Abstract: Al_2O_3-water nanofluid with 40 nm particle size was produced with various volume concentrations from 0.1% to 0.5% using a two-step method with ultrasonication and without any surfactant. The analysis of zeta potential, particle size and absorbancy indicates that 3 hours of ultrasonic vibration can uniformly disperse the Al_2O_3 nanoparticles in water. In this experimental process, thermal conductivities were mearsured.The results show that the thermal conductivities of Al_2O_3-water nanafluids are all higher than water. The enhancement of thermal conductivity is 5.40%-17.9% in the low volume concentration range of 0.1%-0.5%. Enhancement of thermal conductivity is 7.23%-23% in the temperature range of 17-57 ℃ at a volume fraction of 0.2%. Furthermore, the experimental results show the thermal conductivities of Al_2O_3-water nanofluids increased nonlinearly with the nanoparticle volume concentration and temperature increasing.

Key words: nanofluid;preparation;ultrasonic vibration;thermal conductivity