Preparation And Pool Boiling Performance Of Gradient Porous Copper Structure

Boiling is a common phase-change heat transfer process in daily human life.Since the middle of the 20th century,with the miniaturization and high integration of electronic components,heat dissipation becomes a problem and has attracted more and more attention.Boiling heat transfer is considered as an efficient and stable heat transfer method because of its huge latent heat change in the phase change process,low superheat,high heat transfer coefficient（HTC）,and small temperature difference.Therefore,it is widely used in aerospace,nuclear reactors,energy recycling,refrigeration machinery,high-power electronic components and so on.In this paper,a simple electrochemical cathode deposition method was used to prepare uniform honeycomb porous copper surfaces and porous copper surfaces with a radial diameter gradient.For uniform porous surface,the effects of different deposition conditions（current density,deposition time）on surface properties of structures（pore diameter,thickness and morphology）was investigated.For a porous surface with a radial diameter gradient,the effects of different injection speeds and the heights of the needle from the cathode on the gradient range of the gradient porous structures were studied.The results show that as the current density and deposition time increase,the overall average pore diameter and deposition thickness of uniform porous structures increase gradually.For the porous structures with a radial diameter gradient,the height of the needle from the cathode has little effect on the gradient range,which is about 60¹20μm.The pore diameter gradually increases from the center to the edge.The scanning electron microscope（SEM）images show that the pore walls of the honeycomb porous structures are composed of dendrites,which can provide additional nucleation sites and enhance liquid absorption for boiling heat transfer.Gradient porous structure with a larger gradient range（Sample#G1）was selected for the following experiments.The average pore diameters of the uniform porous structures Sample#U1,Sample#U2 and Sample#U3 in the control group are about 50μm,80μm and 120μm,respectively.A pool boiling experiment system was built to conduct pool boiling experiments on radial gradient structures,uniform structures,and pure copper surfaces.Moreover,a high-speed camera was used to study bubble dynamics.For the uniform structures,the larger pore diameters they have,the lower superheat and the higher heat transfer coefficient they have.For the porous structures with a radial diameter gradient,especially on high heat flux conditions,Sample#G1 can further increase the heat transfer coefficient（HTC）.Sample#G1 has the highest Maximum saturated boiling heat transfer coefficient(h_MNB),which is 16.1 W cm^–22 K^–1,1.3 times that of Sample#U3,1.5times that of Sample#U1,3.5 times that of then plain copper surface.The heat transfer mechanism was analyzed from the four boiling processes.For the rewetting process,the porous structures with a radial diameter gradient have superior boiling performance than the uniform structures.Sample#G1 has a smaller porous diameter at the center than at the edge,so the porosity is also radially distributed.The wickability at the center is stronger,promoting the water’s replenishment from the edge to the center.The capillary wicking experiment confirmed this assumption.The K/R_eff of Sample#G1 is 2.89,1.1 times that of Sample#U3,2.3 times that of Sample#U1.It means that the porous surfaces with a radial diameter gradient have the superior rewetting ability.For the nucleating process,compared with the pure copper surface,the honeycomb-like porous structures can reduceΔT_ONB and q_ONB.The honeycomb-like porous structures have a large number of pores and dendrites on the pore wall,which are easy to activate the nucleation of bubbles.Besides,the sample with the radial diameter gradient has the lowerΔT_ONB and q_ONB than the samples with the uniform diameter,which means the radial gradient porous structure can accelerate the arrival of the onset of nucleate boiling.The bubble dynamics during the bubble growth and departure process showed that the bubble detachment diameter of Sample#G1 is larger than that of the uniform structures at 2 w·cm^-2（up2）.Because of the equivalent bubble growth period T,the larger bubble detachment diameter,the faster bubble grows.Therefore,the radial gradient structures can take away the heat faster and enhance the boiling heat transfer.