Numerical characterization of heat transfer deterioration in supercritical natural circulation loop and role of loop inclination

Thermalhydraulics of any supercritical natural circulation loop (sNCL) is strongly influenced by the choice of operating conditions and geometric configurations, and can experience severe deterioration in heat transfer characteristics and circulation rate under certain situations. Present study focuses on systematic identification of a quantifiable criterion for prior identification of such deterioration in a carbon dioxide-driven rectangular loop, subjected to a wide range of operating conditions and also with particular emphasis on loop inclination. Four distinct regimes of operation have been identified, primarily based on the temperature levels in the adiabatic arms. Operation is desirable to be restricted within the enhanced heat transfer regime, which offers large heat transfer rate and moderate level of fluid temperature. Inclination to vertical lowers the driving potential, thereby reducing the flow rate and forcing an early induction into the deteriorated heat transfer regime. Highest circulation rate corresponds to the heater power capable of equating the average loop temperature to the pseudocritical value at that pressure. Strong thermal asymmetry and density stratification has been observed in the horizontal sections, with local recirculation and non-uniform radial velocity profiles. Buoyancy parameter has been identified as a capable dimensionless quantity to predict the appearance of deterioration a priori. It can also be coupled with the average friction factor to quantify the critical power level, and a correlation has been suggested for the same.