Experimental study on thermal conductivity of lubricant containing nanoparticles

In this study, the Al₂O₃ nanofluids produced from the direct synthesis method were used as the experimental samples, and ultrasonic vibration was used for dispersing the nanoparticles into three types of different weight fractions (1.0, 1.5, 2.0 wt.%). The base solvent was the lubricant of R-134a refrigeration system. The objectives of this study were to discuss the dependence of thermal conductivity of Al₂O₃ nanofluids on the temperature (20-40 °C) under different weight fractions. This experiment used the thermostatic bath to stabilize the temperature of sample, and the transient hot-wire method to measure the thermal conductivity in the nanofluids of different weight fractions and sample temperature. The results showed that the thermal conductivities were enhanced by 2.0%, 4.6%, and 2.5% when the nanoparticles of Al₂O₃ of 1.0, 1.5, and 2.0 wt.% were added at 40 °C. Among them, the optimal enhancement of the thermal conductivity was at 1.5 wt.%. The enhancement of thermal conductivity did not grow with the increase of weight ratios, and it was obviously different from the general nanofluids with lubricant as the basic solvent. Besides, thermal conductivities increased from 1.5 to 4.6% when the sample temperatures were from 20 °C to 40 °C at 1.5 wt.%, and the trend of growth rates of the thermal conductivity was proportional to temperature. From the results, it can be realized that temperature has greater effects than weight fraction on the increase of thermal conductivities. Thus, it is better for nanpfluids of Al₂O₃ to be applied in the high temperature field than in the low temperature field.