Influence of secondary flow on the thermal hydraulic mixing characteristics in a T-junction area with a 90 degree bend upstream

Full text of publication follows: In a process of hot and cold fluid mixing, temperature fluctuation in the fluid can occur due to finite heat conduction. When this fluctuation becomes due to unstable fluid mixing process, the surrounding structure would be damaged by high-cycle thermal fatigue. This phenomenon is observed in various nuclear power plants, which means it important to investigate the relation between the fluid mixing process and thermal fatigue. When there exists a 90-degree bend in the upstream of the mixing area, the fluid mixing process becomes more complex due to the occurrence of a secondary flow. In this study, therefore, the flow field in the mixing area and temperature fluctuation near the wall are measured with a PIV measuring system and thin thermocouples, respectively to clarify the influence of the secondary flow on the non-isothermal fluid mixing in a T-junction with a 90-degree bend upstream. The parameters used in the experiment are a flow velocity ratio of the branch pipe flow to the main pipe flow, a aperture ratio, a curvature ratio, and a temperature difference between the branch pipe and main pipe flows. The flow velocities of the main and the branch pipes are 0.3∼0.9 m/s and 0.3∼1.2 m/s, respectively. In both of these flow conditions the flow becomes turbulent completely. The flow velocity ratios are 0.25∼3.00. The aperture ratios are 0.139, 0.194, and 0.287 in ascending order of the inner diameter of branch pipe. The curvature ratios of 90-degree bend are 1.41 and 1.00. The temperature difference of both flow is 40 degrees Celsius. The experimental result obtained for flow visualization in the longitudinal section of the T-junction suggests that a jet flow flowing out from the branch pipe moves unsteady, whose mixing patterns are basically categorized into 3 types. On the other hand, through measuring the flow field in the cross section of the T-junction, it is clarified that the secondary flow fluctuates unsteadily and has strong influence on the jet flow movement itself. And also the analysis of the velocity variation indicates that its prominent frequency almost corresponds to the frequency of the unsteady behavior of the secondary flow. Furthermore from the wall temperature measurement, it is clarified that the temperature fluctuation is also caused by the unsteady secondary. Based on these experimental results, the wall temperature fluctuation intensity in the T-junction with the 90-degree bend upstream is generalized by using the flow velocity ratio, the aperture ratio, and the curvature ratio for each mixing pattern. Furthermore, it is also evaluated that the existence of the 90- degree bend upstream strongly affects decay characteristics of the temperature fluctuation in the downstream direction. (authors)