Steady state and stability characteristics of single-phase natural circulation in a rectangular loop with different heater and cooler orientations

Effect of the heater and cooler orientations on the single-phase natural circulation was studied in a rectangular loop. From steady state considerations, the maximum flow for a specified operating condition is achievable for the orientation with both the heater and the cooler horizontal. However, this orientation is found to be least stable and the orientation with both heater and cooler vertical is found to be most stable. Three oscillatory modes were observed in the experiments, two of which are observable only for certain heat addition paths. A hysteresis (conditionally stable) regime wherein stable or unstable flow can prevail depending on the operating procedure is also observed. Generalized steady state equation valid for all the orientations is possible whereas generalized stability equation is possible for a given orientation and flow direction. Steady state flow rate can be predicted with reasonable accuracy using the generalized equation. The stability map predicted by the generalized equation is found to be highly conservative in that its unstable zone is much larger than that observed in the experiments. Both the linear and the nonlinear stability analyses gave the same stability map for identical initial conditions. With nonlinear analysis, it was possible to reproduce the observed unstable oscillatory modes albeit at significantly different power levels. Also, the predicted limit cycles were significantly different in shape which is attributed to the multidimensional nature of the flow especially during the low flow part of the oscillation cycle.