Characterizations and Convective Heat Transfer Performance of Nanofluids

In recent years, many experimental studies have reported anomalous thermal conductivity enhancement and heat transfer increase in liquid suspensions of nanoparticles. In order to understand the mechanism of this phenomenon and examine the possible applications of nanofluids in heat transfer, the present study experimentally investigated thermal, rheological and heat transfer properties of nanofluids.;In the first part of the work, several types of suspensions of near spherical nanoparticles and base fluids were examined. The results show that particles in suspensions without stabilizers agglomerate over time. The thermal conductivity and viscosity of a range of nanofluids were measured. These measurements indicate that the thermal conductivities of nanofluids are in the range predicted using effective medium theory. For example, Bruggeman predicted a 13% thermal conductivity increase for a 3.86% concentration of particles by volume; our experimental measurement indicated a 15% increase for this concentration. Viscosity measurements indicate that dispersions with larger agglomeration experience a larger increase in shear thinning. The results also suggest that finer particles and a narrow particle size distribution should result in a large viscosity increase.;The second part of this study examined heat transfer performance of nanofluids in both laminar and transitional flows. Within experimental uncertainty, the non-dimensional heat transfer behavior of nanofluids in laminar flow region was the same as for base fluids without particles. The laminar flow data indicates that nanoparticles migrate from regions of high shear rate to regions of low shear rate, causing them to migrate away from the boundaries of pipe flow. For transitional flow (2,600 migrate away from the boundaries of pipe flow. For transitional flow (2,600<Re<9,000),a respective Nusselt number increase of 16% and 50% was observed for 2.6% and 5.1% Al2O3 concentrations in DI water, compared with base fluid results. However, pressure drop measurements showed that pumping power was increased by more than five times for the 2.6% concentration. An examination of the ratio of heat transfer enhancement to the pumping power increase (termed as merit parameter) as a function of Reynolds numbers indicates that the increase in pumping power is much greater than the corresponding heat transfer enhancement. This study additionally showed that concentrations of ND50-Syltherm800, TiO2-water and Al2O3-water nanofluids did not enhance convection heat transfer. Hence, the effect of nanoparticles on the heat transfer properties of a nanofluid appears dependent on the particular type of nanoparticle employed.