In this letter, we report an experimental correlation [Eqs. (1a) and (1b) or (1c)] for the thermal conductivity of Al2O3 nanofluids as a function of nanoparticle size (ranging from 11nmto150nm nominal diameters) over a wide range of temperature (from 21to71°C). Following the previously proposed conjecture from the theoretical point-of-view (Jang and Choi, 2004), it is experimentally validated that the Brownian motion of nanoparticles constitutes a key mechanism of the thermal conductivity enhancement with increasing temperature and decreasing nanoparticle sizes.

Exact comparisons of the thermal conductivities of the base fluid and a nanofluid are very important in the early stages of nanofluid development. A simple procedure of measuring the thermal conductivity of the two fluids by the transient hot wire method and numerically dividing these values is used for this purpose. However, because the experiments are not performed simultaneously and the physical properties of the measurement system are sometimes not properly known, large errors are incurred during the evaluation process. This article proposes a new apparatus for thermal conductivity comparison where the working principle is mainly based on relative measurement rather than absolute measurement. The measuring circuit and data processing steps are explained in detail; a validation test was performed using the well-known glycerine and engine oil.Corresponding Author, shinpyo@kgu.ac.kr 2015 The Korean Society of Mechanical Engineers Ⓒ

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