Experimental study on flow characteristics of a closed natural circulation system

As an effective and reliable heat transport system, the closed natural circulation system is widely used in the energy sector. During startup, these systems may experience severe temperature and pressure fluctuations because the flow is affected by multiple interacting factors. In this paper, an experimental study was conducted in a closed natural-circulation loop to explore the influence patterns and mechanisms of different initial states on the system's startup and operating characteristics. The experimental results indicate that the system's startup process requires traversing a flow instability zone. Two primary types of flow instability exist in the system: boiling flow instability and compound flow instability. Parameters such as water inventory, power, and loop resistance have a significant impact on the system's flow instability. Appropriately increasing the water inventory and loop resistance, as well as reducing the initial power, can effectively mitigate flow instability during the startup process. The experiments reveal that when the water inventory is low, it is difficult to promptly establish an effective natural circulation to remove heat. Drastic power variations can induce flow oscillations and may even lead to the interruption of natural circulation flow. Additionally, the steam-discharge experiments further show that the system is more stable at higher pressure.