Heat transfer enhancement of ferrofluid flow in a solar absorber tube under non-uniform magnetic field created by a periodic current-carrying wire

A novel configuration for heat transfer enhancement in parabolic trough solar collector absorbers by the use of a ferrofluid and an external magnetic field generated by a current-carrying wire is analyzed using numerical simulation. This new configuration consists of a wire which varies its position periodically along the tube. The analysis is focused on the study of the thermal hydraulic characteristics, Nusselt number and friction factor, and the average flow patterns in the turbulent flow regime (15.10(3)<= Re <= 250.10(3)). In addition, different parameters of the periodic wire are analyzed: wire pitch and position angle. Ferrofluid Fe3O4/Therminol 66 is considered for this application. Firstly, the effect of an increase on the magnetic field intensity is studied. The periodic wire configuration shows a higher increment of the Nusselt number in comparison to the straight wire for the same magnetic field increase. The results reveal that the presence of a non-uniform magnetic field generates a disturbed flow with a high velocity region near the current-carrying wire. The periodic wire configuration leads to a spatially-periodic behavior that increases the average Nusselt number and the friction factor, in comparison to a straight wire. Among the studied cases, the position angle theta = 30 degrees and the pitch length l = 3D are found to provide the maximum Nusselt number (Nu = 244.25 for Re = 15000).