Pilot numerical study of new design of passive residual heat removal heat exchanger tubes

A passive residual heat removal heat exchanger (PRHR HX) is an important component of the passive safety system in an advanced nuclear power plant. In the current design of PRHR HXs with C-shaped tubes, thermal stratification occurs, and it inhibits heat transfer process. Here, we propose replacing these tubes with spiral tubes not only to enhance the heat transfer performance, but also increase cooling capacity. In this pilot study, CFD was used to compute the single-phase natural convection of both designs for comparison and analysis. The computation of a PRHR HX with C-shape tubes is validated with experimental data, and the use of Boussinesq approximation is justified. The computational results of simplified models suggest that with the same inlet and outlet temperatures (difference <0.001%), the new design with a spiral-shape tube not only alleviates the thermal stratification by 10.57% up to 600 s but also improves heat transfer by 75.5 W/m(2).K. In addition, the proposed design has a smaller pressure drop by 7.19%, and the volume of the proposed design is smaller by 22.0%. It suggests that the proposed design not only saves material of tubes, but also results in more water in the water tank to cool the reactor core in emergencies, increasing cooling capacity of a PRHR HX. Finally, sensitivity analysis suggests that the influence of inlet velocity oscillation on both designs was similar regarding the magnitude of pressure drop, the average heat transfer coefficient, and the outlet temperature. The results obtained may be used to inspire new design for PRHR HX tubes.