| The natural circulation relies on density difference and effective height between the hot and cold sections in the loop to provide driving force,the heat generated in the reactor core can be discharged without external driving force,thus ensuring the safety of the reactor.Therefore,the third generation of advanced PWR usually has the ability of natural circulation.In the application of small marine reactor,because of the low driving pressure head of natural circulation system,it is easy to be affected by wind and wave,which threaten the safety of reactor.In this paper,the influence of ocean motion on natural circulation,the flow resistance and heat transfer characteristics are analyzed by multi-scale coupling method.The influence of motion conditions on two-phase flow pattern was analyzed by using impedance void meter and visualization method.Research has been carried out in the following areas:(1)Based on the analysis of the forces acting on the three typical motions of inclining,heaving and rolling,the inertial forces introduced by non-uniform motion and the change of gravity components caused by the transformation of the spatial position are considered,and the centrifugal and Coriolis forces caused by rolling motion.RELAP5 is modified,and UDF of Fluent is used to introduce the motion condition,the data exchange between the two programs is realized,and a multi-scale coupling calculation and analysis tool is established,it is verified by the actual system experiment.(2)The flow and heat transfer characteristics in the channel under moving conditions were studied by the coupled program.Under the action of buoyancy,the mixed convection is formed in the inclined channel,and the local heat transfer capacity is greatly enhanced.The flow and heat transfer characteristics in the channel are mainly affected by the flow fluctuation caused by the heaving.In the range of the parameters studied,the flow fluctuation is in the transition region.The cross-section velocity distribution presents“circular distribution”,which causes the flow resistance characteristics to change with the flow fluctuation.The cross-section temperature distribution also has similar characteristic.Under rolling condition,the flow and heat transfer characteristics are affected by both mixed convection and flow fluctuation.The relative magnitude of the two effects is related to the specific motion parameters.It can be concluded that the mixed convection will enhance the time-averaged heat transfer,while the flow fluctuation will not affect the time-averaged heat transfer.When the natural circulation flow fluctuates to a very low level,the natural convection in the axial direction causes the acceleration of the boundary layer,which leads to the sudden increase of the resistance.(3)The transient characteristics of natural circulation driving force under different motion conditions are analyzed by the coupling program.Under the inclined condition,the contribution of the horizontal section to the natural circulation driving force is determined by the different tilt direction,so the flow variation of natural circulation system is different under the condition of inclination and anti-inclination.The driving force,flow rate,and resistance of natural circulation all show approximately sine wave under the condition of heaving.At the beginning of the heaving,the natural circulation driving force changes firstly,and then affects the flow rate and resistance.Therefore,the driving force change phase is before flow rate and flow resistance.The phase difference between the driving force and the resistance leads to the fluctuation of the driving force larger than that of the resistance.The phase difference between the flow rate and the driving force increases with the decrease of the period,the heating power and the secondary side temperature.The fluctuation range of flow rate increases with the increase of amplitude,the decrease of period,the increase of heating power and the increase of secondary side temperature.The mean value of flow rate is not affected by the heaving motion,but increases with the increase of heating power and secondary side temperature.Two kinds of flow fluctuation forms under rolling condition are defined,which are"BIMODAL"with large period and"SINE"under small period.By analyzing the driving force of the rolling motion,the source of two different wave patterns and the possible relationship between them and the spatial structure of the loop are explained.The fluctuation of flow rate increases with the decrease of rolling period,the increase of rolling angle,the decrease of heating power and secondary side temperature.The mean value of the total driving force and the flow rate is determined by the gravity driving force generated by the vertical pipeline.Although the reduction of rolling period has no effect on the mean value of total driving force,it makes the mean value of resistance decrease,and the flow rate increase.The increase of rolling amplitude decreases the mean value of the total driving force,so the mean value of the system flow decreases.With the increase of heating power and secondary side temperature,the total driving force,and flow rate increase.(4)Based on the flat-plate void meter and the visualization method,the macro-distribution characteristics of two-phase in the narrow rectangular channel under static,inclined,and rolling conditions were studied.The gas and liquid apparent velocity ranges were 0.05-20 m/s and 0.15-2 m/s,respectively.The results of flow pattern classification under tilting and rolling conditions show that tilting and rolling have no significant effect on the transition from bubbly flow to slug flow.However,the transition of slug-churn flow and churn-annular flow has an visible lag.(5)The evaluation of the wall heat flux partition model(WFPM)in the framework of two-phase flow model shows that the cross-sectional average void fraction,the cross-sectional average temperature and the wall temperature can be predicted accurately when the proper combination off-N_a-D_b is selected,but it is not good at predicting the radial distribution of void fraction.Two-phase flow and heat transfer characteristics and system characteristics in narrow rectangular channel under natural circulation condition were analyzed by coupled program.The buoyancy force causes the vapor to accumulate or diffuse upward and downward,which leads to the increase or decrease of the wall temperature and the decrease or increase of the wall shear stress.Under the condition of forced circulation,the effect of rolling is similar to that of licline,I.E.It is related to the direction of heating surface,and the flow and heat transfer characteristics vary periodically.However,due to the existence of fluid inertia,the change of parameters has phase difference and obvious multi-dimensional effect,and the change of local parameters is quite different from the spatial average.In the natural circulation condition,the flow and heat transfer characteristics are mainly affected by the flow fluctuation,regardless of the rolling or the heaving motion.In two-phase natural circulation system,the time average of the additional driving(AD)is still negligible compared with the gravity driving(GD),so the time-average characteristic of two-phase natural circulation is still controlled by GD.For the transient characteristics of two-phase natural circulation,the influence of GD is enhanced.Under most rolling conditions,the two-phase natural circulation shows"BIMODAL"fluctuation. |
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