1-D modelling and CFD study of the transient behaviour of a single phase Coupled Natural Circulation Loop

A Coupled Natural Circulation Loop (CNCL) consists of two Natural Circulation Loops (NCL) coupled thermally via a common heat exchanger. The transient modelling of such systems that have practical relevance has not been reported in the literature to the best of the author's knowledge. The present work aims to bridge this gap and investigate the dynamic characteristics of a CNCL system using a 1-D mathematical model. The validation of the model is accomplished by comparison of the results obtained via 3-D CFD simulation. Both horizontal and vertical CNCL systems have been considered for this study and behaviour of the system for parallel and counter flow configurations in the heat exchanger section is elaborated. Transient and steady state CFD analysis has been conducted to analyse CNCL system for different heater and cooler orientations and flow initialisation. The behaviour of the CNCL system is then examined by carrying a thorough parametric study employing the validated 1-D single phase CNCL model with liquid sodium as the operating fluid. The CNCL orientation (vertical or horizontal) coupled with the heater and cooler configuration determines the system dynamics and behaviour. The CNCL system also exhibits chaotic flow oscillations at high heat loads.