In practical engineering, a T-junction has large size and strict parameter conditions, so it is necessary to model the actual working conditions in order to conduct experimental research. A vertical T-junction model was established with the momentum ratio as the main similarity criterion and high temperature heat-conducting oil as the medium to ensure the thermal similarity between the prototype and the model in terms of temperature difference, velocity ratio and diameter ratio. Large Eddy Simulation was used to validate the prototype and the model, and the time distributions of temperature and velocity were compared. The results show that the flow field and temperature field of the prototype and model T-junction are similar, and it is therefore feasible to use heat transfer oil as the medium and the momentum ratio similarity to model the mixed cold and hot fluids in a T-junction.

Injecting gas into the natural circulation loop of a liquid lead bismuth eutectic (LBE) can effectively improve its natural circulation ability. In order to investigate the effect of varying gas injection volumes on the heat transfer characteristics of liquid LBE flowing in the circuit and the efficiency of gas lift, numerical simulations were carried out to investigate the effect of seven different gas injection rate conditions ranging from 0 to 750 NL/h on the velocity field, gas-phase volume fraction, friction pressure drop, heat transfer characteristics, and lifting efficiency of the circuit. The calculation results show that the liquid LBE mass flow rate increases from fast to slow and then decreases slightly with the increasing gas injection rate; the gas phase volume fraction in the rising pipe and the falling pipe increases with increasing gas injection rate; the total pressure drop decreases with increasing gas injection rate, and the rate of decrease slows down, and the pressure drop in the rising pipe decreases rapidly and then rises slightly; the change of the Nu number is affected by the mass flow rate of liquid LBE and the volume fraction of the gas phase; the Nu number increases with increasing Re number in the case of low gas injection rates, and decreases with increasing volume fraction of the gas phase in the case of high gas injection rates; the efficiency of the air-lift shows decreases with increasing injection rate, and the rate of decrease slows down gradually.

To investigate the effects of the diameter ratio on the mixing process and the thermal fluctuations of hot and cold fluids in a T-junction, an experimental setup was constructed that simultaneously measuring the temperatures of the outer wall surface, the near inner wall surface, and the fluid. Experiments were conducted for diameter ratios (D_m/D_b) of 1 and 2.5, corresponding to impinging jet and deflecting jet flow types, respectively, and the temperature data were normalized for analysis. The experimental results indicate that when the change in pipe diameter ratio alters the momentum ratio and thereby affects the flow type, impinging jets exhibit more pronounced thermal stratification and greater temperature fluctuations than deflecting jets. Power spectral density (PSD) analysis reveals that due to the thermal capacitance of the pipe wall, the main frequency of the wall energy shifts to a lower frequency than that of the fluid. Additionally, the PSD of both the pipe wall and the fluid in deflecting jets is generally lower than the corresponding values in impinging jets. Therefore, when the flow type in the T-junction is an impinging jet, reducing the branch diameter can effectively decrease thermal fluctuations in the pipeline, thereby preventing thermal fatigue failure.

In a liquid metal fast reactor, the mixing of coolant from the core outlet in the upper plenum induces temperature fluctuations in the adjacent wall temperature field, which can lead to thermal fatigue of structural components. This study uses computational fluid dynamics (CFD) software and employs large eddy simulation (LES) to numerically simulate temperature fluctuations in a parallel three-nozzle model. The accuracy of the simulation method is first validated against existing experimental data. A subsequent comparative analysis examines the frequency and amplitude of temperature fluctuations for three liquid metal coolants: lead-bismuth eutectic (LBE), liquid lead (Pb), and liquid sodium (Na). The results show that, in the parallel three-nozzle model, temperature fluctuations are more pronounced for lead-based coolants, increasing the risk of thermal fatigue in structural components.