| The design and operation of offshore floating nuclear power plants need to consider the influence of the marine environment on the transient characteristics of thermal-hydraulic parameters in the reactor core.Its design criteria and operating procedures cannot follow the standards of land-based nuclear power plants.Under marine conditions,the two-phase flow phenomenon,the formation and evolution mechanism of bubbles,and the fundamental of boiling heat transfer in the reactor core are more complicated due to the inertial force caused by the rolling motion.The influence and mechanism of the rolling motion are still unclear.In this thesis,a numerical model of the subcooled flow boiling phenomenon in the reactor core under marine conditions is built by using the sliding mesh model and interface-tracking technique.The two-phase flow characteristics in the subchannel,the bare rod bundle,and the rod bundle with spacer grids under vertical,inclined,and rolling conditions are studied.Taking the vertical condition as the standard condition,the variation of macroscopic parameters such as the void fraction and vapor adhesion ratio in the channel under the inclined and the rolling condition are analyzed.The interaction between the microscopic phenomena,such as transverse flow field,bubble behaviors,and vapor-liquid phase distribution,is explained.The influence mechanism of marine conditions on the characteristics of two-phase flow in the reactor core is revealed.In subchannel and the bare rod bundle under the vertical condition,boiling promotes the transverse flow.A pair of secondary vortices are formed in the rod gap regions.Under the influence of transverse flow,the bubbles accumulate in the rod gap regions.There is almost no crossflow between subchannels in the bare rod bundle.Besides,in each subchannel of the bare rod bundle,the transverse flow,void fraction,vapor adhesion ratio,and vapor-liquid phase distribution are almost the same as those in the single subchannel.In the subchannel and the bare rod bundle under the inclined condition,the component of the buoyancy force on the cross-section is the fundamental cause affecting the two-phase flow characteristics.As the inclination angle increases,the component of the buoyancy force on the cross-section increases,and the difference of the two-phase flow characteristics between the inclined and the vertical conditions increases.Under the inclined condition,void fraction,vapor adhesion ratio,and vaporliquid phase distribution in quadrants of the subchannel and the bare rod bundle are not the same.However,the effect of the inclined condition on the two-phase flow characteristics is very limited in the flow boiling with a high flow rate and high subcooling.In the subchannel and the bare rod bundle under the rolling condition,boiling and rolling motion jointly affect the two-phase flow characteristics,with the latter effect dominating.The Euler force varying periodically is the fundamental cause for the periodic variation of the transverse flow,void fraction,vapor adhesion ratio,and vaporliquid phase distribution in the rolling condition.As the maximum angle increases and the period decreases,the maximum of the Euler force increases,and its maximum influence on the two-phase flow characteristics increases.The two-phase flow characteristics in the inclined and the rolling conditions are quite different.In the rod bundle with spacer grids,the mixing vanes greatly promote the transverse flow and play a dominant role in the vapor-liquid phase distribution.Comparing the vertical and the rolling conditions,the microscopic phenomena such as the transverse flow field and vapor-liquid phase distribution in the rod bundle with spacer grids are almost the same,and the macroscopic parameters such as void fraction and vapor adhesion ratio are approximately equal in both the whole domain and the quadrants.Under the effect of mixing vanes,the effects of boiling and rolling motions on the two-phase flow characteristics are negligible.In summary,the influence of marine conditions on the rod bundle with spacer grids is different from that on the subchannel and the bare rod bundle.The two-phase flow characteristics are affected by boiling,rolling motion,and mixing vane.The effect of the mixing vanes is very strong,which plays a dominant role in the influence of transverse flow,void fraction,vapor adhesion ratio,and vapor-liquid phase distribution,and almost completely covers the effect of boiling and rolling motion.This research has a strong theoretical significance and application value and will help to formulate and improve the design standards and operation specifications of offshore floating nuclear power plants and ensure the safe and economic operation of nuclear power units. |
