Numerical Study On Phase Characteristics In Rod Bundle Under Rolling Condition

The marine nuclear power platform is an effective way to solve the energy supply at sea,but the influence of the marine environment on the reactor puts forward higher requirements for its design and safe operation.In this paper,numerical simulation and corresponding theoretical analysis about the phase characteristics in high pressure within typical rod bundle are carried out under the rolling conditions.The rule of rolling motion and interphase transmission are analyzed and added to three-dimensional computational fluid dynamics software Fluent by the user-defined function,which realize the calculation of the subcooled boiling condition under the rolling condition.The applicability and accuracy of the subcooled boiling model and the rolling effect are validated by the experimental data in the literatures.At the same time,the influence of the rolling motion on the single characteristic is similar to that of the signal input and output,based on the research and analysis of signal processing and system analysis,the general formula of the rolling model for the single characteristic fluctuation is deduced,and the fast fourier transform method is used to analyze the rolling data in order to depth study the impact of the rolling motion on the relevant characteristics.The VOF model was used to simulate the vapor-water two-phase adiabatic flow in the high pressure operating environment under static and rolling conditions,which use the interfacial characteristics of the vapor-water two-phase adiabatic flow to reflect the interfacial characteristics under boiling conditions.The bubble interface is more distorted and irregular due to the decrease of the surface tension under high pressure and the influence of the narrow and complex geometrical structure,the distribution parameter and the critical flow velocity of its trend change is also smaller due to the decrease of the density difference between the two phases.Rolling causes the bubble to sway in the channel with the rolling motion,the phase interface is more distorted,the distribution parameter and the interface area concentration fluctuate with the rolling motion,and the change of the gravitational component is the most important factorThe study of the local phase in the rod bundle of the rolling motion is carried out by using the Eulerian two fluid model.The numerical calculation of the subcooled boiling flow within rod bundle in the high pressure operating environment under static and rolling conditions is carried out,the influence of simple spacer grid on the two-phase characteristic is also investigated.The local void fraction sways with the rolling motion in space distribution and fluctuates with the rolling motion in time distribution.the situations of each local point are not the same,the dominant factors of the impact of its fluctuations are different,which is mainly due to the different geometric location result to different flow field and the value of local void fraction.Rolling motion will also affect the nonuniformity of the wall temperature of the fuel rod,the effect of the rolling motion on the wall temperature for different subcooled levels will vary depending on the heat transfer factors at different locations,rolling motion will exacerbate the nonuniformity of wall temperature around the fuel rod in high subcooled level condition and improve wall temperature unevenness in low subcooled level condition.The simple spacer grid has a significant effect on the velocity distribution,which changes the void fraction near the spacer grid.The position of spacer grid plays an important role in controlling the overall void fraction of the channel.Rolling motion has no obvious effect on the velocity distribution in the spacer grid,the changes of void fraction under the space grid is mainly due to the fact that the addition of the space grid affects the two-phase characteristics such as local void fraction and interphase heat transfer which affect the fluctuation law of local void fraction.