Study On Boiling Heat Transfer Characteristics Of Surfactant-modified Nano-refrigerants In Microchannel

Microchannel heat exchangers are widely used in the field of electronics industry,heat exchanger and so on.While strengthening the heat transfer is one of the effective ways to use some additives to change the physical properties of the experimental medium,it is the fact that the shortage of research works on adding different surfactant-modified nano-refrigerants into the flow boiling heat transfer of microchannel at present.Therefore,it is of great significance to explore the phase change heat transfer characteristics of surfactant-modified nano-refrigerants in microchannels.Firstly,the effects of three different types and concentration of surfactants on the dispersivity of nano-refrigerants were studied.Two step methods were used to prepare different concentrations of anionic SDBS-Al₂O₃/R141b,cationic CTAB-Al₂O₃/R141b and non-ionic Span80-Al₂O₃/R141b as surfactant nano-refrigerants?0wt%,0.1wt%,0.2wt%,0.3wt%,0.4wt%?.Based on the measured particle size and zeta potential by using nano-particle analyzer,the optimal dispersion concentration and the suboptimal dispersion concentration of six kinds of surfactant nano-refrigerants are obtained.And the TEM photographs are taken under the projective electron microscope for each the experimental medium.The results shows that the dispersion order of size is 0.3wt%CTAB,0.4wt%CTAB,0.2wt%Span80,0.1wt%Span80,0.2wt%SDBS and 0.3wt%SDBS.Secondly,the above six mediums and pure nano-refrigerants are seen as experimental samples.And the flow boiling experiments are carried out in microchannel with hydraulic diameter of 1.33mm.The experimental results show that the maximum heat transfer coefficient of the saturated boiling zone can be increased by 30.9%,22.8%,18%,12.6%,11.1%and 7.49%when the surfactant concentration is 0.2wt%SDBS,0.3wt%SDBS,0.3wt%CTAB,0.4wt%CTAB,0.2wt%Span80 and 0.1wt%Span80.Obviously,there is the best surfactant type and concentration.Considering the influence of surfactants,the surfactant strengthening factor SEF is introduced and a new correlation is proposed to predict the boiling heat transfer coefficient based on surfactant-modified nano-refrigerant in the microchannel.There are some aspects affected SEF by theoretical researches:1)the surfactant can reduce the surface tension of the liquid and promote the bubbles detachment.2)when a small amount of nano particles deposited in the surfactant nano-refrigerants,the hydrophilicity of heat transfer wall is enhanced,the bubble detachment diameter is reduced further and the bubble detachment frequency is increased.Meantime,the roughness of the heat transfer wall can be increased,the density of the vaporization core is increased and the disturbance is enhanced to strengthen the heat transfer.3)the larger concentration of surfactant can lead the viscosity of the experimental medium to increase,inhibit the micro convection on the heating wall and increase the superheat required in the process of phase change heat transfer,which is not conducive to heat transfer.Finally,it is found that the surfactant nano-refrigerants added into the fluid can reduce the two-phase friction pressure drop and there is little effect on the fluctuation of pressure drop in import and export.Besides,in the low heat flow density,adding the surfactant nano-refrigerant can not only increase the fluctuation range of the pressure difference slightly,but also improve the critical heat flux?CHF?.In brief,surfactants can improve the phase change heat transfer characteristics in microchannel.