Study Of Pool Boiling Characteristics Of Water Outside A Smooth Tube In A Confined Space

The heat transfer mechanism of fluid boiling in confined space is very complex,involving not only convective heat transfer and phase change heat in classical pool boiling,but also special vapor bubble behavior and flow field motion.At present,the research on boiling in confined space at home and abroad mainly focuses on the experimental investigation stage,and it is difficult to get the accurate boiling heat transfer mechanism only through the experimental macroscopic analysis of its heat transfer characteristics.Especially,the microscopic phenomena such as vapor bubble behavior and physical field state in some small size slits(d<2mm)cannot be accurately observed and obtained.On the other hand,the current research on the confined space is mainly focused on the boiling on the plate heat exchange surface,and the research on the fluid heat exchange outside the tube is relatively little,so there is an urgent need to start the research on the boiling heat exchange mechanism of the fluid in the confined space inside the tubular annular slit.In this paper,the VOF multiphase flow model and Lee’s heat and mass transfer model are mainly used to numerically simulate the boiling characteristics of water pools in open and confined spaces outside horizontal and vertical circular tubes.Firstly,the accuracy of the numerical model is verified by comparing the simulation results with the experimental data.Secondly,based on this,a total of eight sets of numerical models for pool boiling heat exchange outside horizontal and vertical pipes under open space horizontal pipes,open space vertical pipes,and restricted spaces with different narrow slit sizes(2mm,4mm,6mm)were established.The effects of pipe arrangement orientation on bubble behavior,flow field motion and heat transfer characteristics were analyzed and compared.The results of the study show that the horizontal off-pipe pool boiling in restricted space can be divided into three regions,namely the diffuse vapor bubble region,the deformed vapor bubble region and the stacked vapor column region.The boiling of the vertical extra-tubular pool can also be divided into three regions,namely the isolated vapor bubble region,the deformed vapor bubble region and the glued vapor block region.At low superheat,the heat transfer performance outside the vertical tube is better than that of the horizontal tube.However,with the increase of wall superheat,the heat transfer performance of horizontal tube will be better than that of vertical tube.In this paper,when comparing the behavior of vapor bubbles and flow field characteristics under different confined space dimensions,it is found that the boiling heat transfer in the off-pipe pool under confined space is influenced by three mechanisms: liquid perturbation,thin liquid film evaporation,and vapor bubble agglomeration.At low superheat,the heat transfer capacity of the tube increases as the size of the confined space decreases,but this enhancement of heat transfer is conditional,i.e.,at a certain superheat,the heat exchanger tube will undergo deterioration of heat transfer when the size of the confined space is too small.This study is an important guide for the selection of heat exchanger tube arrangement direction and confined space size to improve the heat transfer capacity of boiling heat exchangers and to improve the existing heat exchanger structure.