Study On Flow Resistance Characteristics In Rod Bundle Under Typical Ocean Conditions

The core fuel assemblies of most pressurized water reactors are in the form of rod bundles.Understanding flow resistance characteristics in the rod bundle channel is important for the design and safe operation of the reactor.Due to its inherent safety,natural circulation is widely applied in advanced nuclear reactor systems.Compared with the forced circulation,natural circulation is more susceptible to external factors.For example,natural circulation flow resistance characteristics may change under ocean conditions,thus affecting the normal operation of the circulation system.In this thesis,the natural circulation flow resistance in a rod bundle channel under typical ocean conditions was studied in detail.Single-phase natural circulation flow resistance characteristics in a rod bundle channel under typical ocean conditions were experimentally studied.The transition Reynolds numbers of forced circulation and natural circulation flow are both approximately 800.Compared to vertical conditions,inclination increases the friction factors and the main reason is the radial convection spurred by the buoyancy.The convective strength is positively correlated with the inclination angle and negatively correlated with the Reynolds number.Under rolling motion conditions,there is no obvious phase shift between the natural circulation frictional pressure drop and the flow rate.The friction factors variation with the Reynolds number and the flow regime change during the rolling motion process are the main mechanisms of the friction factors fluctuation.Additionally,the dimensionless correlation of instantaneous friction factors was obtained by dimensional analysis and polynomial fitting.The cycle-averaged resistance results indicate that the flow laminarization during the rolling motion process is responsible for the increase of cycle-averaged frictional resistance.The CFD software Fluent was used to simulate the single-phase flow in the rod bundle channel under typical ocean conditions.It is found that the near-wall secondary flow influences the local wall shear stress distribution,while the axial flow velocity is the main factor affecting the wall shear stress.The main mechanism by which the inclination increases the frictional resistance is that the natural convection spurred by the thermal stratification changes the flow velocity distribution of each subchannel and enlarge the radial secondary flow region,thus flattening the shear stress distribution near the rod wall surface.Under rolling motion conditions,the thickness variation range of the boundary layer increases with the decreasing flow rate,which is primarily responsible for the result that the relative variation amplitude of the wall shear stress increases with the decreasing flow rate.Single-phase flow resistance characteristics in a rod bundle channel under pulsating flow conditions were experimentally studied.The phase of the frictional pressure drop lagging behind the flow rate is determined by the change rate of the flow rate.The faster the flow rate changes,the more the friction pressure drop lags and the larger the phase shift is.The friction factors variation with the Reynolds number and the flow regime change are still the main mechanisms of the friction factors fluctuation.Combined with the experimental and numerical simulation results under rolling motion conditions and the analysis of flow resistance under pulsating flow conditions,the flow rate fluctuation spurred by rolling motion is principally responsible for the frictional pressure drops variation under rolling motion conditions.Different rolling motion startup directions resulted in different instantaneous flow rate change directions,while barely affecting the long-term response characteristics of the system.Two-phase natural circulation flow resistance characteristics in a rod bundle channel under typical ocean conditions were experimentally studied.The calculation correlation of ONB positions in the rod bundle channel and the variation law of various pressure drops with the heat flux were obtained.According to the experimental results,a few common homogeneous flow models and separated flow models were evaluated and the Chisholm model was modified.Under both vertical and inclined conditions,the phenomenon that the flow rate decreased with the increasing heat flux occurred since the growth rate of the frictional pressure drops was greater than the driving force growth rate when the mass quality was sufficiently large.Through the dimensional analysis,factors affecting the instantaneous two-phase frictional pressure drops were obtained and the transient calculation correlation of the two-phase natural circulation flow resistance was proposed.It is found that the most significant factor affecting the frictional resistance is the angular acceleration among rolling motion parameters and the liquid Reynolds number and the mass quality among thermal parameters.In this thesis,the variation law of natural circulation flow resistance in a rod bundle channel under typical ocean conditions was obtained through experimental and numerical studies.The influence mechanisms of different factors on the flow resistance under rolling motion conditions were analyzed by the variable-controlling approach,providing new ideas and references for subsequent studies on the flow resistance under ocean conditions.