Effect of carbon nanotube concentration on cooling behaviors of oil-based nanofluids during the immersion quenching

Oil-based nanofluids including 0.75%—1.75% (mass fraction) carbon nanotubes (CNTs) without any surfactants have been synthesized by a two-step process. The probes machined from 45^# steel with 22mm diameter and 50mm length are quenched in the as-synthesized CNT nanofluids for testing the cooling behaviors of the nanofluids. The laser diffraction particle size analyzer, scanning electronic microscope (SEM), X-ray diffractometer and transmission electron microscope (TEM) are used to characterize the quality and distribution of CNTs in the nanofluids. The wettability and viscosity of 30^# oil and oil-based CNT nanofluids are measured by a goniometer at 15 ?C and a rotational type viscometer at 40 ?C, respectively. The results show that the cooling efficiency of the oil-based CNT nanofluids is better than that of 30^# oil. Moreover, the cooling rate of the naonofluids increases with the further increase of the CNT concentration. When the mass fraction of CNTs increases to 1.75%, the cooling rate of the naonofluids reaches a maximum at 760 ?C and it is increased by 77.8% as compared to that of base oil. The improved cooling rate of oil by CNTs is mainly due to the uniform distribution and excellent thermal conductivity of CNTs.