## 3 Investigations Focusing on Flow Regime Characteristics
### Investigations on the Effect of Coolant Boiling Caused by Accumulated Debris
#### 3.4.2 Experimental Studies Using the Bottom-Heated Method  
Aimed at checking the applicability of the gas-injection approach in simulating coolant boiling due to the decay heat of debris, the experimental investigations using a bottom-heated approach to induce liquid boiling were carried out [71]. The experimental system is shown in Figure 16b. Compared to the experimental system shown in Figure 11, a heating tube with a U shape, which can supply heating power (\(P\)), was installed at the tank bottom. For the purpose of monitoring the transient temperatures in the pool, four thermocouples were installed at the axial positions of 15, 30, 45, and 60 cm. For each experiment, \(W_{\text{tank}}\) and \(V_{\text{p}}\) were fixed at 60 mm and 10 L, respectively; while \(d_{\text{n}}\), \(H_{\text{w}}\), and \(P\) varied within the ranges of 30-40 mm, 45-80 cm, and 0-2580 W, respectively. In the experiments, pre-heated particles at 383K comprising different materials (glass, Al\({}_{2}\)O\({}_{3}\), and ZrO\({}_{2}\)), diameters (0.125-6.0 mm), and shapes (sphere, irregular shape, and cylinder) were employed. In total, there were 72 sets of experiments conducted. Figure 18: Influence of gas flow rate on DBF behavior in cases of different flow regimes. (**a**) Regime I; (**b**) Regime II; and (**c**) Regime IV [70]. In light of the qualitative and quantitative experimental observations and similar to the gas-injection conditions, it was found that the steam bubbles produced from the tank bottom could generally play a role in enhancing the pool convection and weakening the particle inertia under boiling conditions. In addition, the aforementioned effects of different particle-related parameters (e.g., particle shape, density, and size) on the regime transitions were verified again under the bottom-heated boiling conditions. However, the influence of \(H_{\text{w}}\) on the characteristic angles for the particle beds was found to be inconsistent under boiling conditions in comparison to that under non-boiling conditions, which likely indicates two points concerning the mechanisms of DBF behavior in bubbling (including gas-injection and bottom-heated boiling) cases:  
1. For the non-bubbling cases, pool convection is triggered by the falling particle jet and, thus, is limited within the center area. Therefore, the particles are driven away inside a relatively small region (such as around the two apexes of the bed). While for bubbling cases, due to the rather uniform distributions of bubbles in the pool, the influence area of pool convection is deemed to be wider. 2. Although increasing the water depth was found to generally enhance pool convection under non-bubbling conditions, for boiling conditions at a constant heating power, due to the larger water mass and the potentially enhanced heat dissipation resulting from larger heat-transfer areas within the environment, a higher water depth might also lead to reduced bubbling rate, thereby reducing the overall intensity of pool convection to some extent [71]. To quantitatively verify the applicability of the gas-injection approach to simulate coolant boiling, the influence of the bubbling approach was studied in different flow regimes by estimating the effective bubbling rates in accordance with measured characteristic quantities (namely, the maximum height of particles suspended and measured angles). Figure 19 shows the effect of the bubbling method in Regime IV. It was confirmed that, regardless of the particle type, the values of the characteristic quantities under the bottom-heated boiling condition at \(P=2580\) W were comparable to those under the gas-injection one within a relatively restricted range of \(Q_{\text{g}}\) (10-20 L/min), revealing the probable existence of an equivalent quantity that can compare the two bubbling methods.