Oscillatory instability of convection in ferromagnetic nanofluid and in transformer oil

Abstract

Stability of a mechanical equilibrium of ferromagnetic nanofluid and transformer oil in a spherical cavity carved inside a Plexiglas block heated from below is investigated experimentally. It is shown that in a fluid left at rest prior to the start of experiment the onset of convection is delayed and the convective motion arises abruptly acquiring a finite equilibrium amplitude at supercritical temperature differences. Convection has an oscillatory character associated with a precession of a vortex axis in the equatorial plane. A hysteresis is detected in the reverse transition to a quiescent state. Oscillations in the investigated ferro-nanofluid have been observed during the complete experimental runs (up to several weeks). Similar oscillations in transformer oil have been detected only in the beginning of experimental runs. They were observed to eventually decay resulting in a stationary convection.