Possible explanations of the deviations in the thermal conductivity measurements

Ever since the report of the abnormal thermal conductivity enhancement of nanofluids by the late S.U.S. Choi, many researchers have observed the extraordinarily high thermal conductivity. However, there are deviations between research groups in the effective thermal conductivity measurements, which leaded researchers to doubt about the using nanofluids as an alternative heat transfer fluid. In this study, the cautions in thermal conductivity measurements using transient hot-wire method, and the non-equilibrium nature of nanofluids were elucidated as possible explanations of the deviations. It was found to be very important to avoid the impacts of finite wire thermal capacity and natural convection during the thermal conductivity measurement using transient hot-wire method, and the particle contents as well as its size distribution in base fluids should be always monitored and reported as the measurement results of nanofluid thermal conductivities.