In the present experimental study, the flow behavior in an open two-phase natural circulation loop was examined. A test loop consisting of a visual heater and riser sections and a condenser section was constructed. Freon-113 was used as the test fluid. Heat flux, inlet subcooling, inlet- and exit-restrictions, liquid charging level and the cross-sectional area of the heater section were taken as the variables. Primarily, the circulation modes and the two-phase flow patterns in the riser section were observed. And the circulation rate(heater inlet velocity), void fraction inside the riser section, and the amplitude and period of flow oscillations were measured.
Four types of circulation modes were identified according to the heat flux and inlet subcooling conditions. They are denoted as periodic circulation (A), continuous circulation, periodic circulation (B), and multi-mode circulation, respectively. Among these modes, only the continuous circulation exhibits no significant flow oscillations and is considered to be the stable operation mode. Periodic circulation (A) is characterized by the large-amplitude flow oscillations with incubation periods(no boiling) between them, while periodic circulation (B) being featured by the flow oscillations with continuous boiling inside the heater section. And, multi-mode circulation undergoes a combined behavior of periodic circulations (A) and (B).
At low inlet subcooling condition, the circulation mode varies from periodic circulation (A) to periodic circulation (B) through continuous circulation mode with increasing of the heat flux. At high inlet subcooling, multi-mode circulation occurs at between the regions of periodic circulations (A) and (B). These aspects are represented on the instability map in the plane of heat flux and inlet subcooling.
In general, the circulation rate increases and then decreases with increasing of the heat flux. The amplitude of velocity fluctuation in periodic circulations (A) and (B) generally increases with the heat flux, while the oscillation period decreases. Parametric study also shows that the increases of the inlet-restriction and the cross-sectional area of the heater section and/or the decrease of the exit-restriction broaden the range of continuous circulation mode, and stabilize the system. Also, when the liquid charging level is lowered or the inlet subcooling is decreased, the circulation mode becomes stable.
The experimental results are in general agreement with the analytical results based on the two-phase homogeneous equilibrium assumption.
