Nanoparticle morphology effects in heat and mass transfer processes .

摘要

Nanoparticle morphology is thought to be an important factor influencing heat and mass transfer rates in liquid systems. How nanoparticles mechanistically increase heat and mass transfer rates is not well understood. Both dispersed nanoparticles and aggregated nanoparticles are thought to play important roles. Dispersed nanoparticles and associated nanoparticle Brownian movements are purported to cause the enhancements by mixing mechanisms whereas aggregated nanoparticles are purported to cause enhancements by forming highly conductive paths. In this study, morphologies of nanoparticle were quantified in laboratory studies and related to laboratory measured heat and mass transfer rates.;Brownian motion effects were experimentally explored by studying effects of non-aggregating spherical SiO2 nanoparticles on oxygen and NaCl mass transfer rates. No mass transfer enhancements were found in the presence of nanoparticles. Oxygen transfer rates were actually diminished by 33% at the highest nanoparticle volume fraction; this is attributed to solution viscosity effects (22% reduction associated with lower liquid film transfer coefficients) and the obstruction effects of impermeable nanoparticle (8% reduction). No evidence was found in our study to substantiate the purported Brownian motion micro/nano-scale convection effects often used by others to explain increased heat and mass transfer rates associated with nanoparticles in liquid systems. This finding indicates that heat transfer enhancements, for some nanofluids, may be primarily caused by other mechanisms including nanoparticle aggregation forming highly conductive paths. In additional studies, different aggregated morphologies of Al2O3 nanoparticle and associated thermal conductivity were investigated. Morphologies were measured with imaging techniques and light scattering methods and quantified using aspect ratios and fractal dimensions. Results showed that highest thermal conductivity enhancement was 29% for 6.4% diffusion limited aggregation (DLA) nanofluids. DLA thermal conductivity enhancements could be predicted with effective medium theory using aspect ratio of 3.65, and the enhancements of dispersed nanoparticle and reaction limited aggregation (RLA) were in agreement with effective medium theory prediction with aspect ratios 1.89 and 1.73. However, because nanofluid parameters tested were limited, the ability to predict enhancement based upon the effective medium theory with aspect ratios considered for aggregates was verified only for the nanofluid and conditions tested in our studies.;As a final study, the morphology characterization techniques developed to study heat transfer mechanisms were extended and employed to quantify nanoparticle morphology effects on nanoparticle filtration behaviors. Results showed that aggregate morphologies impacted filtration behaviors, and nanoparticle rejections close to 1 were possible even when membrane pore sizes greatly exceed nanoparticle size. There was no difference in terms of rejection between different aggregate structures. However, filtration resistance of compacted aggregates was 43% higher than that of loose aggregates due to the dense fractal structure of cake fouling layer aggregates. Prediction of cake layer fouling resistances ratio was within 7% difference when using fractal dimension measurements of the aggregates.