Abstract One of the passive safety systems employed in nuclear reactors is a large water pool positioned at an elevation, which acts as a heat sink to remove decay heat from the reactor core in the event of an accident using the natural circulation phenomena. A large mass of water at an elevated position has concerns regarding the safety of support structure, and therefore an alternative methodology where the heat sink is positioned just below the heater section is investigated in this study. Results of an experimental and numerical investigation for the thermal-hydraulic behavior of single-phase natural circulation in a rectangular loop of height 4300 mm and width 1900 mm with the heat sink located at an elevation just below the heater section is reported here. The heat sink was a finite one, a constant volume insulated tank filled with water, whereas the heat source was a constant heat flux type. The loop was constructed with circular pipe elements with a nominal inner diameter equal to 40 mm. The heater section was 1m in height and three input power values equal to 1.15 kW, 1.55 kW, and 2 kW were used in the study. The flow behavior in a single and twin parallel heater system with a single riser was studied. The loop fluid is initially in a single-phase and becomes two-phase as the heat sink temperature rises, but only the results for the loop fluid in the single-phase regime are presented in the current study. The measured mass flow rate, after an initial transient, settles down to values which slowly decreases with time. The average temperature of the heat sink is shown to rise continuously with increasing time, demonstrating the effectiveness of the heat sink in removing the energy input to the working fluid in the heater section. However, the heat sink removes only 60–70% of the input heat while the rest is either lost to the ambient or utilized to raise the enthalpy of the working fluid. The Nusselt numbers in the heater section are also presented which steadily rise after an initial transient. In addition, the values are much higher than those for the corresponding Reynolds number values in forced convection due to the influence of buoyancy-induced convection. The Nusselt number is well correlated by the expression Nu ‾ = 1.35 ( Ra * ‾ ) 0.3 for 2 × 10 5 ≤ Ra * ‾ ≤ 6.5 × 10 5 and 600 ≤ Re ≤ 100 0 . Numerical predictions for loop flow rate, temperature variation, and sink temperature using the RELAP5/MOD3.2 code for the experimental configuration of the current study are presented which compare well with the measurements the maximum deviation being less than 10%. Numerical results for pressurizing the working fluid and permitting boiling in the heat sink to drive the system towards steady-state operation are also presented.

中文翻译：

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矩形回路中的单相自然循环，在加热器部分下方有一个有限容量的散热器

摘要 核反应堆中采用的非能动安全系统之一是位于高处的大型水池，它充当散热器，在发生事故时利用自然循环现象从反应堆堆芯中去除衰变热。高处的大量水会影响支撑结构的安全性，因此本研究研究了一种替代方法，其中散热器位于加热器部分的正下方。此处报告了在高 4300 毫米、宽 1900 毫米的矩形回路中单相自然循环的热工水力行为的实验和数值研究结果，其中散热器位于加热器部分正下方的标高处。散热器是一个有限的，一个装满水的恒定体积的绝缘水箱，而热源是恒定热通量类型。该回路由标称内径等于 40 毫米的圆形管元件构成。加热器部分高 1m，研究中使用了三个输入功率值，分别为 1.15 kW、1.55 kW 和 2 kW。研究了具有单个立管的单和双并联加热器系统中的流动行为。回路流体最初为单相，随着散热器温度的升高变为两相，但目前的研究中只提供了单相状态的回路流体的结果。测得的质量流量在初始瞬变后稳定到随时间缓慢下降的值。显示散热器的平均温度随着时间的增加而不断上升，证明散热器在去除加热部分工作流体的能量输入方面的有效性。然而，散热器仅去除 60-70% 的输入热量，而其余的要么散失到环境中，要么用于提高工作流体的焓。加热器部分的 Nusselt 数也在初始瞬变后稳定上升。此外，由于浮力引起的对流的影响，这些值远高于强制对流中相应雷诺数的值。Nusselt 数与表达式 Nu ‾ = 1.35 ( Ra * ‾ ) 0.3 密切相关，对于 2 × 10 5 ≤ Ra * ‾ ≤ 6.5 × 10 5 和 600 ≤ Re ≤ 100 0 。使用 RELAP5/MOD3 对回路流速、温度变化和汇温度进行数值预测。给出了当前研究的实验配置的 2 代码，它们与测量结果相比很好，最大偏差小于 10%。还提供了对工作流体加压和允许在散热器中沸腾以驱动系统向稳态运行的数值结果。