| Nuclear safety is an important issue that cannot be ignored in the process of nuclear energy utilization in nuclear power plants.As an important part of nuclear power plants,the primary circuit system is directly connected with the nuclear energy generation device,and its stable operation is related to the safety of the whole nuclear power plant.Therefore,it is extremely important to predict the transient flow change and evaluate the dynamic safety performance of the system in accident conditions.In this paper,the“Hualong One" nuclear power system is taken as the research object.The flow resistance of the main components of the nuclear power system is equivalently adjusted and matched by numerical calculation method.A threedimensional closed simplified model of the system was established,which provides a model support for the study of the internal flow pattern and the change of important parameters of the main pipe in the nuclear power system when the main pump accident occurs,and also provides ideas for the simplification of other similar complex components.The main contents and conclusions are as follows:(1)An equivalent resistance element with circular arc gradual contraction and expansion is designed,which is easy to control the flow resistance.The flow characteristics and resistance characteristics of the fluid in a single,series and parallel tube are obtained by numerical calculation.The comparison results show that the circular arc resistance element with gradual contraction and expansion has less influence on the downstream of the tube,and its resistance is easier to control,so the element is selected as the equivalent resistance element.The relationship between the element length,the shrinkage ratio and the resistance coefficient C,the flow characteristics in the tube is obtained by calculation.Excluding the influence of Reynolds number on the resistance coefficient of the element,the smaller the element length and shrinkage ratio are,the smaller the ζ of a single element is,the greater the change of the axial velocity and turbulent kinetic energy in the middle is,and the larger the length of the downstream flow field recovery zone is.In order to obtain the quantitative relationship,ignoring the length of the component with weak correlation,the exponential function relationship between ζ and shrinkage ratio is verified when Reynolds number is greater than 3 × 107 and shrinkage ratio is in the range of 0.5～0.9.Finally,when two resistance elements with the same structural parameters are connected in series,it is found that with the increase of the relative distance,the influence of the front element on the rear element is gradually weakened.When the equivalent resistance elements with the same structural parameters are paralleled,the tube diameter has little effect on the total resistance coefficient.The more the number of parallel branches,the greater the pressure drop;when the number of branches is the same,the smaller the ratio of the total cross-sectional area of branches to the total crosssectional area of pipelines is,the larger the total pressure drop is.(2)The flow characteristics and resistance characteristics of the fluid in the U-bend tube used for simplifying the tube cluster of complex components in different structural parameters are revealed.The internal fluid flow characteristics of U-shaped elbow are obtained by numerical calculation:within 0°～90° of elbow,a high pressure zone is formed on the outside of elbow,and the low pressure reflux zone is formed on the inside.In the range of 90°～180°,there is a low-speed zone inside the elbow,and the turbulent kinetic energy of the inner wall is greater than that of the outer wall.The influence of different diameter and radius ratios on the internal flow and C of U-bend pipe is as follows:the larger the radius ratio is,the smaller the velocity gradient between different angle sections is,and the high-speed region with axisymmetric distribution on the same angle section gradually spreads upward.When Reynolds number is greater than 3 ×107,with the increase of radius ratio,ζ decreases first,and then gradually tends to be constant;when the diameter increases,ζ almost linearly decreases.Finally,the numerical results are fitted to obtain the quantitative function relationship between C and structural parameters in the range of diameter 416～680 mm and radius ratio 1～2.5.When the local resistance element with length equal to 680 mm and shrinkage ratio equal to 0.5 is connected in series before the U-shaped elbow with diameter of 680 mm,radius ratio of 0.882 and straight pipe length of 9000 mm,C does not change with the change of relative distance when the relative distance is greater than 3.5.When it is connected in series after the elbow,the change of ζ has no obvious regular characteristics when the pipe is less than 9000 mm.(3)The idea of equivalent substitution used to simplify and dynamically adjust the complex tube cluster structure of steam generator and reactor pressure vessel in the nuclear power system,which meets the pressure drop requirements of the system.According to the previous study on the resistance characteristics of equivalent resistance element and U-bend pipe,combined with the given size and pressure drop requirements,the steam generator is simplified into three U-shaped elbows with a diameter of 680 mm,a length of 9000 mm and a radius ratio of 0.882.The target pressure drop is satisfied by adding five resistance elements with a diameter ratio of 0.5 and one with diameter ratio of 0.7 or 0.65.The relative distance a/D between adjacent elements is 1.The relative errors of the two schemes are-0.84%and 0.86%.The reactor pressure vessel was simplified into two U-shaped elbows with a diameter of 680 mm,a length of 5000 mm and a radius ratio of 1.One resistance element with a diameter ratio of 0.7 and one diameter ratio of 0.65 or only one diameter ratio of 0.6 was added to the elbow to replace the flow resistance of the original model.The relative errors of the two schemes were-1.5%and-0.8%.The adjusted component model is put into the system for calculation and verification,and the relative error between the obtained pressure drop and the target value is less than 5%.It is indicating that the simplified model of resistance adjustment by this method can meet the requirements of nuclear power system.Similarly,the idea of equivalent flow resistance can also be applied to the simplification of other similar complex tubular clusters with certain universality. |
PDF Full Text Request