Author : Giwa Solomon Olanrewaju Giwa
Publisher : Unknown
Release : 2019
ISBN : 0987650XXX
Language : En, Es, Fr & De
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Book Description :
Over two decades of extensive research on nanofluids have established them as a better cooling media than traditional fluids such as ethylene glycol (EG) and water. Recently, hybrid nanofluids have emerged as advanced thermal transport media with improved thermal and fluid properties relative to nanofluids. Experimentally, limited studies have been carried out on the thermo- and thermomagnetic convection heat transfer of nanofluids in cavities. However, there is a dearth of documentation on the thermo- and thermomagnetic convection of hybrid nanofluids in cavities in the public domain. In this study, the thermo-convection heat transfer (Qav) performance of three magnetic hybrid nanofluids (MHNFs) contained in a rectangular cavity was experimentally investigated with and without magnetic stimuli. Aqueous MWCNT-ferrofluid (AMF) [MWCNT-Fe2O3/deionised water (DIW)], aqueous Al2O3-ferrofluid (AAF) [Al2O3-Fe2O3/DIW] and bi-aqueous Al2O3-ferrofluid (BAAF) [Al2O3-Fe2O3/EG-DIW] were formulated for volume concentrations (©ʻ℗+¿30́8) of 0.05 to 0.40 vol.%. Key nanofluid formulation parameters of dispersion fraction, sonication time and amplitude were optimised to improve stability of the MHNFs. Stability, characterisation and thermal properties (IÌ2℗ơ and IÌ2℗ð at 20-40 AÌ2℗ʻC) of the MHNFs were carried out using standard instruments. AMF, BAAF, AAF, DIW and EG-DIW were charged into a rectangular cavity subjected to differential heating of the opposite vertical walls under varying IÌ20́+T of 20 to 35 AÌ2℗ʻC. Samples of AMF, BAAF and AAF with the highest heat transfer were thereafter charged into the cavity where the walls (bottom, top and side) were exposed to magnetic stimuli (4.89 aÌ22́Ơ0́− 21.95 mT). Stable MHNFs were formulated according to the optimised parameters as verified using an ultraviolet visible spectrophotometer and visible inspection techniques. The images of the transmission electron microscope for the MHNFs showed an even suspension of the nanoparticles into DIW and EG-DIW. An increase in temperature and ©ʻ℗+¿30́8 was observed to enhance IÌ2℗ðeff of AMF, BAAF and AAF by 3.83% to 14.17%, 2.14% to 12.56% and 2.21% to 10.51% respectively. Temperature rise detracted IÌ2℗ơeff and ©ʻ℗+¿30́8 enhanced it for AMF, BAAF and AAF with augmentation of 11.83% to 28.79%, 1.66% to 13.33% and 4.55% to 20.43% respectively. With the MHNFs, higher IÌ2℗ðeff and lower IÌ2℗ơeff were recorded in comparison with the monoparticle nanofluids of Fe2O3, which made the MHNFs beneficial for convective heat transfer studies. Additionally, models were developed for predicting the IÌ2℗ðeff and IÌ2℗ơeff of AMF, BAAF and AAF from the obtained experimental data. Without magnetic stimuli, the Qav of AMF, BAAF and AAF was enhanced at ©ʻ℗+¿30́8 aÌ20́ʻ℗Þ 0.20 vol.% and attenuation was the case beyond ©ʻ℗+¿30́8 = 0.20 vol.%. Optimum Qav enhancements of 11.2%, 10.09% and 10.79% were achieved for AMF (at 0.05 vol.%), BAAF (at 0.05 vol.%) and AAF (at 0.10 vol.%) respectively. Models were proposed for estimating the Nuav of AMF, BAAF and AAF. The vertical imposition of the magnetic stimuli on the sidewall of the cavity led to maximum enhancements of Qav by 4.48%, 4.02% and 4.31% for the AMF, BAAF and AAF samples respectively. These values were recorded for magnetic stimuli of 21.95 mT for AMF and AAF, and 11.84 mT for AAF. The MHNFs were observed to yield higher Qav than monoparticle nanofluids of Fe2O3 with and without magnetic stimuli. Conclusively, the IÌ2℗ðeff, IÌ2℗ơeff and Qav of AMF, BAAF and AAF were found to be better than those of the monoparticle nanofluids of Fe2O3, which revealed the benefit of NP hybridisation for engineering application, especially in thermo- and thermomagnetic convection studies.