Flow structures and temperature fluctuations in a closed branch pipe connected to a high velocity and high temperature flow in a main pipe were investigated experimentally. A straight pipe and a pipe with a bend were tested for uniform and non-uniform temperature conditions. Visualization of the flow patterns, measurement of the flow velocity and measurement of the liquid and wall temperature distributions were conducted. The flow pattern near the stagnant point in the branch pipe was a spiral flow. The time averaged velocity distributions were linear and the structure of the spiral flow was a simple forced vortex with a vertical circulation. The spiral flow velocity was fluctuated with a long period of several tens seconds. It was shown that large liquid temperature fluctuations were initiated when the spiral flow penetrated into a thermal stratified layer in the bend. The liquid temperature fluctuations caused the wall temperature fluctuations and two types of the wall temperature fluctuations were observed.

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