Investigation of Flow Structure to Prevent Thermal Fatigue in a Downward Branch Pipe With a Closed End

Abstract

Many pipes branch off from the main pipes in power plants. Main flow with high velocity initiates a cavity flow in a downward branch pipe with a closed end. Hot water penetrates into the branch pipe and a thermally stratified layer forms in the branch pipe if the main flow is hot. Fluctuations of the thermally stratified layer may initiate wall temperature fluctuations and thermal fatigue cracks in the branch pipe. Penetration depth of the main flow and the fluctuation characteristics into the branch pipe with a closed end were investigated by experiments. Experiments were conducted for various inner diameters of a branch pipe and main flow velocities under room temperature conditions. Flow structure was observed by test section made of acrylic resin. A tracer method was used to measure the penetration depth of the main flow. The penetration depth of the main flow changed periodically. The maximum penetration depth of the main flow was correlated by the Reynolds number. The fluctuation range and period of the penetration depth were also investigated. Next, the flow patterns on the cross-sectional plane in the branch pipe were observed to investigate the fluctuation mechanism of penetration depth. Three flow patterns were observed on the cross-sectional plane in the branch pipe. They were flow parallel to the cross-sectional direction, flow consisting of small vortexes and large swirl flow. The generation period of the large swirl flow was nearly equal to the fluctuation period of the penetration depth. The fluctuation range of the penetration depth and the duration showed similar trends for different inner diameters of the branch pipe. These results showed that the fluctuation of the penetration depth was caused by the periodic generation of the large swirl flow.