Steady state flow and static instability of supercritical natural circulation loops

Abstract

For thermodynamically supercritical loops, explicit correlation for steady state natural circulation flow is not available. While using the subcritical natural circulation flow correlation for supercritical data, it was not able to predict the steady state flow accurately near supercritical pressure condition. A generalized correlation has been proposed to estimate the steady state flow in supercritical natural circulation loops based on a relationship between dimensionless density and dimensionless enthalpy reported in literature. Experiments have been performed with supercritical CO2 and water to validate this generalized correlation. The steady state flow rate data with supercritical CO2 has been found to be in good agreement with the proposed correlation. The correlation has also been validated using limited number of supercritical water data. Subsequently supercritical natural circulation data for different fluids reported in literature has also been compared with the proposed correlation. It is observed that the same generalized correlation is applicable for other fluids also. Sharp change of fluid properties such as density in the critical region gives rise to instability. The instability could be either density wave type or excursive type (Ledinegg or static instability). Several previous researchers have studied density wave type instability in supercritical natural circulation loops, whereas excursive instability is not studied in detail. In the present paper, an analysis has been carried out to predict the threshold of excursive instability for both supercritical water and supercritical CO2. Static instability was not found for CO2 whereas it was found for supercritical water. The effect of pressure is observed to stabilize the loop.