Experimental And Numerical Study On Typical Bubble Motion Characteristics Under Various Conditions

In nuclear power plant equipment,bubbles with different sizes,shapes and thermal states are present due to the frequent occurrence of gas–liquid two-phase flow in nuclear engineering.For nuclear power systems,the operating conditions involving bubbles mainly include the subcooled boiling of partial equipment during normal operation,the saturated boiling in accident scenarios,as well as the bubbling filtration systems.The generation and movement of bubbles have different degrees of influence on the fluid movement and heat transfer process in the device,and the bubbles in different operating conditions and different devices have great differences in size and movement characteristics.Hence,it is necessary to investigate the dynamic behaviors of bubbles under different operating conditions in nuclear engineering,which is profoundly meaningful for ensuring the safe operation of nuclear power systems.The simulated bubbles are classified into cold bubbles and hot bubbles in this paper,depending on whether the bubble source encompasses a heat transfer process.Through a combination of numerical simulation and visualization experiment,the formation,movement and deformation characteristics of bubbles are explored under several typical conditions of nuclear engineering field.The main research contents and achievements are as follows:Initially,numerical simulation analysis is performed on the bubble motion characteristics under different conditions in the cold state while ignoring the heat and mass transfer between air bubbles and water.During this process,the volume of fluid model is used in combination with the continuum surface force model proposed by Brackbill et al.The deformation and velocity characteristics of two side-by-side bubbles are analyzed under the floating condition,the orifice bubbling condition,the swinging condition and the near-wall condition,which are compared with the floating process of single bubble in ideal condition.Such analysis covers the variation processes of bubble periphery flow and pressure fields with the bubble floating motion,the rebound process between horizontally distributed bubbles,the bubble formation processes at varying orifice sizes and pore spacings,as well as the bubble floating processes under the wall force and the additional inertial force resulting from swing motion.As the simulations demonstrate,various operating conditions influence the bubbles primarily through changes in pressure and velocity fields within the fluid around the bubbles.Secondly,utilizing a high-speed camera,a visualization experiment is carried out on the bubbles escaping from wall orifices by building an experimental platform.The velocity characteristics,detachment sizes and floating trajectories of bubbles are obtained in the single-and double-orifice air intake scenarios,and the experimental results under single-and double-orifice cases are compared at different air flow rates and pore sizes.Further,the effects of pore diameter and pore spacing on bubbles are analyzed.Consistency is found between the experimental observations and the numerical simulations,thus verifying the reliability of the numerical models.Then,the VOF model and the User-Defined Functions(UDF)are used to numerically simulate the bubble dynamic behaviors in boiling conditions.The mass transfer of gas–liquid two-phase flow is implemented with the micro-liquid layer vaporization model in conjunction with the Lee phase change model,and the growth and detachment processes of single boiling bubble are simulated reasonably.By processing the simulations,the distribution patterns of flow and temperature fields around the bubbles are derived,as well as the temporal variations of bubble size and shape.Further,the surface tension coefficient and wall contact angle on the bubble characteristics are analyzed.Finally,an experimental platform is designed and built for single-bubble visualization experiment in boiling conditions,where the bubbles are generated by electrical heating.During the experiment,the growth and detachment processes of single boiling bubbles in the water are recorded by high-speed photography,and the surface temperature of heated wall is recorded with thermocouple.The visualization experimental studies are carried out in this paper at three heat fluxes and three liquid subcooling degrees.Based on the experimentally derived bubble parameters,the effects of liquid subcooling and wall heat flux on the growth and detachment characteristics of bubbles are analyzed,followed by statistical analysis of the bubble detachment sizes.As the experimental results reveal,the period prior to bubble detachment from heated wall can be divided into the rapid bubble growth stage and the detachment preparation stage.A greater heat flux indicates a higher degree of subcooling and accordingly,a larger proportion of the rapid bubble growth stage.