Expermental Study On The Thermal Physical Properties And Convection Heat Transfer Of Oil-based Nanofluids

Nanofluid is a multiphase system that nanometer material is dispersed in base liquid stably. Due to the addition of nano-scale particles, the thermal conductivity, heat capacity, viscosity and other characteristics of nanofluid are quite different from the traditional liquids. It can be used as cooling system more efficiently, and has been applied in micro heat exchanger, transportation, aerospace, microelectronics, medicine and so on. Therefore, it is of great significance for improving the economy, reliability and miniaturization of the heat exchange system. The oil-based nanofluid can break the limit of boiling point of water (100℃), and storage or transfer heat at higher temperature.The paper is focused on the thermal physcial characteristics and convective heat transfer characteristics of oil based nanofluid with a series of experimental studies and theoretical analysis. The nanofluid is prepared by two-step method with added surfactant to enhance the stability. The thermal conductivity and rheological behavior are studied following with, the investigation on convection heat transfer based on a closed loop tube nanofluid flow heat transfer platform. The main contents are as follows:1The preparation and thermal conductivity of oil-based nanofluid.The two-step method with surfacant, mechanical stirring and ultrasonic vibration were used to enhance its dispersion stability. The experimental results show that the nanofluids prepared by two-steps method have been able to meet the requirements of stability.Nanoparticles dispersed in liquid can significantly improve its thermal conductivity. The effects of different factors on thermal conductivity, including nanoparticles species, particle size, volume fraction and temperature, are studied by experiments and theroretical models. The results show that the thermal conductivity is influenced by the thermal conductivity of nanoparticles and particle shape, and the effect of agglomeration is also need to be considered. The semiempirical H-C formula which is based on the shape factor is used to predict the thermal conductivity of nanofluid.2The rheological behavior of oil-based nanofluid.The thermal conductivity and rheological behavior are two important factors influencing the heat transfer characteristics. Due to the small size effect of nanometer particles, the properties of the base fluid; change significantly. This paper studies the influence of nanoparticle type, particle size, volume fraction, temperature and rotational speed.The CNT-oil nanofluid is non Newton fluid as the shape of nano-particle is far from spherical shape, while the Al2O3-Oil nanofluid is Newton fluid in our experiment. With the increase of particles volume and the decrease of fluid temperature, Newtonian fluids are likely to be Non-Newtonian fluid, presenting shear-thinning phenomenon. The experiments show that viscosity decreases greatly with the increased temperature, and the results fit the theoretical formula well. Moreover, the viscosity is more sensitive to temperature at lower temperature. The experimental results are generally higher than the traditional relationship between volume fraction and viscosity, mainly because of the effect of agglomeration. The modified formula indicates that the effect of particle size on the viscosity is related to the volume fraction.3The convective heat transfer of oil-based nanofluid. A closed cycle tube system is built to measure heat transfer coefficient of nanofluids at high temperature under laminar flow conditions. The influences of flow rate and heating power on the average heat transfer coefficient are investigated. The results show that the average heat transfer coefficient is related to two aspects: thermal conductivity and viscosity. Therefore, the performance of heat transfer doesn’t change progressively with the change of volume fraction or heating power. Moreover, based on the experimental data, the convective heat transfer correlation was matched. Compared with the formular for classic forced convection, the fitting correlations has a higher sensitivity to Re number, it is mainly becaused of the high viscosity of oil-based nanometer fluid and relatively high temperature condition in the paper.