We perform a systematic experimental study of heat conduction in aqueous suspensions of aluminum oxide nanoparticles at volume concentrations up to 10%. We develop a micro-hotwire device to reduce experimental errors resulting from spatial or temporal temperature inhomogeneity within a sample. The volume concentration dependence of the thermal conductivity can be explained using the effective medium model with a physically reasonable set of parameters. The average particle size as well as the thermal conductivity is measured as a function of sample sonication time and temperature. The size of particles/aggregates in our nanofluid samples is much greater than the nominal particle size reported by the manufacturers and does not change appreciably with sonication for up to 24 hours. Our data do not reveal any anomalous enhancement in the thermal conductivity or strong temperature dependence reported in other previous studies. The discrepancy may reflect subtle differences in nanopowders or nanofluid preparation procedures that result in drastic difference in the size or shape of suspended particles/aggregates.

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