Sintering Mechanism And Boiling Heat Transfer Performance Of Porous Structure

Heat transfer enhancement is a major problem faced by modern science and technology,comparing to single-phase convective heat transfer, gas/liquid two-phase boiling heattransfer can take a lot of heat. It is the main solution to the future of cooling and thermalmanagement. Pool boiling heat transfer does not require external drivers. The heat transfer inthe liquid is mainly through producing bubbles which caused by vaporization and thismovement disturbances surrounding liquid thus enhanced heat transfer. It can take a lot ofheat through two-phase heat transfer during boiling phase transition process to enhancedboiling heat transfer. It also has the potential for high heat flow can be passed. But it still cannot meet the requirements for the average surface. Therefore it is need to come up with a newtype of surface structure to strengthen the boiling heat transfer.Improving surface structure of boiling heat transfer is the main method of enhancementof boiling heat transfer. Excellent surface of enhancement boiling heat transfer should have agreater surface area and more activation cavity. In order to obtain greater expansion area,using sintering mold to produce a sintered porous structure in high temperature conditionsclose together with the substrate by fusion of granular copper powder. Also there are smallpores between particles. Due to the dense core porosity of the porous surface which hasgeometry complex shape can produce more steam channel, and tiny pores can provide a largercapillary forces which make it easier for liquid replenishment. Thereby it makes the gas-liquidinterface form a more complex flow. At the same time a porous structure having a geometryshape can more increase the contact area between solids and fluids, make the porous surfacehaving more core of vaporization. Therefore it can significantly improve the heat transferefficiency of the pool boiling and reduce the boiling superheat. And it can delay critical heatflux (CHF).Sintered porous column composite structure not only increases the contact surface areabetween the liquid and the sintered surface, and the sintered porous columns can also provideadditional tunnels to the liquid while boiling. Thus contributing to liquid is sucked into porousstructure channel. It also has a role in hindering the heating surface forming the coveringvapor film. It can be effectively separated from the gas-liquid relative motion at high heatflux.Sintered porous micro channel composite structure not only increases the surface area ofthe sintered porous structure during boiling, can also provide a steam channel, ensure thesteam to escape from the channel of the slot with less resistance. And it is easy to make bubbles out. While the liquid flows in the porous layer through the action of capillary forces.Thus, it can provide gas-liquid two-phase channel when the liquid is violently boiling,separate gas-liquid relative flow. Gas-liquid flow will become more orderly. So that theboiling heat transfer of the porous surface is more stable.Four kinds of sintered porous structure enhancement of boiling heat transfer are designedand manufactured:1, sintered surface porous tube structure;2, sintered porous interleavedmicro-channel composite structure;3, sintered porous column composite structure;4, sinteredporous Radial Micro channel composite structure. And the heat transfer mechanism of fourkinds of enhancement of boiling heat transfer structure is analyzed.The manufacturing process is analyzed, including mold manufacturing method ofsintered porous structure, processing the substrate, sintering process and sintering method,EDM wire cutting technology and methods. The sintering properties are studied such ascopper powder morphology, sintering thickness, shrinkage, porosity, permeability, thermalconductivity, etc., and the bond strength of sintered porous structure was studied by the waysof ultrasonic vibration. The testing device of boiling heat transfer enhancement experiments isdesigned. The principle of the apparatus and test method are analyzed. Overall experimentaltest platform is build. The collection process of experimental data is introduced. And the erroranalysis of the results of data collection is done. Finally, the experimental tests of the differentsintered porous structure are carried out.The enhancement heat transfer effect of sintered surface porous tube in alcohol reachedabout3-10times the smooth tube. Heat flux and heat transfer coefficient of sintered tube wassignificantly greater than the smooth tube. Sintered tube is easier than the smooth tube intothe boiling state. During the experiment it is found that the smaller the sintered copper powderparticle size, the greater the heat flux of the sintered pipe, the better the heat transferperformance.Sintered porous surface grooving strengthens the effect of boiling heat transfer. The heattransfer coefficient of sintered porous interleaved micro-channel composite structure can beincreased2-3times compared to the smooth surface. Bidirectional cross-channel has betterheat transfer effect than unidirectional micro channel. It can form a stable liquidreplenishment and gas escape network. Therefore, it is a new direction for enhanced boilingheat transfer that machining channels on a sintered porous surface to form a micro-compositestructure, making sintered porous structure more complex and having a three-dimensionalnature.Within the range of0~120w/cm2the measured heat flux density, sintering columnar structures have different properties at the bottom on the basis of the different thickness of theporous layer such as0.5mm or1mm. For a bottom thickness of1mm porous columnarstructure it can enhance heat transfer effect. But as for a bottom thickness of0.5mm porouscolumnar structure，its boiling heat transfer enhancement effect is not obvious.To the porouscolumn structure, whose bottom of the porous layer having a thickness of1mm, porouscolumn height of2mm, the heat transfer efficiency comes up with to1.5times the lattercomparing to the porous layer which having no porous column. But the column height is notas high as possible. At the same time different column diameters were analyzed and foundcolumn diameter having little effect on boiling heat transfer effects.To the sintered porous radial micro channel composite structure, whose bottom of theporous layer having a thickness of1mm, the heat transfer effect of the porous micro channelstructure is significantly better than non-channel porous surface. The effect of differentmicro-channel number and micro-channel angle on heat transfer is analyzed. The more thenumber of micro-channel, the enhancement effect is more obvious. The heat transfer effect of90°cross-channel is better than45°. To the porous micro channel structure, whose bottom ofthe porous layer having a thickness of2mm, the impact of micro-channel depth and width isanalyzed. Experiments show that the heat transfer effect of the depth of2mm micro channel isbetter than that of1mm. The heat transfer effect of the width of0.8mm micro channel is betterthan that of0.4mm. Finally, the bubble shape of sintered porous composite structure duringboiling heat transfer process under different heat flux is analyzed.