Experimental research on the two-phase turbulent mixing between rod bundle subchannels with spacer grid

The turbulent mixing between rod bundle subchannels is a crucial phenomenon affecting the calculation accuracy of thermal-hydraulic parameters in reactor cores. Currently, studies on two-phase turbulent mixing mainly focused on the effects of flow regimes, rod bundle arrangement and gap spacing, etc., but seldom addressed the influence of spacer grid. In this paper, experiment of the two-phase turbulent mixing characteristics between rod bundle subchannels with spacer grid was conducted with tracer and visualization method. In the low void fraction region, the effects of spacer grid in enhancing phasic turbulent mixing was confirmed, but the enhancement would tend to disappear for large void fraction. It was also found that the effects of spacer grid on gas phase turbulent mixing were larger than that of liquid phase. Studies showed that with the increase of void fraction in the region of 0-0.65, the dimensionless liquid phase turbulent mixing rate was almost constant but decreased slightly for void fraction greater than 0.5, while the dimensionless gas phase turbulent mixing rate would increase. Existing models were also assessed with the experimental data, and it was found that Bues model could generally predict the total turbulent mixing of two-phase flow well if proper theta max was provided, and Carlucci model could also predict the data of liquid phase turbulent mixing well, but may overestimate the data of gas phase turbulent mixing rate to a large extent.