Abstract
An experimental study on countercurrent flow limitation (CCFL) in vertical pipes is carried out. Effects of upper tank geometry and water levels in the upper and lower tanks on CCFL characteristics are investigated for air-water two-phase flows at room temperature and atmospheric pressure. The following conclusions are obtained: (1) CCFL characteristics for different pipe diameters are well correlated using the Kutateladze number if the tank geometry and the water levels are the same; (2) CCFL occurs at the junction between the pipe and the upper tank both for the rectangular and cylindrical tanks, and CCFL with the cylindrical tank occurs not only at the junction but also inside the pipe at high gas flow rates and small pipe diameters; (3) the flow rate of water entering into the vertical pipe at the junction to the rectangular upper tank is lower than that to the cylindrical tank because of the presence of low frequency first-mode sloshing in the rectangular tank; (4) increases in the water level in the upper tank and in the air volume in the lower tank increase water penetration into the pipe, and therefore, they mitigate the flow limitation.
Highlights
During a PWR plant outage for maintenance and refueling, the reactor coolant is cooled by a residual heat removal (RHR) system
Flow visualization with the high-speed video camera showed that, under all the test conditions, the countercurrent flow limitation (CCFL) with the rectangular tank occurred only at the junction between the vertical pipe and the upper tank, that is, some water flows into the pipe from the upper tank, but the remaining water does not and returns to the reservoir, and the water entering the pipe forms liquid film and flows down to the lower tank without flooding in the pipe
The CCFL occurs only at the junction at low JG∗, whereas at high JG∗ and D ≤ 45 mm, some water penetrating into the vertical pipe intermittently flowed back into the upper tank due to flooding, that is, the CCFL occurs at the junction and inside the pipe
Summary
During a PWR plant outage for maintenance and refueling, the reactor coolant is cooled by a residual heat removal (RHR) system. The reactor coolant level is kept around the primary loop center in order to carry out operations like aeration, attachment, or detachment of the steam generator (SG) nozzle dam. This operation mode is called mid-loop operation. Steam generated in the reactor core and water condensed in a pressurizer due to heat transfer to its vessel wall may form a countercurrent flow in a surge line connecting the hot leg and the pressurizer. Understanding the characteristics of countercurrent flow limitation (CCFL) in the surge line is of great importance for safety evaluation of the mid-loop operation
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