The objective of this study is to investigate the quenching characteristics of a high temperature spherical stainless steel in water-based silica nanofluid and water at saturated conditions under atmospheric pressure. The experiments are carried out with a small (10 mm) stainless steel sphere with initial temperature 500°C that was suddenly immersed into the nanofluid or water. The temperature history in the center of sphere was accurately recorded. Heat transfer rate from sphere to fluid was calculated by assuming lumped system. The effect of size and concentration of nanofluid on quenching process and critical heat flux (CHF) were evaluated. The polished sphere was quenched 5 times in water or nanofluid. Boiling and quenching curves of the sphere in nanofluid with different concentrations and sizes were compared with water. The results show that the quenching behavior in nanofluids is nearly identical to that of pure water but strongly depend on the concentration and size of nanoparticles during the repetitive quenching tests. By increasing the size and the concentrations, CHF increases and cooling duration shortens. After the repetition tests in nanofluids, deposition of nanoparticle occurs on the boiling surface. In order to investigate particles deposition on boiling surface before and after the tests, SEM micrographs were taken from boiling surface which clearly show the changes in surface due to deposition of nanoparticle. It appears that surface roughness increase and wettability enhancement due to nanoparticle deposition are responsible for improved boiling performance while coolant type shows minor effect on quenching performance. Key words:
