抄録
Temperature fluctuation of thermal striping are considered in the hot and cold water mixing tees under plant piping systems, for example residual heat removal system of light water reactor (LWR). In order to establish the Japan Society of Mechanical Engineers (JSME) guidelines concerning high-cycle thermal fatigue of piping systems for thermal striping in the mixing tee, experimental studies of thermal striping with scale models of mixing tees with hot and cold water were planned. This paper reports test results of Type A of collision type mixing tee in which water flows from both sides of a main pipe to a branch pipe. As a first step, visualization test with acrylic pipes tests were carried out as in order to understand the flow pattern characteristics and fluid temperature fluctuation. As a second step, temperature fluctuation measurement tests with metal pipes were carried out to measure temperature fluctuations of fluid and pipe wall. Unsteady heat transfer coefficient was also evaluated based on these results. Parameters in the tests were the velocity ratio K, which is the ratio of the velocity in one side of the main pipe to that in the other side of main pipe, and main flow velocity. The ranges of the velocity ratio and the main flow velocity were determined based on the investigation of the piping design conditions of actual plants. The effect of the velocity ratio K was examined. From the visualization test results for K=0.2,large temperature fluctuation occurs near the junction. As the distance downstream from the tee junction increases, the mean temperature approaches the bulk temperature and temperature fluctuating decreases at downstream. From the visualization test results for K=1,boundary surface of hot and cold water is made in the center of the mixing tee. Large temperature fluctuations occurs at near the junction, but temperature fluctuations are comparatively large in comparison with those for K=0.2 at the downstream. The effect of the main flow velocity were examined for K=0.2. Non-dimensional mean temperature distribution and its fluctuation are almost equal for different main flow velocity, the effect of the main flow velocity is considered to be small. The effects of the upstream piping elements were examined. As the upstream piping element, a diffuser, a globe valve with full open condition or a short elbow was installed at twice the length of the main pipe diameter from the center of a tee. From the profiles of the mean and fluctuating temperatures for the diffuser, valve and elbow, temperature fluctuations are decreased in comparison with the base case without upstream piping elements. With the temperature fluctuation measurement of pipe wall, unsteady heat transfer coefficient was evaluated. Unsteady heat transfer coefficient is found to be larger near the junction in the base case K=0.2,the values are two to three times as that of calculated using empirical formula for steady turbulent flow in a pipe.
