Thermal Transport of Nanofluids in a Minichannel
Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage, electronics cooling to thermal processing of materials. However, the thermal transport mechanisms for nanofluids are far from being well understood. In particular, a consensus is lacking on if and how the dispersed nanoparticles alter the single-phase and two-phase heat transfer mechanisms of nanofluids in forced convective flows, and the applicability of established fluid mechanics and heat transfer theories for predicting thermal transport of nanofluids has also been called into question.
The present research aims at conducting a systematic study of single-phase convective heat transfer and two-phase flow boiling of nanofluids in a circular minichannel. The goals are to experimentally characterize the effective thermophysical properties, pressure drop and heat transfer behaviors of nanofluids with respect to their constituent base fluids, and to explore the effects of the particle-fluid interactions on the convective transport physics in nanofluids.
In this work, both aqueous Al2O3-water and non-aqueous Al2O3-Polyalphaolefin (PAO) nanofluids were synthesized. The effective thermophysical properties of the nanofluids, especially the viscosity and thermal conductivity, were first measured and compared to predictions from the effective medium theories (EMTs). Special attention was given to the effects of the nanoparticle size and aspect ratio, the aggregation and the dispersion state of the nanoparticles. Once these effects were considered properly, no abnormal changes were found in the effective thermophysical properties of nanofluids. Then, an experimental investigation was conducted to study the single-phase forced convective heat transfer of the nanofluids through a circular minichannel. The friction factor and convective heat transfer coefficient were measured for the nanofluids over a wide range of Reynolds number, covering the laminar, transition and early stage of fully developed turbulent flows. Some interesting phenomena were observed, such as the developing flow behaviors and delayed transition to turbulence for Al2O3-water nanofluids, and the abnormal heat transfer characteristics of Al2O3-PAO nanofluids, which can be attributed to the nanoparticle-fluid interaction and the shear-induced alignment and orientational motion of nanoparticles. Based on the convective heat transfer experimental data, the thermal performance and effectiveness of nanofluids for practical cooling applications were critically evaluated using different figures of merit (FOM), and it was found the nanofluids do not offer any enhanced thermal performance when the increased pumping power consumption and the negative effect of reduced specific heat on heat transfer are taken into account. And lastly, convective flow boiling and two-phase flow of Al2O3-water nanofluids were studied with an emphasis on the effect of nanoparticle deposition on two-phase flow instabilities. It was discovered the onset of flow instabilities can be suppressed by the nanofluids.