Experimental investigation on non-buoyant and buoyant flow mixing in a horizontally aligned T-junction at isothermal conditions

Abstract

Periodically occurring temperature fluctuations in nuclear power piping systems have been linked to cracking and coolant leakage accidents. T-junctions where hot and cold fluids mix are particularly vulnerable. To study thermal mixing flow phenomena, an isothermal operating test bench was built using a sugar respectively a salt solution to model the cold fluid in the T-junction configuration. The experiments aimed to predict the flow phenomena of mixed fluids with different fluid densities. Different types of investigations of non-buoyant and buoyant flow mixing were conducted using qualitative and quantitative measurement techniques, such as flow visualization, planar laser-induced fluorescence method, and wire mesh sensor measurements. Jet-formation and mixing behavior (up- and downstream) were characterized by the momentum ratio and a Richardson number. Additionally, non-buoyant flow conditions as well as buoyant flow with unstable/stable stratification have been summarized and clustered in non-dimensional flow maps. Results showed that wall- and deflecting-jet formations are a risk pattern for thermal fatigue. These were identified based on mean and fluctuating mixing scalar distribution as well as spectral analysis of the mixing scalar fluctuations. The peak frequencies in the non-buoyant case were linked to the axial motion of the Kármán-like vortices, while in the buoyant mixing case, the shear layer vortices resulting from Kelvin-Helmholtz instabilities were responsible.