Thermal–hydraulic characteristics of a single-phase natural circulation loop with water and Al2O3 nanofluids

A natural circulation system operates on the basis of natural laws like gravity and buoyancy. Although natural circulation is a benign gift of nature for applications to several heat removal systems due to their simplicity in design, elimination of hazards related to pumps, better flow distribution, cost reduction, etc. however, the potential threat of flow instabilities still eludes for its wide applications. Although addition of local losses (orificing) may suppress instabilities, however, it is accompanied by significant flow reduction which is detrimental to the natural circulation heat removal capability. In this paper, we have demonstrated experimentally, with nanofluids, not only the flow instabilities are suppressed but also the natural circulation flow rate is enhanced. The increase in steady natural circulation flow rate due to addition of nanoparticles is found to be a function of its concentration in water. The flow instabilities are found to occur with water alone only during a sudden power addition from cold condition, step increase in power and step decrease in power (step back conditions). With a small concentration of Al2O3 nanofluids, these instabilities are found to be suppressed significantly.