Study On Enhanced Nucleate Boiling Heat Transfer With Composite Porous Structure Driven By Liquid-phase Reflow

The boiling heat transfer enhancement technology of porous media is one of the focus in the future energy-saving field.In the process of gas-liquid phase change heat transfer,micro-nano scale porpus structure comes into focus because of effectively bubble nucleation and driving liquid reflow.The research object is to strengthen the phase change heat transfer technology on the porous surface,using the great advantage of reducing the wall superheat and inhibiting film boiling,etc.Constructing metal porous structure as a substrate,studing hydrophilic/hydrophobic functional partitioning to drive liquid reflow,and building capillary structure on copper foam strengthen boiling heat transfer by forming a micro-nano capillary/porous composite structure.The study of wettability and capillary force as liquid phase driving force to strengthen the boiling heat transfer.Further more,the study could enrich and improve the boiling heat transfer mechanism,provide technical support for the realization and implementation of the goals of"emission peak"and"carbon neutrality".To investigate the effects of hydrophilic and hydrophobic regions on the bubble growth characteristics,period and detachment diameter,a surface structure with a porous structure as the substrate and hydrophilic-hydrophobic regions distributed at intervals was designed.The porous surface of hydrophobic functional partitioning was produced by secondary sintering,and the boiling performance of the surface was tested by using DI water as working fluid.The results showed that onset of nucleation boiling is significantly lower on the hydrophobic functional partitioned porous surface,which is 6 K lower than that of the smooth surface and about 1 K lower than that of the conventional hydrophilic porous surface.The heat transfer coefficient in the boiling preliminary stage(heat flux below 250 k W/m~2)is about 20%higher than that of a completely hydrophilic surface.The heat transfer performance at higher the heat flux(more than 350 k W/m~2)is comparable to that of a hydrophilic porous surface;the critical heat flow density is 400 k W/m~2 higher than that of a plain surface,which is 3.1 times higher than that of a plain surface.The hydrophobic structure can reduce the nucleation barriers for bubble formation,which can produce bubble nucleation at lower superheat.Further more,the hydrophilic structure has high capillary suction,which can promote liquid reflow,and the micro-liquid layer can be rapidly replenished with liquid after evaporation,delaying the onset of membrane boiling.The synergistic effect of hydrophilic and hydrophobic porous structures strengthens the boiling heat transfer.Two kinds of morphological characteristics capillary composite copper foam porous structures were designed to investigate the effects of the micro-nano scale on bubble generation,growth,detachment and liquid reflow during boiling heat transfer.A porous structure composed of micro-nano scale pores was prepared by depositing linearly structured carbon nanotubes using copper foam as the substrate.The boiling heat transfer performancesv was investigated with DI water as the working fluid.The results showed that the critical heat flux of the composite porous surface of carbon nanotube/copper foam is 238 k W/m~2 higher than that of copper foam,and 44%higher than that of copper foam,and it is 595 k W/m~2higher than that of plain surface,and 4.1 times higher than that of plain surface.The boiling heat transfer coefficient is about 35%higher than that of copper foam,and 3 times higher than that of plain surface.The presence of carbon nanotubes/copper foam composite porous surface due to the intertwining of carbon nanotubes to form nano-scale mesh pores which can strengthen the surface capillary suction,accelerate the liquid reflow,delay the film boiling phenomenon and increase the critical heat flux.The presence of carbon nanotubes agglomerate to form micro-clusters which can promote the increase nucleation sites.The synergistic effect of micro and nano scale effectively enhances the boiling heat transfer.Spherical titanium dioxide nanoparticles were deposited onto a copper foam skeleton to construct a micro-nano composite porous surface and to investigate its the boiling heat transfer performances with DI water as the working fluid.Th results showed that the highest critical heat flux of the composite surface is 717 k W/m~2,which is 170 k W/m~2higher than that of the copper foam surface,improve about 31%than that of the copper foam surface.The heat transfer coefficient is 150%than that of the copper foam.Similarly,with the synergistic effect of micro-nano scale structures,the micron-scale structure facilitates bubble nucleation and the nano-scale capillary structure accelerates liquid reflow,both of them synergistically enhance the boiling heat transfer performance.