Experimental investigation on the thermal mixing characteristics at a 90° T-Junction with varied temperature differences

Abstract

At the Fluid-Structure-Interaction (FSI) facility at the University of Stuttgart, thermal mixing experiments have been conducted to investigate thermal fatigue (a.k.a. thermal stripping) effects close to a weld connection in the piping material downstream of a 90° T-junction. For the mixing conditions close to the reality in nuclear power plants, have been performed with a cold flow of 20°C in the branch pipe (DN40) and a hot flow of max. 280°C in the main pipe (DN80) at 75bars. A geometric weld seam model is installed in the mixing zone downstream the T-junction to represent the rimmed root of a weld seam.The T-junction experiments in this work are performed with either temporally cyclical or continuous mixing processes. By a cyclical cold flow injection, an Artificially Induced TEmperature Change (AIPTEC) is created in the mixing region downstream of the T-junction. The experiments are conducted with different flow temperatures in the main pipe of 240°C, 200°C and 160°C, while the flow temperature in the branch pipe line was held constant at 20°C. Micro-thermocouples are installed in pipe modules in the mixing region downstream the T-junction and the flow inlet regions upstream of the T-junction to capture the temperature in the near-wall fluid with a sampling rate of 100Hz. The circumferential temperature distribution in the mixing region has been reconstructed from the near-wall temperature data. The thermocouple measurement data are processed by spectral analysis and presented as power-spectrum density (PSD) diagrams. The results of the analyses are summarized in the form of spectrum patterns. The spectrum patterns show information necessary for a characterization of the thermal mixing in relation to thermal fatigue. A pipe inner wall area with high potential for material damage due to thermal fatigue could be identified in the spectrum patterns. The results show that the change of the temperature difference has a significant influence on the stability of the thermal stratification in the mixing flow, as well as the phenomenon of reverse flow. It therefore has a significant influence on the thermal fatigue in downstream of the T-junction.