Numerical simulation of thermal stratification in an elbow branch pipe of a tee junction with and without leakage

Thermal stratification can cause thermal fatigue of the piping system of a nuclear power plant. One of the regions most at risk of suffering from thermal fatigue is a small elbow pipe branching off from the main pipe of the coolant loop for the drain or letdown system in the chemical and volume control system (CVCS). This work focuses on a fundamental description of the thermal stratification caused by turbulent penetration and buoyancy effects in the elbow branch pipe using large-eddy simulations (LESs). The LES results for the normalized temperature, mean temperature, and root-mean square (RMS) temperature were found to be in good agreement with the available experimental data which confirms that LES can predict the thermal stratification in a closed elbow branch pipe where cold fluids are stagnant. Subsequently, the flow and heat transfer were numerically predicted using LES when leakage occurred in the elbow branch pipe. The numerical results show that the thermal stratification region is pushed towards the horizontal part and may remain there for a long time for a leakage ratio of 1%. However, thermal stratification is quickly eliminated for a leakage ratio of 5%, although there is a higher power spectrum density (PSD) of the temperature in the early stages of the leakage. It may be concluded that a small leakage ratio can result in the elbow branch pipe being at high risk of thermal fatigue caused by thermal stratification.