Experimental Study On Effect Of Surface Modification On Pool Boiling Heat Transfer Characteristics In Metal Foam

Metal foam is a kind of heterogeneous composite porous material with a large number of continuous or discontinuous pore structure,which is based on metal or alloy.It has the characteristics of large specific surface area,high porosity and complex three-dimensional structure Based on the boiling heat transfer enhancement by surface modification,the surface modified metal foam is expected to be used in the cooling and cooling of Very-large-scale integration chips.In order to clarify the pool boiling heat transfer characteristics in surface modified metal foam,an experimental rig was set up.Hydrophobically and hydrophilically modified metal foams were prepared by the combination of alkali-assisted oxidation and Molecular self-assembly technology.The pool boiling heat transfer characteristics of the surface modified metal foams were measured experimentally Based on the experiments,the correlation of pool boiling heat transfer coefficient in metal foam was developed.The main results are as follows:(1)As the pore density increases from 5PPI to 40PPI,the boiling heat transfer in metal foam pool is enhanced by about 20%to 25%;as the porosity of the metal foam decreases from 95%to 85%,the pool boiling heat transfer coefficient of water in metal foam is reduced by 30%～35%.When using R141b refrigerant as working fluid for pool boiling heat transfer,the critical heat flux density increases by 21.5%as the pore density increases from 5PPI to 40PPI.As the porosity of the metal foam decreases from 95%to85%,the critical heat flux density decreases by 8.15%.As the thickness of the metal foam increases from 4mm to 8mm,the metal foam’s ability to strengthen the pool boiling heat transfer is firstly strengthened and then weakened.When the thickness is about 6mm,the heat transfer coefficient and the critical heat flux reach the maximum value,and the metal foam strengthens the heat transfer capacity optimally.The pool boiling heat transfer performance of water and R141b refrigerant in metal foam are both enhanced as pore density increases.But with the increase of the porosity,they show the opposite change:the pool boiling heat transfer performance of water decreases while the refrigerant pool boiling heat transfer performance increases.(2)The pool boiling heat transfer characteristics in hydrophobic modified metal foams were investigated experimentally.The effect of hydrophobic modification on the incipient boiling superheat of pool boiling heat transfer in metal foam decreases with the increase of pore density and porosity.The pool boiling heat transfer coefficient can be increased by 3%-30%by hydrophobic modification,and the enhancement effect of hydrophobic modification on pool boiling heat transfer is gradually weakened with the increase of heat flux density.When the heat flux is lower than 4×10~5W/m~2,the effect of hydrophobic modification on heat transfer of metal foam with pore density of 40PPI is the best,while the effect of hydrophobic modification on metal foam with pore density of 20PPI is the highest when the heat flux density is higher than 4×10~5W/m~2.The porosity has no significant effect on the incipient boiling superheat.Hydrophobic modification cannot increase the critical heat flux of pool boiling heat transfer in metal foam.(3)The pool boiling heat transfer characteristics in hydrophilic modified metal foams were investigated experimentally.The pool boiling heat transfer characteristics of hydrophilic and hydrophobic modified metal foams are consistent with the structural changes of the metal foams.The heat transfer performances increase with the increase of pore density and porosity Hydrophilic modification has no significant effect on the incipient boiling superheat,and the enhanced heat transfer ability was lower than that of hydrophobic modification at low heat flux.The pool boiling heat transfer coefficient can be increased by 0%-15%with the increase of the heat flux,and the enhancement of pool boiling heat transfer is enhanced with the increase of the heat flux The enhanced heat transfer capacity at high heat flux is higher than that of hydrophobic modified metal foams.Hydrophilic modification can increase the critical heat flux of pool boiling heat transfer in metal foam by 40%.(4)A correlation was developed to predict the boiling heat transfer coefficient in surface modified metal foams.For 90%of the experimental data,the prediction deviation of the correlation is within±30%,and the average deviation is 13.4%.