Fundamental evaluation of the effect of pipe diameter, loop length and local losses on steady-state single-phase natural circulation in square loops using the 1D network code Flownex

Abstract

In this study, steady-state single-phase natural circulation of water in square loops is simulated using a 1D network code. Fundamental explanations are given for the behaviour of the buoyancy force and mass flow rate. The methodology and model are validated by the very good agreement with the generalized correlation of the predicted steady-state Reynolds numbers, as a function of the modified Grashof numbers and loop geometry numbers. For a given pipe diameter and loop length, it is shown that the mass flow rate and buoyancy force increases when the power increases. However, due to the effect of the increase in the mass flow rate, the actual increase in the temperature rise over the heater, which is responsible for the buoyancy force, is only 66% of that associated with the increase in the power. Increasing the pipe diameter increases the mass flow rate significantly and decreases the buoyancy force decreases markedly, due to the significant decrease in the frictional resistance. For a given power and pipe diameter, when no local losses are assumed, the mass flow rate is independent of the loop length. When local losses are accounted for, the mass flow rate, however, increases with increasing loop length until it reaches an asymptotic value equal to the mass flow rate for no local losses. For a given power, pipe diameter and loop length, the mass flow rate increases when the inlet temperature increases, due the non-linear nature of the density of the water as a function of temperature.