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This work reviews experimental data and models for the thermal conductivity of nanoparticle suspensions and examines the effect of the properties of the two phases on the effective thermal conductivity of the heterogeneous system. A model is presented for the effective thermal conductivity of nanofluids that takes into account the temperature dependence of the thermal conductivities of the individual phases, as well as the size dependence of the thermal conductivity of the dispersed phase. We demonstrate that this model can be used to calculate the thermal conductivity of nanofluids over a wide range of particle sizes, particle volume fractions, and temperatures. The model can also be used to validate experimental thermal conductivity data for nanofluids containing semiconductor or insulator particles and confirm the size dependence of the thermal conductivity of nanoparticles. V V C 2010 American Institute of Chemical Engineers AIChE J, 56: [3243][3244][3245][3246][3247][3248][3249][3250][3251][3252][3253][3254][3255][3256] 2010

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We present experimental evidence of negative thermal conductivity enhancement in nanofluids consisting of 2 nm titania nanoparticles dispersed in 50% ͑w/w͒ water+ ethylene glycol. This behavior is unlike that of other nanofluids, which have been shown to exhibit positive thermal conductivity enhancements. Our results for titania nanofluids suggest that the thermal conductivity of 2 nm titania nanoparticles is smaller than the thermal conductivity of the base fluid at the same temperature, indicating a dramatic decrease in the thermal conductivity of titania particles as the particle size becomes of the same order as the phonon mean free path. Although such a decrease has been predicted for semiconductor nanoparticles by theory and simulation, experimental evidence has hitherto been lacking. Our results provide indirect experimental evidence for this decrease in metal oxide particles, and validate our previous work on alumina nanofluids that showed an exponential decrease in the thermal conductivity of alumina particles with decreasing particle size, from a limiting value for large ͑micron-sized͒ particles.

We present data for the thermal conductivity of nanofluids consisting of two sizes of ceria nanoparticles dispersed in water at 298 K. We also demonstrate that the effective thermal conductivity of these heterogeneous nanofluids can be described by a simple “mixing” rule that incorporates the size dependence of the thermal conductivity of the nanoparticles themselves. Our data follow the same trends with particle size as shown for alumina nanofluids in our previous work, and provide additional validation for that data.