Schematic of the experimental system: (**a**) layout of the laboratory, (**b**) dry sand and steel reinforcing rods in the boxes, (**c**) measuring points in the mixture.