Experimental Study On Boiling Heat Transfer Properties Of Graphene-related Nanofluids

This thesis designed and built with condenser closed loop of boiling heat transfer experiment device,the boiling heat transfer performance of graphene based nano fluid are studied,including the critical heat flux(CHF)and boiling heat transfer coefficient(HTC),etc.,and by using high speed camera on bubble nucleation,growth,from the form of visual research;Scanning electron microscopy(SEM)was used to study the deposition morphology of the heated surface.The contact angles of different liquids were studied by using the contact Angle measuring instrument,and the factors affecting the boiling heat transfer performance of nanofluids were explored by using bubble morphology,surface morphology and contact angles.The boiling heat transfer properties of Si O₂,GNs,Si O₂/GNs,Fe₃O₄-GNs,Fe₃O₄-GNs/Al₂O₃ and Fe₃O₄-GNs/Si O₂ nanofluids at 0.0005wt%,0.001wt%,0.002wt%and 0.003wt%are all higher than those of ultra-pure water.When the mass concentration of Si O₂ and GNs nanofluids is 0.002wt%,the boiling heat transfer performance is the best,and CHF is 135.89W/cm² and 151.73W/cm²,respectively,which is23.4%and 37.8%higher than that of CHF(110.09W/cm²)in super pure water.The maximum HTC values were 5.90W/(cm²·K)and 7.12W/(cm²·K),respectively,which were 15.7%and 39.6%higher than the maximum HTC values of 5.10W/(cm²·K)for ultra-pure water.The CHF and maximum HTC of Si O₂/GNs,Fe₃O₄-GNs,Fe₃O₄-GNs/Al₂O₃ and Fe₃O₄-GNs/Si O₂ nanofluids increased with the increase of mass concentration and reached the maximum value at 0.003wt%.CHF was 163.94W/cm²,170.21W/cm²,180.23W/cm² and 185.29W/cm²,respectively,which were 48.9%,54.6%,63.7%and 68.3%higher than that of ultra-pure water.The maximum HTC values were 8.17W/(cm²·K),8.78W/(cm²·K),9.73W/(cm²·K)and 10.23W/(cm²·K),respectively,which were 60.2%,72.2%,90.8%and 100.5%higher than the maximum HTC values of ultra-pure water.Different nanoparticles have a great influence on the boiling heat transfer performance,and with the increase of mass concentration,the growth range of boiling heat transfer performance of different nanofluids has a great difference.For functionalized graphene nanofluids,when the mass concentration is lower than 0.002wt%,the boiling heat transfer performance of nanofluids is significantly affected by the concentration.When the concentration was higher than 0.002wt%,it was less affected.At the same concentration,the boiling heat transfer performance of Fe₃O₄-GNs/Al₂O₃ and Fe₃O₄-GNs/Si O₂ mixed nanofluids has a small difference,but the boiling heat transfer performance of both is significantly higher than that of Fe₃O₄-GNs nanofluids alone.In this thesis,the factors affecting the boiling heat transfer performance of different nanofluids were explored through the surface morphology,contact Angle and bubble morphology.The original morphology of the heated surface is constantly changed during the boiling process of the nanofluid.The deposition of nanoparticles adds many irregular concave and convex structures on the original smooth surface,and increases the density of the vaporized core,thus increasing CHF and HTC.Among them,the two-dimensional layered structure formed by GNs deposition significantly improves the thermal conductivity of the heating surface and can reduce the temperature of local hot spots.The deposition of Fe₃O₄-GNs nanoparticles on the heated surface formed many gaps and grooves,forming a dense porous medium,which significantly increased the surface roughness.Compared with Fe₃O₄-GNs nanofluids,the Fe₃O₄-GNs mixed with oxide nanofluids deposited with more spherical small particles,forming more grooves and gaps,the void density of honeycomb structure is higher,resulting in more vaporization core points,the heating surface and the contact surface area of the nanofluids increased significantly,and the capillary absorption effect was stronger.After boiling,Si O₂,Fe₃O₄-GNs and their mixed nanofluids with oxides have reduced contact Angle and enhanced hydrophilicity,which prolongs the drying time of liquid layer,thus improving CHF.However,the phenomenon that the hydrophilicity of GNs deposited surface decreases while the boiling heat transfer performance increases indicates that the wettability is not the decisive factor affecting the boiling heat transfer performance.In addition,nanofluids boiling after deposit the heating surface more emphasis,vaporization divided bubble,which changes the bubble size,and gathered in the bubble at the bottom of the nanoparticles increases the bubble at the bottom of the liquid layer,the structure of the separation pressure in the bubble diameter decreases,and improve the bubble generation and from frequency,is also one of the reasons for HTC and CHF increases.