Liu, Ze-Yu
(2021)
Investigation of thermal properties of citric acid modified magnetic nanofluids and distributions of drops deposition patterns affected by thermal strategies.
PhD thesis, University of Nottingham.
Abstract
Over the past decades, the increasing interests in nanofluids offer more potential to control various performances in different applications. Magnetic nanofluids (MNFs) have been extensively studied in many thermal applications due to both the magnetism and the fluidity. Both stability and thermal properties are important indicators in the process of applications. Moreover, poor colloidal stability leads to particle aggregations even precipitations, which results in clogging of flow channels and the pressure drop. Consequently, nanofluids studied in this way cannot be reliable for practical applications due to unstable physical properties. The first aim of this work is to investigate the synthesis procedure and thermal properties of well-dispersed MNFs. In terms of the synthesis, citric acid (CA) modified MNFs are prepared by a two-step method. The synthesis results show that the stability of CA modified MNFs is sensitive to pH values. The MNFs exhibit well-dispersed distribution when pH is at neutral values. Meanwhile, thermal conductivity can be predicted by the Maxwell model, and the proper aggregation can improve thermal conductivity. For convective heat transfer, it shows that the convective heat transfer of MNFs increases by 10% when the Reynolds number is over 2500. Meanwhile, a similar enhancement is obtained in the thermal efficiency of the solar collector. The experimental results show that the thermal efficiency of parabolic trough solar collector, 0.05 vol.% of MNFs, has a great potential enhancement, which is 25% higher than that of the water-based absorber and 12% higher than that of the selective surface absorber under the external magnetic field. Apart from that, due to the well-dispersed property, the deposition pattern of MNFs is also studied in the following section.
In the framework of the grant of H2020-MSCA-RISE, the distribution of deposition patterns affected by thermal strategies is also studied. Deposition patterns are widely applied in many industrial applications, such as inkjet printing, liquid biopsy, medical science, etc. Previous researchers have investigated different methods to control patterns by adjusting droplet properties. This section is devoted to experimentally exploring how to control the distribution of deposition formations by thermal strategies without changing chemicals. Firstly, two kinds of droplets, including saline and MNFs droplets, have been experimentally conducted by controlling substrate temperatures, ranging from cooling to heating temperatures. The experimental results show that deposited patterns transit from the uniformity, the ring-like structure, to the dual ring structure with the increasing temperature. Meanwhile, IR camera is used to analyse the distribution of surface temperatures, and 3 µm microbeads are applied to track the flow motion of the solute inside for different temperatures. Secondly, deposition patterns with different wettability are also investigated by the local heating. The whole process of the evaporation is recorded by a CCD camera. Experimental results show that the laser-induced local heating changes particle deposition patterns from the ring-like structure to the dotlike structure on a hydrophilic surface, transporting more saline solvents to the centre. Meanwhile, a hydrophobic surface is also investigated. The results show that particles tend to assemble at the central area, but the pattern is slightly different compared to that on the hydrophilic surface. The experimental investigation shows that thermal strategies have great potential to control the distribution of droplet deposition patterns.
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