Experimental Study On The Effect Of Nanofluids On Boiling Heat Transfer Of Downward Facing Heating Surface

In the late stage of core melted in severe accident,the molten core was relocated to the lower head of the reactor pressure vessel,threatening the integrity of the pressure vessel.The in-vessel retention through external reactor vessel cooling(IVRERVC)program is an important mitigation measure for the severe accidents,which has been currently adopted by many large-scale Advanced Pressure Water Reactors(APWRs).Critical heat flux(CHF)is the most important phenomenon for ERVC.In this paper,based on the detailed analysis of the methods of increasing the CHF at home and abroad,the enhancement of heat transfer and the improvement of the CHF for the downward facing heating surface are experimentally studied by using nanofluids instead of traditional coolants.In this study,heat transfer characteristics of four water-based nanofluids,including alumina,Boehmite,graphene and carbon nanotubes,were mainly investigated.Firstly,nanofluids with different concentrations were prepared by two-step method,and then the dispersion of nanoparticles was detected by ultraviolet spectrophotometry.Then an experimental study on pool boiling of the downward facing heating surface was carried out with a block heating section under atmospheric pressure.The main experimental variables were the type and concentration of nanofluids,and the influence of nanofluids on the heat transfer mechanism and CHF was discussed by analyzing the changes of surface roughness and surface wetting ability after the experiments.The heat transfer mechanism of nanofluids is discussed and analyzed.The experiments results show that,under certain mass concentration cases,the four kinds of nanofluids can effectively enhance the heat transfer.The graphene nanofluid has the best CHF enhancement when the mass concentration is 10 mg/L,which increases by 76.1%compared with reverse osmosis(RO)water case.Due to the poor stability of carbon nanotube nanofluids,the nanoparticles are easy to agglomerate in the boiling process.Only at a mass concentration of 8 mg/L,the CHF enhancement was measured,which increased by 17.6%compared with the RO water condition.The influence of nanofluids on heat transfer mainly includes the deposition of nanoparticles,the synergistic effect between solid and liquid phases of nanofluids,the morphology of nanoparticles,Brownian motion and thermophilic effect.In contrast,the deposition of nanoparticles on the heated surface has the greatest influence.