Advanced flow pattern for describing tangential flow oscillation in thermal-mixing pipe flow at a horizontal T-Junction

Abstract

T-Junction experiments have been performed on the Fluid-Structure-Interaction (FSI) test facility at the University of Stuttgart to investigate the thermal fatigue effect in thermal mixing processes. The previous experimental works have been performed with different working conditions to discuss the influence of flow parameters (e.g. flow rate, flow temperature) on the mixing characteristics. The experiments have been performed with continuous and periodical mixing processes. 48 high-performance micro-thermocouples are installed in six measurement cross-sections close to T-junction to capture the near-wall fluid temperature. The temperature measurement data are analyzed and discussed with the viewpoint of their relevance for thermal fatigue. An unexpected mixing phenomenon, which is defined as the additional low frequencies, has been detected in the results of frequency analysis on the temperature data. Such kind of frequency with the value about 2.66 Hz can be found in most of our experiments. This similarity of the additional low frequency from the experimental work is summarized in this paper and compared with each other. With a small-scale experiment, this phenomenon is identified as harmonic oscillation of thermal stratification in pipe tangential direction, which has never been found in any of the previous T-junction investigations, nor in any literature on fluid mechanics. According to this phenomenon, novel flow patterns are introduced to categorize the mixing characteristics at horizontal T-junction. Among the patterns, the deflecting jet has the lowest possibility to initiate thermal fatigue in the mixing process at horizontal T-junction. With the results of the experimental and numerical investigations, the flow patterns of the thermal mixing processes at the FSI test facility can be summarized in the mixing envelope diagram, which presents the thermal mixing characteristics as the property of the specific horizontal mixing tee. Furthermore, once the mixing envelope has been drawn for a specific mixing tee, it can be used as operation guideline to reduce or avoid thermal fatigue close to the mixing tee.