An experimental investigation on thermal properties of molybdenum disulfide nanofluids

This paper prepared MoS2 water-based and oil-based nanofluids with different mass fractions by two step method, and investigated their stability, thermal conductivity, and surface tension. The thermal conductivity of MoS2 oil-based nanofluids was calculated using the classic model, and compared with experimental result. The results indicate that the stability for MoS2-LB2000 oil nanofluid with a mass fraction of 0.01 wt. % was better than that for MoS2-LB2000 oil nanofluid with a mass fraction of 0.05 wt. %. Compared with base fluids, MoS2 water-based and oil-based nanofluids had higher thermal conductivity and lower surface tension. With the increase of mass fraction, the thermal conductivity and surface tension of MoS2 nanofluids increased and decreased, respectively. The experimental results for the thermal conductivity of MoS2 oil-based nanofluids was higher than their theoretical predictions. Introduction Nanofluids are suspensions obtained by suspending nanoparticles with average sizes below 100nm in base fluids. Since solids conduct heat better than liquids, the dispersed nanoparticles enhance the thermal conductivity of base fluid. A small amount of nanoparticles, when dispersed uniformly and suspended stably in base fluids, can provide exaggerated improvements in the thermal properties of base fluids [1]. It is well known that during the mechanical process, the friction between tool and workpiece generates a lot of heat, which causes the rapid tool wear and decreases the surface quality. Due to the enhanced thermal properties and lubrication abilities, it appears to be a promising approach to use the nanofluids as cutting fluids to reduce the machining temperature. Over the past decade, the most common nanoparticles, studied in the field of thermal nanofluids engineering, are copper (Cu), aluminium oxide (Al2O3), and carbon nanotubes (CNTs). From experimental results, Kole et al. [2] reported that the thermal conductivity of Cu-gear oil nanofluid increases non-linearly with nanoparticles concentration and shows a maximum enhancement of 24% for nanofluid containing 2 vol. % Cu nanoparticles in gear oil. Syam Sundar et al. [3] studied the thermal conductivity of nanofluids by dispersing Al2O3 nanoparticles in water and ethylene glycol at different temperature and volume concentrations, and reported that the maximum thermal conductivity enhancement reaches up to 32.26% for the nanofluid containing 1.5 vol. % Al2O3 at 60°C. Kumaresan et al. [4] reported the largest magnitude of thermal conductivity enhancement about 19.73%, when 0.45 vol. % CNTs was added in water and ethylene glycol at 40°C. Although the nanofluid has many advantages, the stability problem has not obtained very good solution, which has become an obstacle for nanofluids in the field of engineering application. So it is very necessary to study the stability of the suspension. At the same time, many researchers focus on the viscosity, thermal conductivity and other thermal properties of nanofluids in recent years. However, the research reports on the surface tension of the nanofluids are very few. Therefore this paper first adopted two step method to prepare molybdenum disulfide nanofluids, and investigated the effect of mass fraction on the suspension stability. Then TC3010L thermal conductivity meter International Conference on Materials, Environmental and Biological Engineering (MEBE 2015) © 2015. The authors Published by Atlantis Press 881 was used to measure the thermal conductivity of molybdenum disulfide nanofluids, and the experimental results and theoretical predictions were compared. Finally, automatic surface tension meter was used to measure the surface tension of molybdenum disulfide nanofluids, and the influence of mass fraction on surface tension of molybdenum disulfide nanofluids was studied. Experimental details The base fluids used in this research involve deionized water, LB2000 vegetable lubricants, and PriECO6000 unsaturated polyhydric alcohol ester. Molybdenum disulfide nanoparticles, purchased from Nanjing GY Co., Ltd. with the diameters of 30 nm, were used throughout the experiments. The molybdenum disulfide water-based and oil-based nanofluids with different mass fractions ranging from 0 to 0.5 wt. % were prepared by dispersing a certain amount of molybdenum disulfide nanoparticles in base fluids using KQ-100DE ultrasonic cleaner with a 100 W output power and 40 KHZ frequency. The ultrasonication time used was two hours for all of the nanofluids prepared. A U-4100 UV-visible spectrophotometer was employed to measure the absorbance of MoS2-LB2000 nanofluids at regular intervals after preparation, and evaluate the suspension stability. A BZY-1 automatic surface tension meter, which is produced by Shanghai Precision Instrument Co., Ltd., was used to measure the surface tension of molybdenum disulfide nanofluids. TC3010L thermal conductivity measuring instrument, which is produced based on the transient hot-wire method by Xi’an Xiatech Electronic Technology Co. Ltd., was used to measure the thermal conductivity of nanofluids prepared. The measurement range of TC3010L is 0.001-5 W/Km. All of the measurements were taken three times at room temperature and the average of three values was used for analysis. Results and discussion Suspension stability. As the nanoparticles have a huge specific surface area and high surface energy, they are prone to agglomerate, thus affecting the performance of nanoparticles. So the study of dispersion stability of nanofluids is a prerequisite for the application. Figs. 1 and 2 show the UV Vis absorption spectrum and absorbance of MoS2-LB2000 nanofluids with different mass fractions for different days after preparation, respectively. It can be seen from Fig. 1 that there is an obvious peak at the wavelength of 600 and 650 nm, respectively. According to the literature, UV absorption peak of MoS2 lies between 620 and 670 nm. So the absorption peak at 650 nm was used to analyze the stability of suspension. As shown in Figs. 1 and 2, the higher the concentration, the higher the absorption peak. This can be attributed to more nanoparticles suspended in LB2000 vegetable lubricants when increasing mass fraction. With the increase of standing time, the peak value decreased, indicating that some nanoparticles deposited. Furthermore, the reduction in peak value for MoS2-LB2000 nanofluid with the mass fraction of 0.05 wt. % was more than that for MoS2-LB2000 nanofluid with the mass fraction of 0.01 wt. %. This indicates that the stability of the latter was better than the former. Influence of mass fraction on thermal conductivity. The factors affecting the thermal conductivity of nanofluids include mass fraction of nanoparticles, type, size, shape, temperature, etc. However, a lot of research show that the most significant factor for the thermal conductivity of nanofluids is the mass fraction of nanoparticles. Therefore this paper focuses on the effect of mass fraction on thermal conductivity of MoS2 nanofluids. Figs. 3 and 4 show the variation of thermal conductivity of MoS2 water-based and oil-based nanofluids with mass fraction, respectively. As shown in Figs. 3 and 4, compared with base fluids, MoS2 water-based and oil-based nanofluids had higher thermal conductivity. Besides it, with the increase of mass fraction, the thermal conductivity of MoS2 water-based and oil-based nanofluids increased. This is because the larger mass fraction, the more nanoparticles per unit mass fraction of the suspension, thus causing the heat transfer enhancement between particles, and the micro convection enhancement between particle and base fluid. The enhancement in thermal conductivity for MoS2 water-based nanofluids seemed higher