An experimental study on stability and thermal conductivity of water/CNTs nanofluids using different surfactants: A comparison study

Nanofluids have proven their ability to improve the thermal conductivity of base fluids and heat transfer performance in automobiles, heat exchangers, solar collectors and air conditioning systems. However, the stability and optimum operating conditions of nanofluids require further study. Therefore, this study compares the effect of three different types of surfactants; Gum Arabic (GA), polyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS) on the stability and thermo-physical properties of carbon nanotubes (CNTs)/water nanofluids which include: density, viscosity, thermal conductivity and specific heat capacity. Furthermore, the heat transfer characteristics and pressure drop of the optimized nanofluid were investigated using a shell and tube heat exchanger. The stability results demonstrate ratios of (1:0.5) and (1:1) are sufficient to achieve a stable nanofluid for more than 6 months using GA and PVP. Moreover, Mathis TCi thermal conductivity analyzer and differential scanning calorimeter were used to measure thermal conductivity and specific heat capacity of nanofluids. The results demonstrate a significant enhancement of thermal properties, 36% in thermal conductivity and 50% in specific heat capacity using 0.1 wt% of CNT. The experimental results of the heat exchanger demonstrate that the heat transfer rate increases with the concentration of CNT up to 65%, with a maximum corresponding increase in pressure drop of about 15% using 0.5 wt% of CNT. The pumping power calculations indicate that the required power to provide the same amount of heat using nanofluids is one-third of that required for water.