Study On Pool Boiling Heat Transfer Enhancement With Copper Foams

Copper foam has the function of enhancing pool boiling heat transfer as a porous material,which has a wide application prospect in the field of thermal power,electronic,aerospace and so on.In addition to the surface structure feature,wettability has significant influences on pool boiling heat transfer as an important surface characteristic parameter.This thesis mainly studies the influences of copper foam on pool boiling heat transfer performance,including its structure characteristics and wettability.Besides,two-phase flow in the pore structure of copper foam is simulated and the affecting factors are analyzed.Firstly,the copper foam samples with extreme wettability are prepared and the microstructures on the sample surfaces are observed to investigate the relationship between the microstructure and the surface wettability.Petal-like nanosheets are formed on both of the superhydrophobic and superhydrophilic surfaces.The contact between water droplets and the above surfaces are composite contact and wet contact respectively.The optimum concentrations of K₂S₂O₈ and KOH for preparing superhydrophobic copper foam surfaces are0.065mol/L and 2.5mol/L respectively.An experimental instrument is set up to study the pool boiling heat transfer performances of copper foams.The results show that copper foams can enhance boiling heat transfer with large specific surface area and plentiful nucleation sites.However,the resistance of copper foam skeletons to bubbles is detrimental to heat transfer.Therefore,the advantage of copper foam is obvious at low heat flux.With the increase of pore density and thickness of copper foam,the heat transfer area,the number of nucleation sites as well as the resistance to bubbles are increased.Thus there is an optimal pore density and thickness.The optimal pore density is PPI30 and the optimal thickness decreases with the increase of PPI.Among the modified copper foam samples,the superhydrophilic samples have little resistance to bubbles and strong capillary suction which can further enhance heat transfer.The superhydrophobic samples have large resistance to bubbles and numerous easily activated nucleation sites.Their heat transfer performances are gradually weakened with the increase of heat flux.Moreover,copper foams make the boiling hysteresis phenomenon more obvious.The two-dimensional and three-dimensional physical models of gas-liquid two-phase flow in porous media are established by approximating copper foams to porous media.The bubble dynamic behavior in porous media is simulated by the lattice Boltzmann method.The results show that when a bubble moves in well-ordered porous media,its shape changes irregularly and its velocity fluctuates up and down which is related to the gravitational field strength and the wettability of porous media.The bubble deforms more obviously and rises faster with larger gravitational field strength in regular porous media.With the increase of wettability of porous media,the bubble contact angle increases.Meanwhile,the resistance to bubbles decreases and the bubble velocity increases.Conversely,the bubble tends to adhere to hydrophobic skeleton surfaces.With the constant porosity,the increase of average pore size of porous media makes the resistance to bubbles decrease and the bubble velocity increases.