Thermal efficiency of a ferrofluid-based flat-plate solar collector under the effect of non-uniform magnetic field

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• A flat-plate solar collector with ferrofluid under magnetic field is studied. • Without magnetic field, the collector's efficiency increased with flow rate increase. • Without field, the maximum efficiency increase is 39.9% for 0.8 vol% and 0.05 kg/s. • The field effect on the efficiency enhancement increased with decreasing flow rate. • The maximum efficiency enhancement of is 52.15% for 0.8 vol%, 0.0083 kg/s, 1.0 T. Mn–Zn Fe 2 O 4 magnetic nanoparticles have high magnetization capability. They can be stimulated well with an external magnetic field for producing a high magnetic body force, which is added to the momentum equation. Applying an Mn–Zn Fe 2 O 4 ferrofluid in a flat-plate solar collector influenced by a strong magnetic field can be a compelling method in improving the thermal efficiency of the collector compared with other nanofluid-based ones. This experimental investigation, for the first time, presents an Mn–Zn Fe 2 O 4 /water ferrofluid-based flat-plate solar collector, equipped with a set of permanent magnets, to study the impact of the non-uniform magnetic field on the collector efficiency based on ASHRAE Standard. Simultaneous effect of the various parameters like the nanoparticles volume fraction ( φ = 0, 0.2, 0.4, and 0.8%), the fluid flow rate ( m ̇ = 0.0083–0.05 kg/s), and the remanent magnetization of the magnets ( B r = 0.0–1.0 T) on the collector efficiency have been taken into the consideration. According to the results, using the ferrofluid-based collector has a relatively significant impact on the collector efficiency than other nanofluid-based collectors. For the ferrofluid case studies without a magnetic field, the maximum enhancement in the collector efficiency compared to water is 39.9% for the case with φ = 0.8% and m ̇ = 0.05 kg/s. Also, when the magnets are used, the collector efficiency is improved with volume fraction augmentation and decreasing the fluid flow rate. Finally, with applying the magnetic field, the maximum enhancement in the efficiency has been 52.15% obtained for φ = 0.8%, m ̇ = 0.0083 kg/s, and B r = 1.0 T.