Study On Enhanced Pool Boiling Heat Transfer Characteristics Of Gradient Porous Copper And Its Composite Modified Copper Mesh Surface

With the continuous development of modern science and technology,the production and application of electronic products are more and more extensive.In the past three decades,various electronic devices have shown the trend of miniaturization and high density,which has also produced many new problems,and the heat dissipation of electronic components is an important research topic.Pool boiling heat transfer,a phase change heat transfer mode,is more intense in the process of heat transfer,has higher heat transfer efficiency,and has broad application prospects in the field of heat dissipation.There are many factors that affect pool boiling heat transfer,such as the surface properties of heat exchanger and the structure of heat transfer materials.Most of the current research focuses on foam metal,and little attention has been paid to powder sintered porous metals and composite porous metals.Because dense pores and small pore diameter can provide sufficient bubble nucleation points for pool boiling heat transfer,metal nanoparticle formed porous copper is a new porous metal material to enhance boiling heat transfer in recent years.In this thesis,graded porous copper with metal nanoparticles with different thickness and porosity was prepared to reveal the effects of porous copper thickness and porosity on its pool boiling heat transfer.After obtaining the optimum thickness and porosity of gradient porous copper,the copper mesh is further coated on the surface of porous copper,and the copper mesh is hydrophobically modified.The influence of surface coating on pool boiling heat transfer is explored,and the mechanism of strengthening heat transfer is further revealed through bubble dynamics analysis.Firstly,a pool boiling test-bed was designed and built by ourselves.The test bed is composed of heating system,condensation heat exchange system and data image acquisition system.The test bed meets the functions of controllable heating power and real-time data acquisition,which can better complete the steps required by the experiment.After assembly,the reliability of the test-bed was tested.The results show that the error of the test-bed is small and the test bed meets the experimental requirements.The gradient porous copper was prepared by cold pressing method and vacuum sintering method,and its surface was characterized.The characterization results show that the larger the porosity is,the more the pores are connected with each other,so that a highly porous structure is formed in the porous copper.The gradient porous copper has obvious stratification phenomenon due to the different diameter of pore forming agent.This stratification conforms to the law of bubble diameter from small to large in the process of rising inside.The results show that the pool boiling heat transfer performance of the gradient porous copper is the best when the thickness is 2 mm and the porosity is 80%.The ONB is only 2.1℃,which is nearly 8℃smaller than that of smooth copper surface.The minimum bubble separation diameter is nearly 20%smaller than that of smooth surface.Its diameter is about 2.6 mm,and the maximum bubble separation frequency is about 1.3 times that of smooth surface,reaching 30 Hz.This is mainly due to the nucleation sites of porous copper are distributed in the porous structure,which can be activated at the beginning of pool boiling to produce bubbles to take away heat,and the gradient structure is conducive to the rise and escape of bubbles and reduce the escape resistance of bubbles.On the basis of gradient porous copper,copper mesh with different pore sizes was further covered on its surface,and the copper mesh was modified and hydrophobic treated.The results show that the surface of the modified copper mesh is uneven due to Fe Cl₃ etching,which is easy to produce a large number of nucleation sites,and can be activated in the early stage of boiling to produce bubbles.Compared with the porous copper of unmodified surface composite copper mesh,the porous copper of surface composite copper mesh modified by surface copper mesh has strong hydrophobic ability,and the contact angle increases by about 75°to 101°.Pool boiling experiments show that in the stage of high heat flux,the convective heat transfer coefficient of surface composite copper mesh porous copper is higher than gradient porous copper,and the surface composite copper mesh porous copper with 100 mesh copper mesh shows the best pool boiling heat transfer performance in this study,and its convective heat transfer coefficient can reach 7.3×10⁴ W/m²·K。The copper mesh effectively prevents the bubble merging,reduces the bubble separation size,accelerates the bubble escape,and increases the bubble separation frequency.Compared with the porous copper without copper mesh,the bubble separation diameter of the porous copper with copper mesh decreases and the bubble separation frequency increases,in which the smallest bubble separation diameter decreases by nearly 30%,The maximum bubble separation frequency increases by 50%,and the maximum frequency can reach 40 Hz.As there are nucleation sites on the surface of copper mesh,with the increase of heat flux,these nucleation sites are gradually activated in the later stage to produce bubbles to take away heat.When the surface of the copper pressure net stacking bed is hydrophobic copper net,its ONB decreases by about 30%to 1.6℃,the hydrophobic composite porous copper begin to appear bubbles when the heat flux is about 8×10⁴W/m²,which is lower than the composite porous copper with non-hydrophobic surface.Thanks to the large roughness of the hydrophobic copper mesh,a large number of nucleation sites on its surface can be activated at the beginning of boiling.In the later stage of boiling,due to the hydrophobicity of its surface,bubbles are more likely to stay on the surface and form a gas film,which increases the bubble escape resistance.Therefore,when q>5×10⁵W/m²,the pool boiling performance of the composite porous copper with the hydrophobic copper mesh is weakened in the later stage.