Numerical investigation of turbulent thermal stratification at a horizontally oriented 90° pipe-elbow with varied elbow radiuses

Abstract

Thermal stratification is one of the major phenomena in thermal-mixing pipe-flow, when flow streams with a large temperature difference are mixed. A new phenomenon named as tangential oscillation of thermal stratification has been found in previous investigations, which can create a regular temperature change on the pipe wall and lead to material damage due to thermal fatigue. The flow behavior and conditions for initiation of tangential oscillation that promote temperature changes and fatigue damage are still unknown. This paper aims at analyzing the effect of elbow radius on thermal stratification initiation and flow behavior at horizontally oriented 90° pipe-elbows. The thermal stratification flow and heat transfer conditions are studied by computational fluid dynamics analyses with the Large-Eddy Simulation method. The results confirm that the tangential oscillation is initiated at the elbows, and its stability, which is quantified by the Richardson-number, decreases along the flow stream at the pipe-elbow. The phenomenon of turbulent mixing enhancement has been additionally found at the elbows by comparing the temperature fluctuations and vortex structures among the studied cases. Mixing enhancement takes the lead in the competition with tangential oscillation at the elbow with the smallest radius, and creates a higher temperature fluctuation downstream of the elbow. The temperature monitor data in the flow region is analyzed to evaluate the fatigue life of the structural material. It is shown that the pipe-elbow with the smallest radius of 0.8D has a fatigue-life time of 80 days. The tangential oscillation takes the lead with the elbow radius of 2D, and reduces the fatigue life to about 15% of that with the biggest radius of 3.2D.