In a natural circulation loop, flow is induced by density difference of the fluid between the riser and the downcomer sections. There is no mechanically driving parts to induce the loopwise circulation, and the only source of the driving force is the heat input. There are many systems adopting the concept of two-phase natural circulation, such as thermosyphon reboilers, waste heat recovery systems, solar water heating systems, and geothermal power plants. In two-phase natural circulation, various types of flow instabilities may occur depending on the system geometry, thermal properties of the working fluid and the operating conditions. Understanding of the two-phase natural circulation behavior is also very important in analyzing the hypothetical loss of coolant accident (LOCA) in nuclear power plants.
In the present study, behavior of two-phase natural circulation in closed loops which are characterized by constant-loop volume was examined. One-dimensional homogeneous two-phase model was adopted for the analyses, and the time-averaged equation and the characteristic equation were obtained by using the perturbation method with linearization. Loopwise steady circulation rates were calculated, and conditions for flow oscillations were predicted. The parameters considered were the heat flux, coolant temperature and the overall heat transfer coefficient at the condenser, locations of the heater and the condenser, charged mass within the loop, area ratio of the cross-sectional area of the condenser section to that of the other sections and the flow restrictions in the liquid- and two-phase regions. Also the flow characteristics of the closed loop were compared with those of the semi-closed and the open loops. In addition, time variations of the circulation rate were examined using the finite difference method.
With the closed loop, the dynamically unstable region in the plane of the heat flux vs. the coolant temperature shows a convex shape. However, the excursive (static) instability region which used to occur with the open and the semi-closed loops doesn't appear. The stability of the closed loop system turned out to be highly sensitive to the charged mass within the loop. With increasing of the charged mass, unstable region first shrinks down to a minimum and then expands and other unstable regions also appear; this implies that there is a certain charged mass for the maximum stability. The system becomes unstable with the larger cross-sectional area of the condenser, and with the higher elevation of the condenser and/or the heater when the lengths of the heater and the condenser are fixed. The system is destabilized when the heat transfer coefficient at the condenser becomes larger. The system is stabilized with the increase of the flow restriction in the liquid-phase region, and with the decrease of the flow restriction at the two-phase region. Finally a set of the unsteady conservation equations were solved numerically for several cases to verify the appropriateness of adopting the linear perturbation method to analyze the flow instability within the two-phase natural circulation loops.
