Numerical investigations on dynamic characteristics of series coupled single-phase natural circulation loops

On the backdrop of the Fukushima accident, the importance of passive heat removal systems based on coupled natural circulation loops (NCLs) has gained more attention towards nuclear safety. However, the transient behavior of these coupled NCLs is quite complex due to their regenerative feedback effects. In this connection, a numerical model based on the finite volume method is developed to investigate the transient performance of series-coupled NCLs. In this study, a double pipe heat exchanger (HX) is employed, while most of the published works in literature considered only point contact and plate type HXs. The model developed is validated extensively against the benchmarked single and coupled NCL experimental facilities. The model has additional features such as wall conduction, equation of state (EOS) for properties, wall-fluid heat transfer coefficient correlations, free convection type heat exchanger, variable sections, different heater and cooler orientations, etc. The grid and time-step independence studies are carried out for the actual application of the model. The model is then employed to investigate the horizontally and vertically coupled NCLs. The effects of essential parameters such as power, pressure, aspect ratio, wall thickness, wall material, initial flow and temperatures, loop diameter, ambient heat loss and heater-cooler orientations etc., are investigated on the flow and thermal transients of coupled NCLs. Subsequently, a comparison of single and coupled NCLs is carried out. Finally, a sensitivity analysis is carried out by defining the Sensitivity Factor (SF), and the dominant parameters which influence the transients of coupled NCLs are highlighted. The findings of this study can be helpful for the design of coupled NCLs for conventional and nuclear applications for the purpose of safety.