Experimental Study On Pool Boiling Heat Transfer Characteristics Of Surface Modified Porous Copper

The reasonable and effective heat dissipation of electronic equipment is one of the most challenging global problems in the 21st century.Pool boiling phase change heat transfer technology has great potential in solving the heat dissipation of electronic components with small area and large heat flux due to its high efficiency.In this paper,a reasonable pool boiling heat transfer experimental device is designed and built.The copper foam material is selected,the gradient pore density is constructed,and the hydrophilic surface is modified.The influence of the thickness,pore density,hydrophilicity and gradient of the copper foam on its pool boiling heat transfer performance was explored;the porous copper mesh material was selected to explore the effect of super-hydrophilic surface modification on the vertical liquid absorption performance of the multilayer copper mesh;and explored the influence of the number of layers and hydrophilicity on the pool boiling heat transfer performance.The specific research contents are as follows:（1）Selected porous copper foam samples with different pore density,thickness,and wettability,and carried out single-layer porous copper foam pool boiling heat transfer experiments.The results show that the use of high density copper foam can provide a large number of potential vaporization cores,which significantly advances the time when the copper foam sample enters nucleate boiling.However,with the increase of copper foam’s thickness,the bubble detachment resistance increases significantly and the boiling heat transfer performance decreases.In this experiment,the best pore density of 2mm copper foam is 80PPI;the best pore density of 3mm copper foam is 60PPI;the best pore density of 4mm copper foam is 40PPI.The super hydrophilic copper foam sample was treated by chemical oxidation,which delayed slightly at the beginning of nuclear boiling heat transfer.However,with the continuous boiling process,the advantage of super hydrophilicity is fully reflected in the second half,which speeds up the bubble separation process,takes more heat,has better boiling heat transfer performance,and obviously delays the emergence of boiling crisis.In this experiment,the best pore density of 2mm super hydrophilic copper foam is 80PPI;the best pore density of 3mm super hydrophilic copper foam is 80PPI;the best pore density of 4mm super hydrophilic copper foam is 40PPI.The method of super hydrophilic treatment can increase the optimal pore density of the same thickness of copper foam to a certain extent.（2）Under the condition that the lower layer is uniformly 1mm,110PPI copper foam,the upper layer selected porous copper foam with different pore density,thickness and wettability to construct double-layer vertical gradient copper foam samples,and carried out pool boiling heat transfer experiments.The results show that the gradient copper foam sample is well adapted to the movement characteristics of bubbles in the skeleton of the copper foam.In the whole boiling heat transfer process,the copper foam samples with gradients are better than those without gradient,and obviously delays the emergence of boiling heat transfer crisis.For the gradient copper foam with 3mm and4mm,the smaller the upper pore density and the larger the pore size,the better the boiling heat transfer effect.For the super hydrophilic gradient copper foam,the best pore density of the upper copper foam will increase with the total thickness of 3mm due to the excellent bubble detachment performance on the surface.（3）The 1,3,5,10 layers of copper mesh samples prepared by 400 mesh porous copper mesh were selected for super hydrophilic surface modification.The experimental platform was built and carried out the vertical liquid absorption experiment of multilayer copper mesh.The results show that super hydrophilic treatment can significantly enhance the vertical liquid absorption capacity of multilayer copper mesh,and the more layers,the stronger the vertical liquid absorption capacity of super hydrophilic modified multilayer copper mesh.From the same starting time,the maximum wetted area ratios of 1,3,5 and 10 layers of super hydrophilic copper mesh samples are 15%,47%,91%and 98%at 120s,480s,660s and 660s,respectively.Moreover,through the infrared camera observation,the lowest temperature point of the whole experiment always appeared on the surface of the super hydrophilic 10-layer copper mesh sample,and in the whole experiment,the lowest temperature decreased by 1.6℃.（4）The 1,3,5,10 layers of copper mesh samples prepared by 400 mesh porous copper mesh were selected for super hydrophilic surface modification and carried out the multilayer copper mesh pool boiling heat transfer experiment.The results show that the boiling heat transfer performance of 5-layer copper mesh is the best,and the peak boiling heat transfer coefficient reaches 73655 W·m^-2·K^-1,which is 2.1 times of smooth copper surface boiling heat transfer coefficient under approximate heat flux.The boiling heat transfer coefficient of 10-layer copper mesh is slightly lower than that of5-layer copper mesh,and that of 3-layer copper mesh and 1-layer copper mesh is relatively poor.Compared with the samples without super hydrophilic treatment,the boiling heat transfer of the samples with super hydrophilic treatment is weakened in varying degrees,and the more layers there are,the more obvious the weakening effect is.In the super hydrophilic copper mesh samples,the boiling heat transfer effect of 3-layer copper mesh is the best,but it is weaker than that of smooth copper surface at high heat flux,far less than that of 5-layer copper mesh without super hydrophilic treatment.This shows that the strong liquid absorption capacity of super hydrophilic sample increases the bubble detachment resistance,the bubble separation is blocked,and the boiling heat transfer performance decreases to a certain extent.However,from the boiling heat transfer curve in this experiment,it can be found that no obvious deterioration of boiling heat transfer occurs in four different layers of copper mesh samples,no matter whether they are super hydrophilic or not,indicating that porous copper mesh is good material for enhancing pool boiling heat transfer and has a good engineering application prospect.