Конвекция магнитной жидкости в замкнутом гидродинамическом контуре

Abstract

Thermal convection of ferrofluid in a closed side-heated hydrodynamic loop is investigated experimentally when a nonuniform magnetic field is applied to the tube section near the electric heater. The nonuniform magnetic field with a strength of up to 24 kA/m is created by a permanent magnet of the “neodymium-iron-boron” type, equipped with ferrite pole pieces. For temperature measurements, miniature copper-constantan thermocouples are used. The temperature distribution along the circuit and temperature differences on both sides of the heater is measured. The tubes of the loop are cooled by a stream of thermostatic air which ensures a constant heat transfer coefficient at the outer surface of the tubes and an exponential temperature distribution along the circuit. The exponent determined in the experiments is used to provide information about the integral axial heat flux (Nusselt number). The experiments were performed with undecane in the ordinary gravitational convection regime and with medium concentrated magnetic fluid in the combined (gravitational and thermomagnetic) convection regime in the range of Rayleigh numbers 103–104. The estimation of the characteristic magnetic Rayleigh numbers was carried out taking into account the demagnetizing fields. For all modes, the dependence of the Nusselt number normalized to the heat transfer coefficient on the thermal Rayleigh number is plotted. It is shown that thermomagnetic convection increases the intensity of heat exchange by 4–6 times.