Visualized experimental study on pressure drop oscillations of a natural circulation system under low pressure condition

By using deionized water, a visualized experimental study on flow instability phenomenon has been performed in the natural circulation system under condition of low pressure 0.14-0.25 MPa and inlet temperature 45-65 degrees C. The two-phase flow pattern is recorded by high speed camera and the types of flow instability under different heating power are identified and studied. Besides, the influence of heating power, inlet pressure and temperature on the pressure drop oscillations (PDO) is researched. The experimental results in this paper show that the flow has experienced three stages which are single-phase, sub-cooled boiling two-phase and unstable saturated boiling two-phase, with the increase of heating power. For unstable two-phase flow, both density wave oscillations (DWO) and pressure drop oscillations (PDO) with superimposed DWO are observed. Meanwhile, it is found that the mass flow rate oscillation amplitude of PDO increases while the period of that decreases when increasing the heating power. The parameters analysis results show that the critical heating power of PDO increases with system pressure and decreases with inlet temperature of test section. The experimental results can help to un-derstand the flow instability characteristics and improve the flow stability in the natural circulation loop under low pressure condition.