Natural circulation loop (NCL) is mainly used as passive cooling system in nuclear power plants to ensure safety during pump failure and station blackout. Stable operation without flow instability is the most desirable operational mode of natural circulation loop for passive cooling. The present work deals with numerical analysis on the effect of inclination of horizontal leg of a rectangular loop working in single phase and presence of orifice on the thermohydraulic behaviour of NCL, using commercial CFD code ANSYS FLUENT 19.0. The effect of temporal variation of loop mass flow rate and temperature difference across the cooler section is compared for inclined loop with and without orifice. It is observed that the flow stability attained in the loop at inclination h = 308 can be achieved at h = 108 with orifice, with a reduction in the loop mass flow rate of 28.57%. The reduction in loop mass flow rate is accompanied by an increase in temperature difference across the cooler of 46.67%.
Many upcoming new generation reactors employ natural circulation for heat transfer in normal mode of operation. Natural circulation systems are simpler and safer than their forced circulation counterparts. However, these systems are prone to flow instability which are undesirable due to several reasons. In the present work, a rectangular glass loop, wherein cooler is just above the heater, has been considered for experimental and numerical investigation at atmospheric pressure. Heat addition from room conditions has been studied to understand the natural circulation loop dynamics, checking the possibility of occurrence of instability with the new orientation of the heater and the cooler. Experiments were performed at different power levels and coolant flow rates. CFD analyses were performed for all the cases investigated experimentally using the commercial CFD code ANSYS FLUENT 14.0. No instability was observed during the experiments and none during the simulations done for the duration of the experiments.
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