Features of convection flows and heat transfer in magnetic colloids

Undertaken experimental observations and stability analysis have shown that a simple parallel buoyancy-induced flow in a differentially heated vertical layer of ferrofluid that is placed in an external uniform horizontal magnetic field is subject to a number of instabilities leading to the appearance of various convection patterns. Depending on the values of the governing parameters, the instability patterns are found to consist of vertical stationary magneto-convection rolls and/or vertically or obliquely counter-propagating thermo-gravitational or thermo-magnetic waves. Vertical rolls are the most prominent feature of the thermo-magnetic convection while inclined rolls and waves result from the interaction of the magnetic and gravitational mechanisms. Convective flow patterns are characterized by significantly increased heat transfer rates in comparison to pure conduction states. In particular, a significant intensification of an integral heat transfer was observed in convection regimes corresponding to vertical stationary rolls. It is found that these patterns are caused completely by a thermo-magnetic mechanism and are the consequence of an internal magnetic field gradient induced by the temperature dependence of fluid magnetization. These were not detected in earlier experiments and computations.