Study On Wall Heat Flux Partitioning Model In Vertical Rectangular Narrow Channel

The cores of marine nuclear power reactors widely use a plate fuel assembly,which form narrow rectangular coolant channels.Compared with large channels with conventional dimensions,the bubble behavior and heat transfer characteristics in narrow channels with limited space are different.The purpose of this paper is to study the bubble behavior and heat transfer mechanism of subcooled flow boiling in a narrow rectangular channel and to establish a wall heat flux partitioning model suitable for narrow channels.Visualization experiments of subcooled flow boiling in a narrow rectangular channel are carried out by high-speed camera and image post-processing technology.The thermal parameters and bubble images are obtained.The test section used in the experiment is a single-side heated vertical narrow rectangular visualization channel formed by a stainless steel heating plate and visualization glass.The effective heating length is 550 mm.Deionized water is used as the experimental working fluid.In the experiment,the system pressure is 0.2～0.4MPa,the wall heat flux is 110～620 k W/m~2,the inlet subcooling is 20～60 K and the mass flow rate is 200～1600 kg/(m~2·s).According to the visualization bubble image,bubbles existing in the experiment are divided into two types based on the behavior of single bubbles:condensing bubble and sliding bubble.The growth and movement characteristics of each type is studied respectively.Among them,the sliding bubble with long lifetime and sliding distance and a relatively large number is the research focus of this paper.Based on the experimental results under different working conditions,the influence of wall heat flux,mass flow rate,inlet subcooling and system pressure on bubble growth and sliding velocity are studied.Through theoretical analysis of the heat transfer effect during the growth of the sliding bubble,a semi-mechanical model of the bubble growth rate is established.At the same time,an empirical correlation of bubble sliding velocity is established based on the experimental data.In addition,the coalescence between two sliding bubbles are studied,the different stages of bubble coalescence and the changes of bubble shape,diameter and sliding velocity before and after coalescence are analyzed.The nucleation site distribution and nucleation site density are obtained by statistical methods.The influence of thermal parameters on nucleation site density is analyzed.Correlation for nucleation site density is fitted by introducing the inhibition factor based on the experimental data.A detailed force analysis of a single bubble is carried out and the force parameters are determined.According to the change curve of bubble forces with time,it is explained that the bubbles in the narrow rectangular channel depart and slide but does not lift off.The influence of thermal parameters on bubble departure diameter is analyzed from two aspects of energy and force.Based on the force analysis,the prediction models of bubble departure diameter and lift-off diameter in horizontal and vertical directions are established.Aiming at the significant bubble sliding behavior in the narrow rectangular channel,combined with wall boiling model(RPI model)in commercial computational fluid dynamics software,an improved wall heat flux partitioning model is established.The total wall heat flux is divided into four parts:single phase convection,evaporation,departure transient conduction and sliding transient conduction.The improved model is embedded into FLUENT via user-defined functions to performe the numerical simulation of subcooled flow boiling in a narrow rectangular channel.The calculation results of improved model and RPI model are compared with experimental results.The results show that the improved wall heat flux partitioning model prediction results are significantly better than RPI model under test conditons,which verifies the accuracy of the improved model.In addition,the heat flux distribution and bubble dynamics parameters calculation results of the two models under different working conditions are compared and analyzed.In this paper,through the combination of experimental research,theoretical analysis and numerical simulation,the bubble dynamic behavior characteristics in subcooled flow boiling in a narrow rectangular channel are obtained.The correlation for bubble parameters are established and a wall heat flux partitioning model is developed,which provides a reference for thermal-hydraulic design of the reactor core.