The previous experimental facility (Lin et al., 2021) was retrofitted to suit this research. As shown in Figure 1, the experimental facility mainly consists of six devices: high-pressure fan, heater, measuring system, containment, data acquisition system, and computer. The equipment and instruments used in the experiment have been calibrated. According to the calibration result, the measurement error and uncertainty are shown in Table 1. The height of the containment is 2.4 m, which has a platform with a height of 0.77 m at the bottom. The material of the containment shell is 304 stainless steel without coating. The containment consists of two accumulated tanks (ACC) and two core makeup tanks (CMT). The interval of each layer between A-1 and A-7 is 200 mm, and the interval of A-7 and A-8 is 1,000 mm. In addition, another temperature measurement point is set in the heater and near the jet. The size of the containment and the distribution of the measuring points are shown in Figure 2 (units = mm).

Air is used as the working medium to study the influence of different jet sizes and jet positions on temperature distribution. The specific experimental data are shown in Table 2. The jet position corresponding to the horizontal location of the jet at different angles is shown in Figure 3.

The experiment procedure is divided into 4 steps:

1) The initial temperature of the experiment facility is the room temperature of the laboratory, and the cooling condition is natural convection of air.

Figure 4 shows the temperature distribution with height at line 1 (L1) and line 5 (L5) under different jet positions. It can be seen from Figure 4 that the temperature distribution at L1 and L5 at case 2 and case 3 is similar, while the upper temperature of L1 at case 1 is higher than that of the other two cases because the injected hot air directly reaches the position of L5. Then, hot air went through the dome of the containment and downward along the L1. The reason why the bottom temperature (x = 0.0 m) of case 2 and case 3 at L5 is higher than that of x = 1.0 m is that the jet pipe penetrates through the containment from the bottom near L5 in the experimental facility. Although there is insulation outside the jet pipe, the temperature of the air near the jet pipe is a little higher. It can be seen from Figure 4B that the temperature of case 1 is generally higher than that of the other two cases, because in case 1, the jet flow is located near L5, and the temperature measuring point at the lower space of the jet is affected by the jet pipe heat dissipation, while x = 1.5 m is the highest because of the jet core region.

Figure 5 illustrates radial temperature distribution at H = 1.8 m and H = 2.2 m. The temperature of case 1 is generally higher than that of the other two working conditions when H = 1.8 m and H = 2.2 m. This is mainly determined by two factors: The first is that the jet of case 1 is located in a narrow space between the two tanks, which results in the jet in this case being a restricted jet flow, whereas it is a free-developing jet flow in case 2 and case 3. The second is that the measuring point is located in the core region of the jet in case 1, which leads to a peak temperature of x = 0.5 m.

The change of jet position is equivalent to the change of the relative position in the complex space of the containment, thus affecting the flow and heat transfer of the jet.

Figure 6 shows the temperature distribution with height at L1 and L5 under different jet sizes. In the upper space (H ≥ 1.8 m) of the containment, the temperature of case 4 is the lowest, and the temperatures of other cases are similar. The temperature distributions of the 3 cases are similar in the middle space (1.2 m < H < 1.8 m). In the lower space (H ≤ 1.2 m), the temperature of case 4 is the highest, and the temperatures of the other two cases are similar. This is because when the mass flow rate is the same, the larger the diameter of the jet is, the lower the jet velocity. Case 4, with a higher jet velocity, easily forms a large circulation in the containment, which results in a lower temperature of the upper space and a higher temperature of the lower space than other cases. In Figure 6B, only case 1 has a peak temperature. The possible reason for this is that the measuring point (x = 1.4 m) is located in the core region of the jet flow in case 1.

Figure 7 illustrates radial temperature distribution at H = 1.8 m and H = 2.2 m. When H = 1.8 m and x = 0.5 m, the temperature increases with the increase in the jet size. This is because it is close to the jet, and the increase in the jet size leads to a decrease in the velocity, so the temperature is high here. However, when H = 2.2 m, the peak temperature of case 1 is the highest. As the height increases, it needs more kinetics to transfer heat to this position. The smaller the size of the jet, the higher the velocity, and the stronger the heat dissipation around, but at the same time, the cone of the jet core region is also smaller, which leads to decreased heat dissipation, and the final temperature depends on which is the main factor.

When the jet size changes, the most important effect is that the jet velocity changes. A large loop is formed throughout the containment vessel because of the high velocity. It only circulates in the upper space of the containment vessel at a moderate velocity. However, when the jet velocity is lower, the jet’s kinetic energy is too small to form a circulation after it reaches the dome of the containment, and the main force is buoyancy.

Steady simulation using ANSYS CFX was performed based on the experimental conditions. The following experimental model (Figure 8) is established, and its mesh quality is shown in Figure 8. A jet size of 25 mm was taken as an example to verify the mesh independence. The total number of grids was selected as 1 million, 2 million, and 3 million, respectively. The temperature of the center in the containment dome (H = 2.0 m) was taken as a reference, and the errors were compared with 3 million grids. As shown in Tables 2, 3, million grids can meet the computing requirements.

In the numerical simulation, the outer wall temperature was set at 15°C. Taking the jet size = 25 mm and jet horizontal location 45° as examples, the experimental results of L1 were compared with the numerical simulation results. According to Figure 9, the trend of temperature variation between the experimental results and the numerical simulation results is similar, but the results of the simulation are generally lower than those of the experiment. The reason is that the simulations used constant wall temperatures, but the temperature of the containment wall increased during the experiment.

Figures 10–13 show the numerical simulation results of jet sections under different jet sizes and jet positions.

It can be seen from Figure 10 that the temperature fields of the three working conditions at the core region are similar, but there is some difference in the temperature field in the middle of the containment. The temperature in this area in Figure 10B is slightly lower than that in the other two working conditions, which indicates that obstacles have an influence on heat transfer and restrain heat transfer in the local area. In addition, the core region in Figure 10B is disturbed by obstacles and tends to tilt slightly toward the side with higher obstacles. Figure 11 shows the velocity vectors in the containment. The velocity distribution is similar between Figures 11A,C, and it is different in Figure 11B because of the obstacles (4 small tanks). For the length of the core region of the jet, Figure 11B is also the largest. This is mainly because the obstacles limited the jet diffused and reduced kinetic dissipation.

It can be seen from Figure 12 that the temperature difference between the upper and lower spaces of Figure 12A is the smallest, and the temperature difference between the upper and lower spaces of Figure 12C is the largest. Furthermore, the jet flow to the dome of the containment and transform to plume are shown in Figure 12C. In addition, both the areas of core regions of temperature in Figure 12A and Figure 12C are smaller than that of Figure 12B. This is because high velocity at the dome surface leads to strong heat exchange with the environment in Figure 12A. The core region of Figure 12C is small because the velocity is too small to transfer heat far in vertical direction. Obstacles (tanks) on both sides block the development of the jet core region. Moreover, the lower the velocity, the easier it is to swerve to the side of the obstacle because of the Coanda effect, which states that a fluid or gas stream will hug a convex contour when directed at a tangent to that surface. It can be seen from Figure 13 that the smaller the jet size, the larger the core region of velocity. Moreover, the larger the core region of velocity, the more intense the entrainment effect, which leads to a difference of the flow field in the dome of the containment.