A comparison of nanofluid thermal conductivity measurements by flash and hot disk techniques

Heat transfer fluids, such as water, mineral oil, and ethylene glycol, always have an important role in many industrial processes. The poor heat transfer properties of these common fluids hamper their effectiveness as heat exchangers. The conversion into nanofluids is considered a suitable solution to increase the heat transfer efficiency of such fluids. Ever since the report of the abnormal thermal conductivity k enhancement of nanofluids by Choi et al. [1], many researchers have tried to explain the mechanisms leading to extraordinarily high thermal conductivity. Accordingly, several theories with an emphasis on different thermal nanofluid mechanisms have appeared to predict enhanced conductivity measurements. The predictive models were developed using the experimentally nanofluid measured thermal conductivity data, provided by many research groups. There are many ways to measure the thermal conductivity of fluids, such as the cylindrical cell method, temperature oscillation method, steady-state parallel-plate method, 3?? method, thermal constants analyser method, thermal comparator method, and transient hot-wire or hot disk method. However, the experimental findings have been controversial and theories do not fully explain the mechanisms of elevated thermal conductivity. Some researchers argued that the anomalous k enhancement data are caused by inaccuracies of thermal measurement methods. In this paper, measurements on thermal conductivities of nanofluid mixtures (alumina/water) by means of two different methods are accomplished, i.e. the flash and the hot disk technique. In the first method, a NETZSCH model LFA 447 NanoFlash is employed, while in the second one a Hot Disk model TPS 2500 S is used. A comparison between the results obtained from the different measurement techniques is done. Two-step method is used to prepare nanofluids mixtures with a nanoparticles volumetric concentration from 0.1% to 4%. Each mixture, at assigned volumetric concentration, is treated with a sonicator for different times and thermal conductivity is measured in the range of temperature from 20°C to 50°C. Moreover, for assigned volumetric concentration and sonication, the stability analysis is performed and thermal conductivity measurements are carried out to determine the effect of sonication time. Results show the thermal conductivity dependence on sonication time and an asymptotic value is evaluated for each volumetric concentration.