Numerical investigation on similarity of isothermal and thermal flow mixing in a horizontal T-junction configuration

Abstract

Turbulent and stratified mixing flows can cause thermal fatigue in nuclear power plant piping systems. An isothermal Mixed-Fluid-Interaction (MFI) test rig is used to investigate mixing phenomena related to High Cycle Thermal Fatigue in nuclear power plant piping systems. Here, the cold heavy branch pipe fluid of the thermal mixing Fluid Structure-Interaction (FSI) facility is modeled by a sugar or salt solution. Due to limited optical accessibility of the FSI facility a numerical similarity comparison of the flow phenomena occurring in both experimental setups (MFI/FSI) is essential. Thus, Large-Eddy Simulations are carried out which are experimentally validated by applying the Wire-Mesh-Sensor measurement technique (MFI) and temperature measurements (FSI). Numerical LES investigation on similarity between two isothermal cases, three thermal mixing cases, and one geometrically similar scaled-up thermal mixing case is performed when the momentum ratio MR and the Richardson number Ri is kept constant. A very good agreement in the spatial distribution of the time averaged mean and rms values as well as statistical fluctuations is observed between the isothermal and thermal cases. Non-dimensional power spectrum density (PSD) of the mixing scalar fluctuation showed an almost unique profile, indicating the same origin for the occurring periodical fluctuation in all cases. The non-dimensional magnitude of the dominant frequency of the mixing scalar fluctuations in terms of the Strouhal number is in the range of 0.38<Sr<0.40. In the context of similarity, it is shown that the momentum ratio and the Richardson number are mainly responsible for the jet-formation and thus for the down- and upstream flow behaviour. The influence of the inlet Reynolds numbers Rem and Reb as well as the density ratio ΓR on the normalized flow characteristics seem to be negligible in comparison.