Thermomagnetic convection in a hydrodynamic loop: the role of drop-like aggregates

Abstract

In order to clarify the effect of drop-like aggregates on the thermomagnetic convection of a ferrofluid in a vertical hydrodynamic circuit (convective loop), two series of experiments were carried out, which differ in the dispersed composition of single-domain particles in the fluid. This particular geometry of the cavity with the ferrofluid was chosen because in a closed hydrodynamic circuit the best conditions are created for thermomagnetic convection and the heat transfer enhancement, provided that a magnetic field is applied to the heated section of the circuit in one of its vertical sections. In this case, the field nonuniformity in the cross section of the channel forming the loop is sufficiently small, and the ponderomotive force acting on the liquid is directed along the tube. The heated section of the circuit, which is affected by a nonuniform magnetic field, ‘works’ as a kind of thermomagnetic pump and a circulation flow occurs in the circuit. In the first series of experiments, we used a ferrofluid containing a coarse fraction of particles capable of causing thermodynamic instability of the ferrofluid in a magnetic field (spinodal decomposition) and the formation of drop-like aggregates. The second series of experiments was carried out with a ferrofluid that had undergone magnetic separation and was free from coarse particles. It is shown that the results of convective experiments differ qualitatively depending on the disperse composition of particles and the presence of drop-like aggregates. A ferrofluid with a coarse fraction completely blocks the global circulation flow, including gravitational convection, while a separated ferrofluid, purified from large particles, increases the intensity of convection in a strong magnetic field and the integral heat flux by 6–7 times.