Thermomagnetic Convection of a Ferrofluid in a Vertical Hydrodynamic Loop: Intensification of Heat Exchange in a Magnetic Field

Experiments with a closed laterally heated hydrodynamic loop located in the vertical plane (convective loop) are carried out to clarify the question of the effect of a magnetic field on convective heat exchange in a ferrofluid. The loop is made of a circular cross-section tube and is blown by a thermostatted air stream, which provides a constant heat-transfer coefficient on the tube surface. To prevent competition between gravitational and thermomagnetic convections, the length of the loop is chosen to be large compared to its inner diameter. It is shown that, in the stationary convection mode, an exponential temperature distribution is established along the loop. The exponent determined in the experiments is used to obtain information on the fluid flow rate through the tube cross section and on the dimensionless integral heat flow—the Nusselt number. Experiments are carried out with kerosene and four samples of a magnetite–kerosene–oleic acid ferrofluid with different magnetic phase concentrations. Part of experiments are carried out in zero magnetic field (under gravitational convection conditions), and another part, under mixed (gravitational and thermomagnetic) convection conditions. In the latter case, a nonuniform magnetic field with an amplitude of 23 kA/m was applied to a section of the loop near the heater. The results of experiments are given in dimensionless variables as a function of the Nusselt number Nu versus the thermal Rayleigh number Ra. It is shown that in zero magnetic field the function Nu = Nu(Ra) is the same for kerosene and all ferrofluids. The switching on of a magnetic field induces a thermomagnetic convection (codirectional with gravitational convection), which increases the convective heat exchange by a factor of more than three depending on the thermal Rayleigh number and the concentration of the magnetic phase.