Numerical Simulation Of Multi-bubble Behavior And Heat Transfer Process In Nuclear Pool Boiling

Boiling heat transfer gains wide attention because of its efficient heat transfer capability,which has been widely used in traditional and modern industries.Boiling is often accompanied by the complicated process of gas-liquid phase transition.Bubble kinetics is a very important branch of boiling and the behavior of bubbles has a crucial influence on heat transfer efficiency.In this work,the Volume Of Fluid method was used to numerically study the growth of finite bubbles in the process of supercooled nucleate pool boiling,and explored the changes of bubble behavior and heat transfer characteristics under different working conditions.First of all,a single bubble model was established to study the growth and separation process of bubbles in one cycle.The influence of gravity,superheat,supercool and other factors on bubble behavior were analyzed.The results show that at the initial stage of bubble growth,the bubble grows rapidly,which is affected by the superheated liquid layer,and the surface tension is dominant at this stage.As the volume of the bubble increases,the growth rate slows down,and buoyancy is dominant at this stage,the buoyancy of the bubble also increases with increasing gravity,and the bubble can leave the wall faster,as a result,the heat flux on the wall also increases.The increase of the degree of superheat and the degree of subcooling increases the temperature gradient between the wall surface and the working fluid,which enhances the convection heat transfer of the wall,thereby improving the heat transfer performance.Then the growth process of double bubble and triple bubbles in one cycle was studied.The influence of factors such as the spacing of the gasification cores and the arrangement of the three bubbles on the bubble behavior was analyzed.The results show that when the supercooling degree is 10 K and the superheating degree is 20 K,the smaller the gasification core spacing is,the greater the force between the bubbles is,the easier the bubbles are to coalesce.When the gasification cores spacing is 0.6 mm and 0.8 mm,the bubbles have already coalesced on the wall.When the spacing is 1 mm,the bubbles will coalesce during the rising process.When the spacing is 2 mm,the bubbles will not coalesce.When three bubbles are arranged in a triangle,the bubbles are more likely to coalesce and break away from the wall,and the bubbles coalesce can enhance the liquid disturbance.Therefore,when three bubbles are arranged in a triangle,the wall heat transfer performance is stronger than when the bubbles are arranged in a straight line.Among the spacing of gasification cores listed,the heat transfer performance is the strongest when the gasification cores are arranged in a triangular pattern with the spacing of 0.6 mm,have the heat transfer performance of the gasification cores is worst when the inline pattern spacing is 2 mm.The average heat flux of the former is 125.5% of the latter,which greatly enhances the heat transfer performance.Finally,the differences in the growth of single-bubble,double-bubble and threebubbles under different arrangements in water and FC-72 were compared when the superheat was 20 K and the subcooling was 5 K.The results show that increasing the number of bubbles is conducive to enhancing heat transfer,and when the working medium is FC-72,the heat flux can increase rapidly,meanwhile,the heat transfer is enhanced.