Role of Buoyancy and Acceleration Parameters in the Heat Transfer Deterioration of Supercritical Carbon-Dioxide Under Natural Circulation

Supercritical fluid (SCFs) may have normal heat transfer (NHT), enhanced heat transfer (EHT) or deteriorated heat transfer (DHT) as they pass through the pseudo-critical region. It is the DHT which poses threat to safe operation of reactor as DHT results in higher surface temperature which may lead to the failure of the fuel clad and cause further detrimental complexities. Literature suggests the buoyancy and acceleration parameters as the main factors causing the DHT in SCFs. There have been many studies in the literature for forced convection systems for investigating DHT, still, there is lack of clarity for DHT criteria to be adapted for safety studies of the reactors. Nonetheless, such studies for natural circulation systems are rare. In the present paper, experiments have been conducted with supercritical carbon-dioxide (SC-CO2) to study the DHT under natural circulation (NC). Experimental results obtained for vertical flow of SC-CO2 at different pressure and cooler orientation have been reported, and the role of buoyancy and acceleration parameters in the DHT has been discussed for NC systems.