Article ID: 20-00527
Nanofluid, a liquid containing choroidal dispersion of nanometer-sized solid particles, enables high-temperature bodies to be cooled more rapidly during quenching than in pure liquid. Drastic rise of the minimum heat flux temperature (TMHF) caused by the layer of nanoparticles formed on the heat transfer surface is the key phenomenon of heat transfer enhancement. In the present work, using alumina, silica, and titanium dioxide as the nanoparticle materials, quenching experiments were carried out to explore the mechanisms of the rise of TMHF in nanofluids; stainless steel 304 and Inconel 718 were used as the materials of the specimen and distilled water was used as the base liquid. In the experiments, TMHF increased in all the nanofluids but the increasing rate was dependent significantly on the nanoparticle material and the nanoparticle layer thickness. To elucidate the mechanisms of the heat transfer enhancement, the relations of TMHF with the three basic surface parameters of roughness, wettability, and wickability were examined but no clear relationship was found. When the metal specimen of higher thermal conductivity is covered with the nanoparticle layer of lower thermal conductivity, the contact temperature during quenching should decrease and the contact duration would be dependent on the thermal properties and thickness of the nanoparticle layer. Assuming that TMHF rises with an increase in the contact duration, a new model describing the rise of TMHF in the nanofluid was proposed.
