Dispersibility And Stability Of Water Based Nanofluids And Its Effect On The Thermal Conductivity

Improving the thermal conductivity of ice storage material in both liquid and solid phase is an effective approach to enhance heat transfer in ice storage tank. In our experiment, TiO2 nanoparticles were added into deionized water to prepare nanofluids to improve the thermal conductivity of water and ice. Dispersant which would influence the thermal conductivity of nanofluid, plays an important role in preparing stable nanofluid, and the mechanism of the dispersant is complicated. The aim of this paper is to explore the effect of dispersant on the dispersibility and stability of nano fluid in liquid phase and the thermal conductivity of nanofluids in ice and liquid phase. The main work and results are summarized as follows:1. Seven rutile TiO2-DW nanofluids with particle weight fraction of 0.7% were used to study the dispersibility and stability of nanofluid, in which the weight ratio of SDS and TiO2 were 0,0.5:10,1:10,3:10,5:10,7:10 and 10:10, respectively. A two-steps method was introduced to prepare the nanofluids. Dispersant (SDS), ultrasonic vibration and PH agent were used in this method. Transmission electron microscopy, particles size analysis and weighing method were introduced to the characterized the dispersibility and stability of nanofluids.2. Comparison was done between a Hot Disk thermal constant analyzer (TPS 2500S, HotDisk AB, Sweden) and a nanoflash system (LFA 447, NETZSCH Instruments Inc) in order to select an appropriate instrument for measuring the thermal conductivity of nanofluids. The experimental results were compared with those of the H-C model and Xuan's model.3. Two theoretical models (one with dispersant and the other without dispersant) available for estimating the thermal conductivity of solid phase nanofluid were established by means of the laws of the minimal thermal resistance and the criteria of equivalent specific thermal conductivity. The effect of dispersant and heat transfer characteristics of nanoparticles were take into account. And nanoparticles in nanofluids were assumed to be without aggregations. Anatase TiO2-DW nanofluids were prepared using the method mentioned above, and measured the thermal conductivity in solid phase by a Hot Disk thermal constant analyzer with a cooling system. Experimental results were compared with theoretical calculations.The results obtained from particles size analysis and weighing method demonstrate that the particle size were smallest when the weight ratio of SDS to nanoparticles was 1:10. The particle size and weight fraction of nanoparticles decreased with time, it is because the sedimentations of particles with bigger size were more quickly. Thermal conductivity ratios of nanofluid to base liquid had no obvious variation with weight ratio of dispersant, but decreased with time. Linear fit of thermal conductivity of nanofluid with weight fraction of nanoparticles was higher than the results of H-C model and Xuan's model.The calculated results of thermal conductivity of nanofluids in solid phase suggest that the thermal conductivity increase with the increase of both particle volume fraction and the thermal conductivity of dispersant and nanoparticles. Calculated results of thermal conductivity of nanofluid without dispersant were higher than that with dispersant. And the difference between them is bigger when particle size is smaller. Difference between the experimental results and calculated results were±2%.It is likely that dispersant has an effect on the dispersibility and stability of nanofluid. And the thermal conductivity of nanofluid was influenced by the dispersibility and stability. The energy transfer processes in nanofluid were complex due to the addition of dispersant. This may lead to the difference between the experimental results and the theoretical results.