Flow reversal in natural circulation systems

Boiling flow in natural circulation systems can be susceptible to thermohydrodynamic instabilities which can be the cause of mechanical damage. This paper presents an analysis of a circulation system where two unequal heated riser circuits are connected to a common downcomer. It can be shown that for such systems a critical heat absorption ratio exists which is dependent on the geometry and the overall heating conditions. Based on computer simulations, the static and dynamic behaviour of the riser systems with emphasis on the lower heat absorption circuit is discussed. Criteria to avoid flow reversal are presented.