Experimental Study On The Bubble Nucleation And Growth Characteristics In The Narrow Heating Surface Of Pool Boiling

Boiling heat transfer is a complex process on phase change,which is formed by the formation of a large amount of bubbles in a solid-liquid interface.A lot of empirical formulas for boiling heat transfer have been summarized in the past several decades.However,most of these empirical formulas do not contain the parameters relating to heater surface properties and bubble nucleation characteristics.In order to reveal the mechanism to bubbles nucleation and growth in boiling heat transfer,this paper has developed the visualized experiment apparatus to observe the bubble structural changes in the bubble nucleation and growth.The main work includes:First of all,the visualized experiment platform for boiling heat transfer has been set up,including the transparent pool,core heating parts,DC power source,data acquisition system and image acquisition system.Among them,the transparent pool and core heating parts are the main components of the experiment apparatus.DC power source is used to provide the constant heat flux to heating core.Data collection system is used to record the temporal temperature variation of the heating parts.Image acquisition system is used to record the image information of the bubble nucleation and growth.The phenomenon of boiling heat transfer on the circle and the narrow surface is observed,respectively.Water is the working medium of the experiment on boiling heat transfer.The effect of different roughness on boiling heat transfer was investigated.The experimental results show that the effect of surface roughness on boiling heat transfer is limited in the natural convection region,and the influence in the nuclear boiling region is obvious.And as the surface roughness increases,the boiling curve moves to the left because of the increased number of active nucleate sites on the wall.At the same time,the process of bubble nucleation and growing on the heated surface is observed on the circle and the narrow surface.Based on the time sequence images,the variation of diameter,volume,height and contact angle with a single and multiple bubbles were analyzed,respectively.The results show that the initial diameter changes of the single bubble nucleation and growth process are the largest,and the height changes gradually.The results show that the diameter changes are the largest in the early stages of a single bubble nucleation and growth.It comes from nothing and the change in height is gradual.In addition,with the bubbles growing,the rate of growth increases in diameter first and then decreases in diameter,while the rate of increase is the opposite in height.The bubbles can be classified into non-coalesced bubbles,half-coalesced bubbles,full-coalesced bubbles.During the bubble coalescence process,there is no sudden increase in bubble height.And in bubble diameter,there is an instantaneous increase in bubble consolidation.