Experimental Investigation On Bubble Coalescence Characteristics Of Subcooled Boiling In Narrow Rectangular Channel

The coalescence of bubbles in rectangular narrow channel is of great significance in microelectronics and nuclear energy fields.Coalescence will change the size distribution and phase interface concentration of bubbles,affecting the heat and mass transfer performance and chemical reaction rate of the equipment.In the thermal design of PWR,to increase the coolant outlet temperature,the subcooling boiling can occur locally,thus improving the heat exchange efficiency.However,with the increase of the steam content,the bubble accumulation on the wall will directly affect the fuel assembly surface’s heat transfer capacity.At present,the characteristics and mechanism of bubble coalescence in narrow rectangular channel are still unclear,so further research is needed.In this paper,the bubble coalescence in narrow rectangular channel is studied by four sides visualization method.The influence of the evolution law of liquid film,condensation and sliding size evolution on coalescence is analyzed,and the bubble collision coalescence model is established.The experimental results show that the bubble aggregation will produce capillary waves on the ring surface,and the capillary wave reaches both ends of the bubble,which causes the edge of the bubble to form an extension and cause instability,thus producing the secondary bubble.It is found that the initial average bubble diameter is greater than 0.87 mm.Besides,the reverse direction of the capillary wave after arriving at both ends of the bubble is the direct reason for bubble oscillation after coalescence,and the difference of surface energy and curvature before and after bubble coalescence is the fundamental reason for bubble oscillation.In addition,the shape fitting of the bubble after coalescence shows that the oscillation mode is the initial value of the collision angle,the oscillation amplitude is 80-90°,the more the initial diameter ratio is close to 1,the smaller the bubble oscillation period is.The smaller the initial impact angle of the bubble is 90°,the smaller the bubble oscillation period is.The results show that the dry spot growth rate of the bubble is exponential,and the curve of dry spot evolution is obtained by data fitting.The size evolution of bubbles will affect the interface collision velocity of the bubble when the bubble coalesces,and the relative size of condensation heat transfer directly determines the direction of bubble size evolution.Therefore,the condensation experiment of bubble interface is carried out based on the narrow rectangular channel,and the interface evolution phenomenon in the process of bubble condensation is studied,and the condensation interface heat exchange model is established.The bubble slip and condensation process size evolution model is established by combining condensation heat transfer with the evaporation of micro liquid layer at the bottom of bubble and evaporation heat exchange of superheated liquid layer.Experimental data verify the model.Based on the bubble size evolution model,the relative interface expansion velocity in the process of bubble collision is studied.The interface collision velocity of bubbles in a collision is obtained by combining the velocity difference between bubbles and the relative expansion velocity.The experimental results show that the critical velocity of the coalescence of steam bubbles is 0.03-0.05 m/s.When the bubble’s interface collision velocity is lower than the critical speed,the bubble will coalesce;when the interface collision speed exceeds the critical speed,the bubble will rebound without coalescence.Besides,the bubble velocity in the narrow rectangular channel is normal distribution,and the collision probability of the bubble is obtained.Finally,the bubble coalescence frequency model is established based on the bubble collision probability and the bubble’s critical velocity.