Numerical simulation of the mixing behaviour of hot and cold fluids in the rectangular T-junction with/without an impeller

Abstract

• The thermal mixing features in a T-junction with/without an impeller are compared at M R = 0.49. • The mixing behaviour with the impellers are analyzed at different blade numbers and diameters. • The temperature uniformity parameter is defined to investigate the mixing uniformity in the T-junction. • The criterion of mixing length is proposed according to the mass mean temperature. Thermal stratification is the main reason for thermal fatigue failure in the rectangular T-junction. An impeller is set in the mixing zone in the rectangular T-junction to improve the mixing behaviour of hot and cold fluids under the deflecting jet with the inflow momentum ratio of M R = 0.49. Blade numbers and blade diameters are investigated for the range of N p = 2 ∼ 4 and D p * = 0.33 ∼ 0.8, respectively. By the application of large-eddy simulation, the flow fields and temperature fields are obtained in this work. The flow fields show the mixing behaviour between hot and cold fluids can be promoted by setting the impeller with a constant rotation speed, and three flow patterns including the wall jet, deflecting jet and impinging jet will be observed in the T-junction, which is effective for breaking the thermal stratification. Temperature fluctuation and temperature gradient calculated by mean temperature are compared with that of no impeller. It is found that by adding the impeller the temperature gradient is reduced while the temperature fluctuations almost keep the same. The criterion of mixing length is proposed according to the mass mean temperature and the mixing lengths for different conditions are obtained, as well as the temperature uniformity parameter is proposed to evaluate the mixing degree in the T-junction. The results indicate that the relationship between the mixing length and blade diameter is not monotonic and the optimal operating point is around D p * = 0.5 for the impeller of 2 blades. By combining the temperature fields and pressure drops, the results indicate among N p = 2 ∼ 4, the impeller of 3 blades is the best choice. Temperature power spectrum density is also investigated and it is possible to control the critical frequency of temperature fluctuation in the duct by changing the blade numbers.