Study On The Thermal Conductivity And Radiation Characteristics Of Nanofluids

Nanofluid is a new kind of suspension which is prepared by adding nanoscaled metallic or nonmetallic particles with definite proportions in base fluid. Nanoparticles have some special characters, such as surface effect, quantum size effect and macroscopic quantum tunnel effect, which enable the improvement of heat transfer coefficient without clogging flow channels or eroding pipelines, therefore nanofluid is also a favorable working fluid for heat transfer.There are two classified processes used to produce nanofluids:one-step and two-step methods. Nanofliud made by one-step method has better stability and dispersion, while two-step method has advantages in manufacture and cost. To investigate the characteristics of nanofluids on thermal conductivity and the radiation. high performance nanofluids were prepared by high shear homogenizer, and the stability of the nanofluids were analyzed by monitoring the size distribution and zeta potential.In this thesis, to investigate the effects of several key factors on viscosity, including particle type, size, shape, and fluid temperature, we detected the viscosity of the nanofluids by Ubbelohde viscometer. Experimental results indicate that the reduction of particle size will enhance the viscosity. Meanwhile, the viscosity increases linearly with the rising of volume concentration of particles, and declines with the drop of fluid temperature, which is similar with that of the base fluid.Hot-wire method was customized and the surface of platinum wire was insulated by evaporation vacuum coating technology and transparent polyurethane insulation spray, so that the thermal conductivity of nanofluids with a pH value in a range of 1-11 can be tested accurately. Ethylene glycol and deionized water were used to calibrate the testing system, showing that the test method has a very high accuracy within 2% error. The thermal conductivity of nanofluids was measured under different conditions. Experimental results demonstrate that the thermal conductivity of nanofluid increases linearly with the rise of volume concentration of particles and decreases with the temperature decline. For the simulation of thermal conductivity, we found most of the theoretical values are much lower than the experimental results, and the reasons might be those models are based on some particular objects or consider limited factors. In this thesis, a methodology is proposed for predicting the effective thermal conductivity of nanofluids based on rheology. The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively, which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids. And the modified H-C equation successfully predicted the effective thermal conductivity of the nanofluids, minimized the gap between the theoretical value and the experimental data.The nanofluids act a special feature on transmissivity. The transmissivity of SiO2 nanofluids with various particle sizes, volume concentrations and optical path were measured by the spectrophotometer based on integral ball principle. Experimental results indicate that the transmissivity of SiO2 nanofluids are affected by the particle size and the volume concentration. Also, the results show that the transmissivity of the SiO2/H2O nanofluid changes with the variation of optical path, which follows the Lambert-Beer law, as the diameter of particles is below 20 nm..However, when the diameter of particles is over 20 nm, the Lambert-Beer law is no longer available.The characteristics of nanofluids are not only associated with the characteristics of particles and temperature of fluid, also related to the pH value of fluid which can affected the state of the nanofluid and the internal structure of particles. In this thesis, experimental studies proceed to investigate the effects of pH value on the microstructure of nanofliud agglomeration, further the effects of agglomeration on the viscosity, thermal conductivity and transmissivity of nanofluid. According to fractal theory, the fractal dimension can provide a more accurate quantitative description of the agglomeration, which is used to predict viscosity and thermal conductivity. A modification of fractal model was established considering of agglomeration effect, and the simulation results fit very well with the experimental results in this case.Because of these excellent features on the heat transfer and radiation characteristics, nanofluid is expected to be a new working fluid in a solar energy PV/T system.