par Banisharif, Alireza
Président du jury Dubois, Frank
Promoteur Van Vaerenbergh, Stefan ;Aghajani, Masoud
Co-Promoteur Rashidi, Alimorad
Publication Non publié, 2023-05-10
Thèse de doctorat
Résumé : The present research examines the thermophysical properties and heat transfer performance of water-based nanofluids, WEG-based nanofluids (50:50 mixture of water and ethylene glycol), and LN2-based nanofluids (Liquid Nitrogen). The nanofluids were produced using a variety of nanoparticles, including Cu purchased from VWR as metallic nanoparticles, Fe3O4 synthesized via precipitation method as metal oxides nanoparticles, MWCNT, and Nano Porous Graphene (NPG) synthesized via a CVD method as nonmetallic (carbon-based) nanoparticles, in the volume concentration 0.01-0.1vol%, using different surfactants. The particles were characterized using several analytical techniques. X-ray powder diffraction (XRD) was employed to determine the crystal structure and identify the phases of metallic and metal oxide nanoparticles. Scanning electron microscopy (SEM) was used to visualize and analyze the morphology and size distribution of the nanoparticles. Specific surface area was measured using the BET method to evaluate the surface properties of the nanoparticles. Dynamic light scattering (DLS) provided information about the size distribution of particles in the nanofluids.The density, viscosity, thermal conductivity, and surface tension of base fluids and nanofluids were measured experimentally and theoretically at temperatures ranging from -20°C to 20°C. Additionally, through the measurement of heat capacity in a temperature range of 80 to 350 K, a set of fascinating data was obtained for various nanoparticles. These data were subsequently used to develop several correlations that can be incorporated into equations for cryogenic nanofluids. The convective heat transfer enhancement of water-based and WEG-based nanofluids in a pipe heat exchanger used for the ethanol condensation process is investigated in this work at relevant temperatures of 20°C and -20°C, respectively. Convective heat transfer enhancement under a laminar regime was evaluated from a well-designed experimental setup. Heat transfer efficiency of LN2-based nanofluids was tested experimentally using a miniature stainless steel cylindrical heater under atmospheric pressure in a very cold region (-196°C) and compared to different correlations in both the nucleate (II region) and film boiling (IV region) regimes. The thermophysical properties of nanofluids are strongly influenced by the type and concentrations of nanoparticles, the type of surfactant, and temperatures, especially at low temperatures, according to the key findings. As the main results, the thermal conductivity of nanofluids increases up to 3-5% (water-based nanofluids) and 4-14% (WEG-based nanofluids). e.g., with 0.1 vol%, the thermal conductivity of Fe3O4 nanofluids increases by nearly 9.5%, and 14.3%, at -10°C and 20°C, respectively. The thermal conductivity enhancement of nanofluids with concentration and temperature was compared to some relevant theoretical models. While no clear proof for physical phenomena involving such an enhancement has been discovered, a reasonable agreement is obtained using a comprehensive model that incorporates effective medium theory, the nanolayer effect of molecules surrounding the solid particle, and Brownian motion of nanoparticles that includes aggregation and nano-convection.The viscosity can increase or decrease with nanoparticle concentration, showing a lubricating effect of nanoparticles coupled with respective surfactants. For all temperatures, except for lower ones (-20°C), Newtonian behavior of water and WEG-based nanofluids are reported in the range -20°C to 20°C. The addition of MWCNT and NPG nanoparticles did not result in any noticeable change in viscosity. On the other hand, the presence of surfactants and L-ascorbic acid as physical and chemical stabilizers in Cu and Fe3O4 nanofluids led to a decrease in viscosity with an increase in nanoparticle content. E.g., it is also found that the dynamic viscosity of Fe3O4 WEG-based nanofluids decreases with nanoparticle content in particular below 0°C, up to 40% at 0.1% in volume. Surface tension decreases by adding the surfactant to the base fluid and then increases with Fe3O4 concentration with nearly 38% and 33% with 0.1% in nanoparticle volume fraction at -20°C and 20°C, respectively. The heat transfer properties, including the heat transfer coefficient and Nusselt number, of nanofluids were found to be significantly higher than those of water and WEG-based fluids (0.05 vol%). This increase was observed to be up to 20% and 55% for Pe 2000-10000 and Gz (Graetz number) 40-300, respectively. For example, at low Pe and Gz numbers, the Nusselt numbers for Cu and Fe3O4 nanofluids in water-based fluids were increased by approximately 18% and 29%, respectively. In addition, the Nusselt numbers for Cu and Fe3O4 nanofluids in WEG-based fluids were enhanced by around 30% and 24%, respectively. Experimental heat transfer properties were shown to be greater than theoretical ones. Increasing in heat transfer coefficient is observed by adding the nanoparticles. Finally, these results are promising in view of Cu water-based and Fe3O4 WEG-based nanofluids use in cooling applications and pipe flow geometry considered. It is also noticeable that the heat transfer coefficient of LN2-based and nanofluids has risen as the Jacob number has increased, and that nanofluids have higher heat transfer coefficients than liquid nitrogen, with the exception of Cu nanofluids at low Jacob numbers. At the maximum Ja numbers (around 7.5), the rises for Cu, Fe3O4, MWCNT, and NPG (0.1 vol%) nanofluids are 22, 40, 42, and 14%, respectively. The average heat transfer coefficient improvements for Cu, Fe3O4, MWCNT, and NPG (0.1 %) LN2-based nanofluids are 77, 35, 22, and 27% in the IVa region (Ra = 0.9-2×109), and 35, 15, 9, and 17% in the IVb region (Ra = 2-9×108) at cryogenic temperatures, respectively. Finally, the best candidate nanofluid for cryogenic applications is Cu nanofluids. Overall, this research provides valuable insights into the thermophysical properties and heat transfer performance of nanofluids, suggesting their potential application in cooling systems.

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