Experimental Investigation On Boiling Heat Transfer Enhanced By Gradient Aperture Porous Copper Packed Bed

With the development of technology,electronic communication,military aerospace and other fields involving all kinds of electronic devices are facing the situation of increasing device integration,sharply increasing power consumption and heat flux.As a result,the thermal management of devices has become an important bottleneck in the development of microelectronic technology.In order to ensure the stability of electronic products,people pay more and more attention to efficient cooling technology.Among them,the phase change heat dissipation technology represented by pool boiling has a very high cooling capacity and has a good application prospect in high heat flux density electronic devices.The boiling heat transfer is closely related to the surface morphology of the heat tran sfer surface.The surface microstructure plays a regulatory role in the process of liquid replenishment and steam removal,which directly affects the boiling heat transfer effect.However,the three-dimensional porous structure used to enhance the pool boiling heat transfer will be affected by the steam resistance,which leads to the accumulation of the vapor and the difficulty in replenishing the working fluid,thus weakening the pool boiling heat transfer performance.In order to promote the departure of vapor and the supplement of working fluid in the three-dimensional porous structure,we prepared the porous copper packed bed with open holes,and compared the boiling heat transfer performance of single-layer and multi-layer porous copper packed bed with gradient pore size.The pool boiling curves and bubble dynamics of the samples were experimentally studied to obtain the mechanism of enhanced boiling heat transfer.Firstly,porous copper packed bed surface was prepared by the space holder-copper powder sintering method,and the water absorption experiment was carried out.The experimental results show that the samples prepared in this work have good water absorption capacity because of the high capillary force.Among them,liquid absorption speed of 4-layer sample is faster than that of 2-layer sample,which is related to the smoother change trend of pore apreture.Good liquid absorption ability can ensure that the working fluid can be replenished in time in the experiment to avoid the dry out phenomenon.In addition,the SEM photographs of the side and bottom of the four-layer sample were analyzed.The results demonstrate that the samples are obviously stratified,highly porous and interconnected.During the preparation process,in addition to the atmospheric pores left by the removal of space holder(Na Cl)particles,the sintering neck between copper powder particles will also form smaller pores.The small holes between the sintering necks are used as minor channels to connect the large holes,which provide a strong liquid absorption capacity for the porous samples.Secondly,a pool boiling heat transfer experimental platform was designed and manufactured.The platform consists of a heating system,boiling chamber,cooling system,visualization and data acquisition system,which can accurately adjust the heating power and study the pool boiling heat transfer characteristics of each sample under different heat flux.The reliability of the experimental platform and the errors of various physical quantities involved are analyzed.The results show that the experimental platform has good thermal insulation,convenient visualization and low error,which can meet the experimental requirements.Finally,the pool boiling heat transfer of the porous copper packed bed w as studied by using deionized water as working fluid at atmospheric pressure.The effects of sample thickness,porosity,aperture,number of layers(pore size change tendency),heat flux and wall superheat on the pool boiling heat transfer were studied,an d the visual bubble dynamics analysis was carried out.The results show that the porous copper packed bed with 2 mm thickness and 70%porosity has better heat transfer effect.Among all the samples,the four layer gradient sample has the best heat transfer effect,the nucleate boiling starting point(about 1℃)is only about 8%of the smooth surface(about 12℃),and the maximum heat transfer coefficient is about 4×10~4W/m~2·K,which is mainly due to the gradient pore structure can promote gas escape and liquid replenishment.In addition,the porous structure provides a large number of nucleation sites and has a strong disturbance effect on the flow field.In terms of bubble dynamics,the bubble departure diameter of the 4-layer sample is almost 50%of that of the smooth surface,and the departure frequency is twice of that of the smooth surface.The reason for this phenomenon is that the gradient pore structure can meet the different pore size requirements of gas escape and liquid supplement.Considering its simple preparation and excellent pool boiling performance,we believe that the gradient pore size porous copper can provide an effective way to enhance boiling heat transfer.