| Cooling is one of the important challenges faced by many industries. Nanofluids, suspensions of high thermal conductivity nanoparticles in conventional heat transfer fluid, are promising candidates for novel heat transfer fluids with unique heat transfer properties. High thermal conductivity, low density and variable aspect ratios make carbon nanoparticles very attractive for nanofluid applications.; The thermal properties of nanofluids are closely related to the microstructure of the dispersions. Rheological measurement is one of the most powerful tools to study the aggregation structure in the dispersions. There are several factors that affect the thermal and rheological properties of solid-liquid dispersions. These include the particle shape, dispersant chemistry, temperature, dispersant concentration, dispersing energy, and nanoparticle loading in the dispersions.; Low aspect ratio graphite nanoparticle dispersions show stronger elasticity but higher sensitivity to shear stress than high aspect ratio nanotube dispersions. The power law indices for 1 wt% graphite nanoparticle dispersion and nanotube dispersion are 1.86 and 0.77 respectively. Dispersant chemistry affects the rheological behaviors of the dispersions; poorly dispersed nanoparticles lead to a several orders of magnitude viscosity increase above the base fluids and cause a two-stage shear thinning behavior.; Abnormal viscosity increases occur in some carbon nanofluids at high temperature (>60°C). The polymer chains of the dispersant contract at high temperature and contribute to the formation of network structures in the nanofluid.; The thermal and rheological properties of nanofluids always vary with the size of agglomerates in the fluids. Fluids with large scale agglomerates have high thermal conductivities and viscosities. Developing practical heat transfer fluids containing nanoparticles may require a balance between the thermal conductivity and viscosity of the dispersions.; The heat transfer coefficients of carbon nanofluids are higher than those of base fluids (up to 22% increase) in laminar flow. However, the results are lower than the values predicted from conventional models developed for homogenous fluids or a new developed model for spherical nanoparticle dispersions. Further study is needed to understand the mechanisms for the heat transfer in carbon nanofluids.; Keywords. Carbon nanofluids, Poly (alpha-olefin), Thermal conductivity, Rheology, Heat transfer coefficient... |
