Flow characteristics during loss of coolant to the heat exchanger (HX) via the secondary loop cold leg of a natural circulation (NC) test facility referred here as loss of heat sink transient (LOHS) in a closed rectangular NC system having a single heated channel at two different heating powers and system pressures has been investigated experimentally. Two LOHS experiments (LOHS-1 and LOHS-2) were conducted in a well dimensioned closed loop NC test facility designed to represent the primary system of a nuclear reactor. Operating conditions were set and maintained at 6.0 kW and 9.0 kW with system pressures of 0.2 MPa and 0.5 MPa respectively for both single-phase and single-to-two-phase flow transition to determine their thermal hydraulic responses at these predetermined parameters. The transients were initiated by closing the cold leg of the secondary loop supplying coolant to the tube-in-shell heat exchanger while NC flow phenomena were monitored at each vital section along the flow map based on the pre-determined inlet operating conditions. LOHS experiment stops when critical heat flux (CHF) occurs or heat exchanger outlet (HX_outlet) temperature rises too high to approximately equal its inlet temperature. Results were acquired using a system-design platform and development environment LabVIEW, programmed in a dedicated computer that was interfaced with the coupled sensors in the measuring devices. Common computational tools which include MATLAB, Ms Excel and Origin software were used for the analysis of the acquired data. Results at the inlet and outlet of heated sections as well as the inlet and outlet of condensers of both tests were analyzed and discussed in details at various flow regimes with the associated phenomena. It is concluded that while stable flow was maintained before transient with slug flow occurring in the heating channel, the system experienced transient stability resulting in steady temperature increase and subsequently drops into an instability region when the transient scenario was activated. In addition, the NC system transited into stable two-phase flow and further loses stability when HX_outlet temperature surges to its inlet value with CHF occurrence. These results are presented to contribute to a more extensive utilization of NC systems and understanding of flow behavior during LOHS in advanced reactors.

冷却剂通过自然循环 (NC) 测试设施的二次回路冷段损失到热交换器 (HX) 期间的流动特性，此处称为具有单个加热通道的封闭矩形 NC 系统中的散热片瞬态损失 (LOHS)在两种不同的加热功率和系统压力下进行了实验研究。两个 LOHS 实验（LOHS - 1 和 LOHS -2) 在设计用于代表核反应堆主系统的尺寸合适的闭环 NC 测试设施中进行。对于单相和单相到两相流过渡，将操作条件设置并保持在 6.0 kW 和 9.0 kW，系统压力分别为 0.2 MPa 和 0.5 MPa，以确定它们在这些预定参数下的热力水力响应。通过关闭向管壳式换热器提供冷却剂的二次回路的冷段来启动瞬态，同时根据预先确定的入口操作条件在沿流图的每个重要部分监测 NC 流动现象。发生临界热通量 (CHF) 或换热器出口（HX_出口）时，LOHS 实验停止) 温度升得太高，几乎等于其入口温度。结果是使用系统设计平台和开发环境 LabVIEW 获得的，在专用计算机中编程，该计算机与测量设备中的耦合传感器接口。常用的计算工具包括MATLAB、Ms Excel 和Origin 软件用于分析所获得的数据。分析和讨论了两种测试的加热段入口和出口处以及冷凝器入口和出口处的结果，并在各种流态下以及相关现象进行了详细讨论。得出的结论是，虽然在瞬态之前保持稳定的流动，但在加热通道中出现段塞流，系统经历了瞬态稳定性，导致温度稳定升高，随后在激活瞬态场景时下降到不稳定区域。此外，NC 系统过渡到稳定的两相流，并在 HX 时进一步失去稳定性。随着 CHF 的出现，_出口温度飙升至其入口值。提出这些结果有助于更广泛地利用 NC 系统和了解先进反应堆中 LOHS 期间的流动行为。