Experimental Study On Thermal Properties Of Nanofluids And The Applications In Direct Absorption Solar Collectors

In this thesis,carbon nanotube nanofluids were prepared by two-step method,and the stability of nanofluids was studied by gravity sedimentation method.Based on the prepared carbon nanotube nanofluids,the effects of temperature,concentration and particle size on the thermal conductivity and viscosity of carbon nanotube nanofluids were investigated using a Hot-disk thermal constant analyzer and an NDJ-5S viscometer.The effects of concentration,particle size and optical path on the permeability of nanofluids were investigated using an ultraviolet-visible spectrophotometer.Based on the study of the stability,thermal properties and optical properties of nanofluids,based on economic rationality and direct absorption solar collectors,the light of carbon nanotube nanofluids under different outdoor natural environment conditions was studied through a self-built experimental platform.Thermal conversion characteristics.Studies have shown that the use of dispersant and ultrasonic vibration can greatly improve the stability of carbon nanotube nanofluids,and in this experiment,gum arabic is used as a dispersant,the ultrasonic oscillation time is 1.5 hours,and the prepared carbon nanotube nanofluid is stable.Good sex,stable in a certain period of time.The temperature,concentration,and particle size of the nanoparticles have an effect on the thermal conductivity and viscosity of the nanofluid,but the degree of influence varies,and the effect of the concentration on the thermal conductivity and viscosity of the nanofluid exhibits a linear change.Studies have shown that only a small amount of carbon nanotubes(0.001 wt%)can greatly increase the absorption rate of sunlight by nanofluids compared with the base solution;as the concentration increases,the nanofluids of carbon nanotubes pass through.The rate is reduced and the absorption peak appears blue shift.Adding a small amount of carbon nanotube nanoparticles to deionized water can greatly improve the photothermal conversion ability and light absorption capacity of the nanofluid;there is micro convection inside the collector tube,and the temperature of the upper nanofluid is higher than the temperature of the lower nanofluid,The temperature of the nanofluid at the wall of the tube is higher than the temperature of the nanofluid at the center of the tube.The photothermal conversion characteristics of carbon nanofluids increase first and then decrease with increasing concentration,and there is an optimal concentration(0.7 wt%)with a conversion rate of 32.03%,but when the concentration is 0.09 wt%,When the photothermal conversion efficiency and thermal conductivity are slightly lower than the optimum concentration(0.7 wt%),the viscosity is lower and the cost performance is higher,which can be used as the optimum concentration in practical applications.