Research On Bubble Group Characteristic Parameters Detection Of Gas-Liquid Two-Phase Flow In Rod Bundle Channel By Laser Imaging Method

Pressurized water reactors are now widely used in nuclear power plants in China.The core fuel assembly rod bundle channel contains a large number of bubbles under operating conditions,forming a gas-liquid two-phase flow with the coolant.Study of the void fraction of two-phase flow,as well as the shape and distribution of bubble group is very important for analyzing the characteristics such as flow,heat transfer and critical heat flow density,as well as constructing a mathematical model of the two-phase flow,and guiding the thermal design of the reactor system.However,due to the characteristics of high temperature,high pressure,high flow rate and high radiation in the actual core operating conditions,it is impossible to directly detect the characteristics of the internal gas-liquid interface area and phase distribution.Therefore,by building a core structure simulation device under the environment of temperature reduction and pressure reduction or normal temperature and normal pressure,the characteristic parameters of bubbles in the fuel bundle channel are studied with the help of relevant detection methods.It is still the mainstream research method to construct the analytical model and interface transport equation of two-phase fluid to predict the pressure distribution and heat transfer characteristics of the actual core fuel assembly.In view of this,this paper studies a method for measuring the characteristics of bubble groups based on laser imaging,aiming at the calculation requirements of the core gas-liquid two-phase flow parameters in the thermal design and modeling of nuclear reactors.The twophase flow is irradiated with a plane laser outside the transparent model core to form a transverse detection section,and the forward light scattering image of the bubble group in the section is obtained by high-speed photography,and the flow characteristics are analyzed by image analysis method.In the experiment,a 3×3 rod bundle channel gas-liquid two-phase flow bubble generation device and a flow channel cross-section bubble moving image acquisition device were built under normal temperature and pressure.A dual-probe conductivity probe bubble velocity and particle size analysis system was used to verify and analyze the measurement data by the image method.Firstly,after preprocessing the collected bubble images,the bubbles are divided into two types: regular bubble group and irregular bubble group by circularity value classification method.For regular bubble groups,direct ellipse fitting is used to extract their characteristic parameters,and for irregular bubble groups,an optimized image segmentation algorithm based on fast radial symmetry transformation is used to reduce the area and number calculation errors caused by bubble image overlap or adhesion.The instantaneous cross-section void fraction measurement was calculated using the ratio of the bubble area of the cross-sectional image to the flow channel cross-section,and the simplified slip ratio model of Milopolsky was used to verify the error of the void fraction measurement value.The results show that the error range is less than 10% under bubbly flow conditions.By integrating the bubble area of the continuous image sequence,the void fraction distribution analysis of the cross-section is completed.In addition,based on the constraint relationship of the bubble "fault profile-velocity-shape",it is proposed to use the longitudinal roundness value to complete the solution of the vertical velocity of the bubble.Combined with the vertical velocity and the coordinate information of the edge point of the fault contour,the bubble three-dimensional ellipsoid reconstruction is completed,realize the three-dimensional shape reconstruction of bubbles and the calculation of average particle size distribution.The relative error between the experimental results and those measured by the probe system is within 10%.Aiming at the detection of characteristic parameters of bubble motion in gas-liquid twophase flow in rod bundle channels,the cross-sectional image analysis system proposed in this paper is superior to the traditional longitudinal image method in terms of range and accuracy.Moreover,the analysis types and dimensions of bubble characteristic parameters are more abundant,which has a certain reference value for studying the phase interface area and phase distribution of gas-liquid two-phase flow in rod bundle channels.