| Optical absorption and heat transfer properties of nanofluids are much different fromthose of the base fluid because of the dispersed nanoparticles,and has attracted considerable attentions in recent years.Four types of stable nanofluids were prepared in this study.The impacts of mass fraction on the optical absorption properties of nanofluids were investigated according to the transmittance measured by spectrophotometer,and the factors influencing the nanofluids stability were also analyzed.Finally,the insolation and optical path experiments were performed to investigate the factors affecting the photo-thermal properties of nanofluids.The main conclusions are as follows:(1)The transmittance of MWCNT-H2 O nanofluids decreased with increasing the mass fraction of MWCNTs from 0.0015 to 0.01%,while the variation of transmittance is quite small for a further increase of mass fraction,indicating 0.01wt% is the critical saturation concentration for photo-absorption in this study.Heating treatment is beneficial to improve the optical absorption capability of MWCNT-H2 O nanofluids for lower mass fraction.(2)Compared to deionized(DI)water,the temperature of MWCNT-H2 O nanofluid was increased about 14.8 °C at the optimal mass fraction of 0.01% after a light irradiation of 45 min.Additionally,the photo-thermal conversion performance was enhanced with increasing the light irradiation cycle at lower concentrations of nanofluid,but it is opposite at higher concentrations due to the CNT agglomeration.(3)The photo-thermal conversion performance of Cu O-H2 O nanofluids were enhanced with increasing the mass fraction.Compared to DI water,the maximum temperature of the Cu O-H2 O inside the single-layer glass tube was increased about 6.0 ?,while the double-layer vacuum glass tube was increased about 9.2 ?.The optical path had greatly influence on the temperature of the nanofluid and the nanofluid temperature decreases as the distance from the liquid surface increases.The addition of Cu O nanoparticles resulted in a higher temperature of the upper fluid and lower temperature of the botom fluid.Temperature difference between surface and the deepest is 36.3 ? after a light irradiation about 45 min.At 1 cm,the nanofluids temperature increases as the concentration increases,instead,it decreases as the concentration increases at 9 cm.At 1 cm,the maximum temperature difference between nanofluid and basefluid is 6.8 ?and the collector efficiency was improved by about 30.4%.(4)The temperature of the mixed nanofluids increases with the increasing of CNTs massfraction at the mass fraction of Cu O 0.01% and 0.025%;however,when the mass fraction of Cu O is 0.05%,0.1% and 0.25%,the temperature increases first and then decreases with the increasing of the CNTs mass fraction.The addition of MPCMs weakens the light absorption properties of MWCNT-H2 O nanofluid,and the temperature rise rate of mixed nanofluids decreases with the increasing of MPCM mass fraction.(5)Outdoor experiments found that the whole process can be divided into rapid warming,slowly warming to steady phase and cooling stage.It was found that all of the three nanofluids have an optimal concentration.MWCNT-H2 O nanofluid is 0.01%consistent with indoor experiments;however,Cu O-H2 O nanofluid is 0.1% and mix nanofluid is 0.0005% CNT and 0.05% Cu O,which is different from the value of indoor experiments.The differences between simulated sunlight and real sunlight,and the short light irradiation time in the indoor experiments are the main reasons for these results. |
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