Experimental Study On Thermal Properties And Photothermal Conversion Properties Of MWCNTs/Novec Nanofluids

Compared with traditional thermal fluids,nanofluids had higher heat conductivity and heat transfer ability,which had been a hot topic in research.However,the research on photothermal conversion was mostly limited to single equipment of heat exchanger and a few of them focused on the whole heat exchanging system.Based on Novec7100,the multi-walled carbon nanotube nanofluid（MWCNTs-N）was prepared.The stability,viscosity and thermal conductivity of MWCNTs-N were investigated,and the photothermal conversion performance of MWCNTs-N in the whole system was investigated through a self-built experimental platform and TRNSYS simulations.MWCNTs-N nanofluids were prepared by two-step method,and the stability of nanofluids was investigated.Like many other nanofluids,the stability of MWCNTs-N could be enhanced by dispersants and ultrasonic oscillations.The stability difference of Novec7100,ethylene glycol and deionized water was analyzed from the theory of sinking velocity.The effects of temperature,volume fraction and substrate on thermal conductivity and viscosity of MWCNTs-N were investigated by the Thermal Constant Analyzer（Hot-Disk）and viscometer（NDJ-5S）.Viscosity was analyzed with classical theoretical model.The results showed that the viscosity and thermal conductivity of MWCNTs-N could be increased by increasing the volume fraction,but the viscosity can be decreased by increasing the temperature.The influence of volume fraction on the thermal conductivity and viscosity of nanofluid was linearization.The photothermal conversion performance of MWCNTs-N was investigated by muffling experiment.The warming rate of MWCNTs-N was faster than base fluid.Under the solar for one hour,the temperature of MWCNTs-N with a volume fraction of 0.05%could rise 27.1°C.MWCNTs-N with a volume fraction of 0.01%was 25.8°C;and base fluids only was 12.9°C.However,MWCNTs-N had lower caloric and thermal efficiency than MWCNTs-H₂O and MWCNTs-EG at the same volume fraction due to the lower specific heat capacity.The model of solar thermal collector system was established by TRNSYS.Under different working conditions,the operation of solar collector was simulated in whole year,and the data of August 25 were selected according to the relation between month and hour.The results showed that the MWCNTs-N was able to increase the useful energy obtained by the collector.The useful energy was from 11,476.04 kJ to 18,906.67 kJ when the volume fraction of MWCNTs-N changed from 0 to 0.05%.It was also found that the locations could affect the heat collection performance of the collector.