3 오메가 방법을 이용한 나노유체 분산안정성 평가를 위한 실험적 연구

Abstract

The nanofluid is a suspension in which nanoparticles are uniformly dispersed in a working fluid such as DI-water, ethylene glycol and oil. It is well known that the thermal conductivity of the fluid can be improved by adding nanoparticles with high thermal conductivity to the working fluid. For more than 20 years, many studies have been carried out around the world to increase the thermal conductivity of nanofluids. However, high concentrations of nanofluids usually have low sedimentation stability because they are aggregated and sedimented. Many physical and chemical methods are being attempted to increase the dispersion stability of nanofluid at high concentration, but technology capable of ensuring dispersion stability at high concentration to the extent that heat transfer performance is significantly improved has not been developed. In addition, even if such technology is developed, the method of quantitatively evaluating the sedimentation stability of nanofluids at high concentrations is limited. Currently, methods for evaluating dispersion stability of nanofluids include UV-Vis spectrophotometer, Zeta potential, sediment photograph capturing, electron microscopy (TEM, SEM), light scattering method, and sedimentation balance method. These measurement methods developed analysis principles based on the unique properties of particles, such as particle scattering intensity, electrophoresis velocity, and gravity sedimentation. But these methods are nanoparticles, concentration, concentration range type of fluid can be distributed according to the stability it is difficult to measure. Since there are many limitations to existing methods, a new analysis theory is needed that can measure sedimentation stability regardless of high concentration and optical properties. 3ω method measures thermal conductivity at the bottom of the specimen, so it has an advantage in identifying the agglomeration and sedimentation characteristics of nanofluids. And since the 3 m method has a high spatial resolution, the temperature amplitude can be quantitatively measured with just one drop of nanofluid. In particular, unlike existing methods, since it is a method using conductive heat transfer rather than an optical characteristic, it is possible to measure high concentration, absorbance material, and transparent samples. So, this paper thermal characterized a particle with a size to precipitate at a constant speed to form a layer with a constant porosity. To find out how the concentration and particle size affect the temperature amplitude with the designed modeling, nanofluids with Al2O3 200 nm particles with 0.1, 0.2, and 0.3 vol. % concentrations and Al2O3 200, 400, and 800 nm particles with 0.2 vol. % were analyzed. In addition, experiments were conducted by dispersion method, concentration, and dispersion time to compare sedimentation stability. The average particle size and porosity were estimated by comparing and analyzing the experimental value and the theoretical value. In addition, for quantitative comparison, the time and the temperature amplitude were non-dimensional, and each experimental value was derived with one constant, the stability constant (S). Through this, sedimentation stability was evaluated.