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

The conversion into nanofluids is considered a suitable solution to increase the heat transfer efficiency of such fluids. Several theories with an emphasis on different thermal nanofluid mechanisms have appeared to predict enhanced conductivity measurements. There are many ways to measure the thermal conductivity of fluids. 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 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.