Effect of geometric parameters on steady-state performance of single-phase NCL with heat loss to ambient

Present study aims at development of a theoretical model to simulate the steady state performance of a rectangular single-phase natural circulation loop and to investigate the role of different geometric parameters on the system behaviour. The system has been considered as a conjugate problem with interaction of the wall with loop fluid, cooling stream and ambient along different sections of the loop. Non-dimensional form of coupled conservation equations have been solved using 1-d numerical techniques. Predicted values exhibited good degree of agreement with corresponding experimental data from the literature. As NCL is a self-sustaining system and it is difficult to control any flow-related parameters from outside once the system is under operation, it is very much essential to analyze the role of different geometric parameters at design level itself. Detailed parametric variation has been attempted to find operating limits of geometric parameters. Consideration of heat loss to ambient from the loop wall has been found to have significant effect on suitability of system dimensions, particularly under cooler atmospheric conditions. A loop with shorter height and smaller diameter yields higher effectiveness, i.e., transfers larger proportion of input energy to the sink, but employing lower circulation rate due to reduced buoyancy. Longer heating section yields enhanced heat transfer from wall to fluid and has a stabilizing effect on the system. Wall materials with higher thermal conductivity have been found to be more effective to avoid large thermal gradients within the system.