Natural convection and pool boiling studies of nanofluids

Abstract

The present PhD thesis sheds light on the physical mechanisms behind nanofluids, by investigating experimentally the heat and mass transfer behaviour of nanofluids under natural convection and pool boiling heat transfer. Nanofluids represent a new class of heat transfer fluids, engineered by dispersing and stably suspending nanoparticles in traditional heat transfer fluids. Nanoparticles have the potential to considerably enhance the heat transfer performance of the base fluids, in concentrations starting from as low as 0.0001 vol.%, with theoretically limited drawbacks compared to milli- or micron-sized particles. Despite the reported promising characteristics of nanofluids, the physics of the underlying processes have not yet thoroughly been established and a controversy exists concerning their capability and applicability in engineering applications. With respect to natural convection heat transfer in a Rayleigh-Bénard cell, it was found that the presence of Al2O3 nanoparticles in deionised water, decreases the heat transfer coefficient of the base fluid, due to the sedimentation of nanoparticles, as a consequence of poor nanofluid stability. Meanwhile, it was shown that when nanofluids that exhibit acidic behaviour are used, the attained stability is superior, and the heat transfer deterioration is minimal. By using a laser-based velocimetry technique, it was found that the presence of a minute amount of Al2O3 nanoparticles in deionised water significantly alters its mass transfer behaviour in the bulk region of turbulent Rayleigh-Bénard convection. The spatially and temporally averaged absolute velocity of nanofluids was higher than that of water alone, while the spatially and temporally averaged turbulent intensity was lower, as measured in the field of view. Regarding subcooled pool boiling heat transfer, it was observed that the suspended Al2O3 nanoparticles in deionised water notably modify the bubble dynamics and nucleation site activity at the heating surface, due to nanoparticle deposition. The combination of traditional heat transfer experiments, novel laser-based velocimetry measurements and a visualisation study revealed the contribution of nanoparticles on the heat and mass transport mechanisms of conventional heat transfer fluids, in a comprehensive manner. The outcome of this thesis is a step towards the evaluation of the applicability of nanofluids in demanding cooling applications, where mixed heat transfer modes are involved, such as those employed in the current generation of experimental fusion reactors and future fusion power plants.