A Visualization Study On The Effect Of Micro-structure Surface To Nucleate Boiling Process

As an efficient heat transfer method,boiling heat transfer has been widely used in high efficiency heat exchanger,fast cooling and energy storage processes.It is a complex process involving gas,liquid and solid phase,meanwhile,the randomness and uncertainty of the boiling process brought great difficulties to its depth research.The boiling phenomenon of different silicon surface was analyzed by using high speed camera and infrared thermal imaging equipment,obtained the bubble departure diameter,bubble detachment time and boiling curve at various heat flux.The local temperature evolution during the process of bubble growing and detaching was also provided.On a smooth surface,the nucleation generating sites have great random and the local temperature evolution shows that the decreasing temperature of wall arrives ten degrees at the moment of bubble detachment,then it rises gradually.In which process the transient heat conduction plays a leading role.On the cavity surface,bubbles generate at the site of the cavity,and the bubble departure diameter tends to be stable for different cavity diameters.Thus,the cavity is considered could reduce the randomness and uncertainty of boiling process.The transient heat conduction also dominates the heat transfer process on the cavity surface the same as the smooth surface.Meanwhile,we first proposed a definition of three dimensional superheat temperature that effects the nuleation site,and it is found that the nuleation site of nucleate boiling on smooth and micro-cavity surface mainly controlled by three-dimensional superheat.Bubble dynamic on micropillar surface is quilt different from smooth and cavity surface.On the uniform micropillar surface and cavity micropillar surface,bubbles firstly generated in the micro pillar gaps,then merging with each other to become a big bubble.The bubble departure diameter has a positive correlation with the detachment period and has little relation with heat flux.The local temperature evolution shows that the bubble temperature is less change due to the thickness liquid layer and the capillary force introduced by micropillars.For the micropillar surface,microlayer evaporation plays a key role in nucleate boiling heat transfer;The bubble on the the channel micropillar surface shows a sliding movement to side spaces obviously.The bubbles generate in the channel are easily attracted away and merged into an irregular large buble at the micropillar region.