Visual Experimental Study On Vapor Bubbles Under Boiling Conditions

Two-phase flow occurs frequently in nuclear power plant equipments,and for nuclear power plant equipments,the main form of gas phase in two-phase flow are bubbles.On the other hand,if the temperature of fuel clad in the core of nuclear reactor is too high,boiling heat transfer will occur.Because of the presence of bubbles which can enhance convective heat transfer between hot and cold fluids,the nucleate boiling heat transfer can transfer more heat than the single-phase liquid.Therefore,in order to reveal the boiling heat transfer mechanism and ensure the nuclear reactor can operate in safety,it is necessary to study the growth characteristics of the bubble under boiling conditions.In this paper,an experimental study on the bubble growth behavior under pool boiling conditions is carried out.The influence of external boundary conditions on the bubble growth characteristics are analyzed according to the experimental data of vaper bubbles,and bubble departure diameters are also statistically analyzed.Author established an experimental platform for the bubble visualization research under boiling conditions.The size of the experimental water tank for simulating the pool boiling phenomenon is 250×150×300 mm.The working liquid used in the experiment is deionized water.Because the experimental condition is subcooled boiling,during the experiment,the water in the water tank should always keep at a certain degree of subcooling.Vaper bubbles are generated by the conductive heating device which located at the bottom of the water tank.The high-speed camera is placed in front of the water tank in order to capture the growth behavior of vaper bubbles.The experimental data of vaper bubbles are processed by the image processing software,so that the basic parameters of vaper bubbles required for the research can be obtained.Several hypothetical models of nucleate boiling are also introduced in this paper.And the force balance model which is suitable for this paper's experimental conditions for the prediction of the detachment diameter of vaper bubbles is given.The feasibility of the experimental platform used in this paper was verified by comparing the experimental results of other scholars.The statistical analysis of the bubble departure diameter data obtained in the experiment was carried out.It is found that the departure diameters of vaper bubbles generated by the same nucleation site are not a fixed value but fluctuate within a certain range,and this fluctuation will decrease with the increase of the heat flux.Most of vaper bubbles' diameters are between 0.2 and 0.3 mm.According to the experimental data,the influence of heat flux on the growth behavior of bubbles are studied.The results show that with the increase of heat flux,the growth rate of vaper bubbles is faster and the growth period is shorter.The stability of bubbles generated at the same nucleation site is increased in the same working conditions,also the nucleate boiling state is more stable.The effect of liquid subcooling on the bubble growth behacior was also studied based on experimental data.It has been found that an increase in the degree of liquid subcooling leads to an increase in the randomness of the size of bubbles generated by the same nucleation site under the same working conditions.The condensation and polymerization phenomena of vaper bubbles are also observed during the experiment.It is found that these two phenomena are related to liquid subcooling and heat flux respectively.In addition,the research also shows that the maximum diameter which a bubble can reach is mainly affected by the initial growth rate of the bubble,and the larger the size of the bubble,the faster the velocity on the vertical direction during the lifting movement.