By accounting for the external and internal force acting on the suspended magnetic nanoparticles and motion characteristics of the suspended magnetic nanoparticles in the magnetic fluids, the three-dimensional microstructure of magnetic fluids is investigated by means of the molecular dynamics simulation method. The distribution of suspended magnetic nanoparticles and microstructure of the magnetic fluid are simulated in both absence and presence of an external magnetic field. The effects of the nanoparticles volume fraction, the dipole-dipole interaction potential and the particle-field interaction potential on the microstructures of the magnetic fluids are discussed. The main results obtained here are summarized as follows. The suspended magnetic nanoparticles tend to aggregate and make the irregular distribution structure in the absence of an external magnetic field. When the magnetic fluid is exposed to a magnetic field, the magnetic nanoparticles suspended in the carrier fluid tend to remain chained-alignment in the direction of the external magnetic field. The tendency of chain-alignment morphology of the suspended magnetic nanoparticles is enhanced with the nanoparticles volume fraction, the dipole-dipole interaction potential and the particle-field interaction potential.

Experimental investigations are carried out to measure the viscosity and the thermal conductivity of the aqueous magnetic fluids in either the absence or the presence of the external magnetic field. The effects of the volume fraction of the suspended magnetic particles, concentration of surfactants and the external magnetic field strength as well as its orientation on the transport properties of the magnetic fluid are analyzed. The experimental results show that the viscosity of the sample magnetic fluids increases with the percentages of the suspended magnetic particles and the surfactants. The viscosity first increases with the magnetic field and finally approaches a constant as the magnetization of the magnetic fluid arrives at a saturation state. For the same magnetic fluid, the viscosity in the magnetic field being perpendicular to the flow direction is bigger than that in the parallel field under the same magnetic field. The thermal conductivity of the sample magnetic fluids is larger than that of pure fluids in both the absence and presence of the external magnetic field. Almost no change in the thermal conductivity of the sample magnetic fluid is found in the magnetic field perpendicular to the temperature gradient. The thermal conductivity of the magnetic fluid increases with the strength of the applied magnetic field being parallel to the temperature gradient.

A 2D Lattice-Boltzmann (LB) model is proposed for analyzing the heat conduction process in the porous media. The effective thermal conductivities of several porous materials are calculated by means of this model. The calculated results are found to be in excellent agreement with the experimental data of the existing references. The factors affecting the effective thermal conductivity of porous materials are discussed. The results show that the effective thermal conductivity is strongly dependent upon the porosity and the pore structure and only has imperceptible dependence on the pore density. Then the correlation for estimating the effective thermal conductivity of the porous material is established. This LB model can be used conveniently to calculate and analyze the heat conduction problems of porous media or other materials with complex geometry boundary in pore scale.

An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correlation for nanofluid under single-phase flows in tubes is established. Comparison between the experimental data and the calculated results indicate that the correlation describes correctly the energy transport of the nanofluid.