## 3 Investigations Focusing on Flow Regime Characteristics
### Investigations on the Effect of Coolant Boiling Caused by Accumulated Debris  
#### 3.4.1 Experimental Studies Using the Gas-Injection Method  
Recognizing that the decay heat of accumulated debris may lead to sodium coolant boiling, which possibly significantly affects DBF behavior, the gas-injection approach was utilized by [PERSON] et al. [71] to simulate the boiling conditions to elucidate their effect. Figure 16a shows the experimental system, which includes additional gas-injection apparatuses in comparison with that shown in Figure 11. For each experiment, nitrogen gas with a controlled flow rate (\(Q_{\text{g}}\) = 0-50 L/min) was injected from the bottom of a liquid pool through an aistrone, which was installed to guarantee a relatively uniform distribution of gas. In order to concentrate on the influence of the injected gas (i.e., the sodium boiling), \(W_{\text{tank}}\), \(V_{\text{P}}\), \(d_{\text{n}}\), and \(H_{\text{w}}\) were set at 60 mm, 10 L, 30 mm, and 60 cm, respectively. In total, 78 experimental sets were carried out by using various types of particles with different shapes, densities, and sizes. Considering different flow regime mechanisms and characteristics for different regimes, the key parametric effect (i.e., the effect of the gas flow rate \(Q_{\text{g}}\)) on DBF was analyzed for each main regime (including Regime I, Regime II, and Regime IV) separately. Figure 17 shows the typical transit DBF behavior in gas-injection cases, while Figure 18 shows the influence of \(Q_{\text{g}}\) on DBF behavior for different flow regimes. In experiments for Regime I, as \(Q_{\text{g}}\) increased, despite no noticeable variations in flow regime or bed shapes (see Figure 17a), an evident increase in the maximum height of particles suspended (see the illustration of the height of particles suspended in the fourth photo of Figure 17a), as shown in Figure 18a, was found due to the larger impetus that drives the upward particle movements (esp. for smaller and lighter particles). Moreover, from Figure 18a, it is also observable that with a similar \(Q_{\text{g}}\), due to the difficulty for the upward gas-liquid flow to suspend the larger-sized (i.e., greater-inertia) particles, the peak value for the height of the particles suspended seemed to decrease when the particle size became larger. In addition, the difference in the height of particles suspended between the cases with 0.18 mm and 0.25 mm particle diameters was observed to be greatly reduced in comparison to that with 0.125 mm and 0.18 mm ones, indicating the non-negligible role of particle inertia in those cases (esp. under conditions without gas injection) and a probable flow regime transition with a further increase in particle size. In regards to Regime II, it was found that with a larger \(Q_{\mathrm{g}}\), a lower-numbered flow regime may emerge as a result of the greatly enhanced pool convection and the effect of larger-rate flows on diminishing the dominance of particle inertia (see Figures 12b and 17b). More quantitatively, as shown in Figure 18b, by measuring the center angle (illustrated in Figure 14a), a gradual increment of the center angle could be found as \(Q_{\mathrm{g}}\) increased, revealing the trends for Regime II transit to Regime I.  
Figure 16: Experimental systems for studies on sodium-boiling effects. (**a**) With gas injection; (**b**) with bottom heating; and (**c**) with the release of heated particles [70; 71; 72]. However, in the case of Regime IV, as displayed in Figure 17c, although the intensity of pool convection was obviously enhanced in the early DBF stage, as the particles gradually accumulated, the accumulated particle bed possessing enough height was able to mitigate the gas-flow confluences, and thus, it was found that flow-regime transitions did not occur under the range of parameters considered.