The Impact of Nanofluids on Single-phase Convective Heat Transfer in Microchannels

The aim of this thesis is to understand the effect of the presence of nanoparticles in the base fluid on single-phase convective heat transfer in microchannels under laminar conditions. Therefore, single-phase flow has been experimentally investigated through silicon rectangular microchannels using both water and several water-based nanofluids and suspensions, i.e., Al2O3-water, TiO2-water, and polystyrene-water in the fluidic loop. The heat transfer performance of the nanofluids and of the water have been evaluated and compared. The overall convective heat transfer coefficient has been obtained simultaneously with the nanoparticle size distribution via Light Extinction Spectroscopy technique (LES).

The LES technique has been first applied to well-dispersed colloids when the fluid is at rest. The assessment of the technique has been numerically and experimentally conducted on PS-water colloids. In other words, the sensitivity analyses on the noise level and on the particle complex refractive index discrepancies have been performed. In particular, experimentally characterized noise profile has been embedded into numerical simulations. Moreover, it has seen that wavelength spectrum limited to high signal-to-noise ratio region improves the LES inversions. Besides, in-situ calibration of the complex refractive index spectrum has enhanced the quality of the LES results. As a result, both mono and bi-modal size distributions have been experimentally retrieved. More specifically, the mono-modal distributions possess the maximal 16% discrepancy for the median diameter, volume mean diameter, number concentration, and dispersity level.

Then, the LES setup has been modified for the convective heat transfer experiments with nanofluids as an in-situ, real-time, and non-intrusive optical measurement technique. During these experiments, nanoparticle clustering and consequent tribological effects at the channel walls have been observed via the LES technique without any fluid sampling. The material loss due to abrasion-erosion-corrosion phenomena of the nanoparticle clusters on the channels could not have been overcome by any means even at low Reynolds numbers despite different types of nanoparticle materials that have been tested. No significant heat transfer enhancement has been obtained with the use of nanofluids compared to water.