Experimental investigation on convection heat transfer characteristics of ferrofluid in a horizontal channel under a non-uniform magnetic field

In this study, the free convection heat transfer behaviour of a diester-based ferrofluid in a horizontal channel under the influence of a permanent magnetic field was studied experimentally. The experiments were conducted by cooling the upper wall while heating the bottom wall of the channel. The results showed that the efficiency and rate of heat transfer were significantly improved when the cooling temperature dropped from 20 °C to −10 °C for a heat load of 12 W. Heat transfer would reach a relative maximum thermal equilibrium state after the cooling temperature reduced to a certain value. The average Nusselt number increased with an increase in the Rayleigh number. However, an increase in the viscosity of the ferrofluid caused by the decrease in the temperature suppressed the flow and heat transfer, with the result that the increase in the Nusselt number was not significant. The local Nusselt number was gradually decreased from the middle towards both sides, and the maximum value corresponded to the minimum temperature of the ferrofluid in the central region of the channel. The mechanism that affects the heat transfer characteristics of the ferrofluid can be attributed to the influence of flow rolls induced by the inhomogeneous Kelvin force on the distribution of particles and the disturbance in the thermal boundary layer.