Film Boiling Heat Transfer on a High Temperature Sphere in Nanofluid

Quenching experiments of a high temperature sphere in Al₂O₃ nanofluids are conducted to investigate the characteristics of film boiling and compared to those in pure water tests. One stainless steel sphere of 10 mm in diameter at the initial temperatures of 1000∼1400 K was tested in the nanofluids of the volume concentrations from 5 to 20% and the degrees of subcooling from 20 to 80 K. The test results show that film boiling heat fluxes and heat transfer rates in nanofluids were lower than those in pure water. The differences of the film boiling heat transfer rates between pure water and nanofluids become larger when the liquid subcooling decreases. Those results suggest that the presence of nanoparticles in liquid enhances vaporization process during the film boiling. The effects of nanoparticle concentrations of more than 5 vol. % on film boiling appear to be insignificant. However, the minimum heat fluxes tend to decrease when the concentration increases. Direct quenching without film boiling was repeatedly observed when an unwashed sphere was employed for quenching tests in nanofluids. It suggests that nanoparticle deposition on the sphere surface prevents the sphere from forming film around the sphere, which consequently promotes the rapid quenching of the hot sphere.