Optical Absorption And Photo-thermal Conversion Performance And Comparative Study Of Graphene Oxide And Mxene Ti₃C₂ Nanofluids Under Different Irradiation Angles

This thesis focuses on the photo-thermal utilization of carbon-based nanofluids in direct absorption solar collectors.The base fluid of nanofluids is deionized water,and the nanomaterials are graphene oxide（GO）and MXene Ti₃C₂.GO has excellent thermal conductivity,hydrophilicity and dispersibility,while MXene Ti₃C₂ has the advantages of huge specific surface area,outstanding electrical conductivity and good optical absorption.Both nanofluids are prepared on the basis of two-step method,and their optical absorption performance is tested by spectrophotometer.Two photo-thermal experimental systems are designed to investigate the photo-thermal conversion properties of nanofluids.At the same time,the influence of the irradiation angle on the photo-thermal conversion perfomance is deeply analyzed,and the properties of the two nanofluids were compared and discussed.The main work and results are obtained as follows:（1）For GO-water nanofluids with the same mass fraction,the optical absorption performance of the nanofluid is stronger with the smaller GO sheet diameter,this is because the smaller diameter of the GO sheet means more GO nanosheets in the base fluid,which can effectively increase the reflection of the incident light and improve the light absorption efficiency.For example,the average transmittance of GO-water nanofluids（100ppm）with GO sheet diameters of 50-200 nm、200-500 nm and>500 nm in the 200-1400nm band is 44.5%、46.1%and 47.3%respectively.The improvement of optical absorption performance and the enhancement of Brownian motion caused by small size make GO-water nanofluids with smaller diameter of GO sheet have better photo-thermal conversion performance.In the forward irradiation experiment,the temperature rise of the GO-water nanofluid（100 ppm）with the above three sheet diameters（compared with that before the experiment）increased by 10.5%、6.9%and 5.6%than that of DI water.（2）In the reverse irradiation experiment,the photo-thermal conversion performance ofthe nanofluids become better with the increase of the irradiation angle.For instance,the temperature rise of the GO-water nanofluid（GO sheet diameter 50-200 nm）at the concentration of 150 ppm under irradiation angle of 30°、45°and 60°is 38.6℃、40.4℃and 41.3℃,Which is 21.0%、24.9%and 26.5%higher than that of DI water.In addition,the enhancement of photo-thermal conversion performance improves the receiver efficiency.The maximum receiver efficiency of MXene Ti₃C₂-water nanofluid with a mass fraction of 50 ppm at irradiation angle of 30°、45°and 60°is 73.7%、77.7%and 78.8%,respectively.Through the simulation analysis of four irradiation angles（30°,45°,60°and90°）,it is found that with the increase of the irradiation angle,the vortex range in the cross section of the tube expands significantly,and the vortex center moves upward along the tube wall The movement indicates that the influence of natural convection increases with the increase of the irradiation angle,which can effectively destroy the temperature distribution on the wall,enhance the uniformity of the temperature distribution of the fluid in the tube and improve the photo-thermal conversion effect.（3）In the wavelength range of 200-1400 nm,the average transmittance of the MXene Ti₃C₂-water nanofluids is lower than that of the GO-water nanofluids with the same mass fraction,and the difference is more obvious at the higher concentration.For example,as the mass fraction increases from 10 ppm to 150 ppm,the average transmittance of GO-water nanofluids（GO sheet diameter 50-200 nm）in the 380-780 nm band decreases from 72.8%to 53.5%,while the average transmittance of MXene Ti₃C₂-water nanofluids decreased from 73.4%to 6.3%.The influence of multiple factors such as optical absorption and dispersion stability results in differences in the photo-thermal conversion performance and receiver efficiency of the two nanofluids.In the reverse irradiation experiment,when the irradiation angle is 30°,the temperature rise of GO-water nanofluid and MXene Ti₃C₂-water nanofluid with a mass fraction of 150 ppm is 24.2%and 36.8%higher than that of DI water;The maximum receiver efficiency of the above two nanofluids with a fraction of 10 ppm is 67.3%and 61.0%,respectively,which is 10.8%and 9.5%higher than that of DI water.The introduction of carbon nanomaterials significantly improves the receiver efficiency,and the receiver efficiency of MXene Ti₃C₂-water nanofluids is higher than that of GO-water nanofluids with the same mass fraction.