Study On The Flow Characteristics Of The Open Natural Circulation System

Reliable passive containment cooling system is one of the most important components of the advanced nuclear reactor designs.An open natural circulation system,which operates under the atmospheric pressure,can be used for the passive heat removal of the double concrete containment.The system consists of an internal heat exchanger,external cooling water tank and the connecting pipes.The heat source of the system is the high-temperature mixtures of the steam and non-condensable gas released into the containment after the accidents,and the heat sink is the water in the tank or the atmospheric environment.The heat inside the containment is removed by the natural circulation developed between the heat source and sink.In this paper,the flow and heat transfer characteristics and the influences of the thermal and geometric parameters on them in the open natural circulation are investigated by using the numerical simulations and theoretical methods,which contributes to provide the references for the engineering designs to effectively improve the heat transfer capability of the system.One dimensional transient thermal calculating code,which has been validated by using the experimental data,is developed to simulate the transient operating behaviors of the open natural circulation system,and the effects of the thermal parameters and the geometric parameters on the flow modes and the flow instability are studied.The results show that seven flow modes are found in the low-pressure system,including the single-phase flow,alternate oscillation between the single-phase and flashing,boiling induced alternate oscillating flow,oscillation between the boiling and flashing,flashing-induced oscillating flow,flashing-induced steady two-phase flow and boiling steady two-phase flow.Flow instability occurs only within a certain range of the steam quality in the open natural circulation.Under the high inlet subcooling conditions,both increasing the riser height,downcomer diameter or riser diameter,and decrease the inlet and outlet resistances of the heat exchanger or the outlet resistance of the riser can increase the mass flow rate and extend the range of the steady single-phase flow.Under the low inlet subcooling conditions,decreasing the riser or downcomer diameter,and increasing the increasing the inlet and outlet resistances of the heat exchanger or the outlet resistance of the riser can expand the steady two-phase flow range.The flow modes of the open natural circulation have great effects on the heat removal capacity.The single-phase mass flow rate and heat transfer increase with the containment pressure,steam fraction and the inlet subcooling increasing,but they are independent of the water level.The two-phase mass flow rate and the heat transfer is dependent on the mass quality.Increasing the riser height can improve the flow and heat transfer of the single-phase and boiling two-phase flow,but it has little influence on the flashing-induced two-phase flow.The effect of the riser and downcomer diameters on the flow and heat transfer is related to the resistance fractions of the sections.The main flow resistance of the flashing-induced two-phase flow is the accelerating pressure drop in the upper rise,and the main flow resistance of the boiling two-phase flow is the frictional pressure drop in the heat exchanger and riser.So increasing the diameter of one section which has the most resistance fraction can improve the flow and heat transfer efficiently.Increasing the height,diameter and number of the heat transfer tube can increase the mass flow rate and the heat transfer capacity of the system,but the variations of the heat transfer area and the coefficient should be considered simultaneously when investigating the effect of the diameter and number of the heat transfer tube on the changing degree of the flow and heat transfer.Flashing phenomenon occurring in the adiabatic riser section is the main induced factor of the two-phase flow in the open natural circulation system.The theoretical relationship between the mass flow rate and heat transfer in the steady flashing two-phase flow is deduced according to the characteristics of the flashing phenomenon firstly.Based on the steady theoretical results,frequency-domain method is used to investigate the flashing-induced flow instability,and finally the simple and feasible scaling method is proposed.Both the theoretical analyses and the scaling method above have been validated by using the transient numerical code or the experimental data and have certain feasibility.