Experimental investigation of surfactants on thermal and heat transfer properties of nanofluids

In recent years, studies on th use of nanofluids, an innovative approach to enhance heat transfer efficiency, have been increasing rapidly. However, some factors limit the use of nanofluids. The most important of these factors is not being able to provide nanofluid stability for a long time, but different methods have been developed accordingly. One of these methods is to use surfactant as a dispersant. Within the scope of this experimental study, it was aimed to determine the effects of three different surfactants on the thermophysical properties of the pure water which was selected as the base fluid, the physical properties and stability of the nanofluids obtained by using nano-sized Al2O3 particles, and the heat transfer characteristic of the prepared nanofluid using a shell and tube heat exchanger. For these purposes, 3 different surfactants, SDS, NP 10, and Tween 80, were mixed to pure water with a ratio of 0.2% 0.4% 0.6% and 0.8% by weight, and solutions were obtained. Density, viscosity, and thermal conductivity measurements of these solutions at 25 °C, 35 °C, 45 °C, 55 °C and 65 °C were carried out. Afterward, using the Al2O3 particles with an average particle size of 18 nm, nanofluids were prepared with a volumetric ratio of 0.3%, 0.5%, 0.7%, 0.9%, and 1.1%. Surfactant and ultrasonic homogenizer were used to increase the stability of nanofluids. Density, viscosity, and thermal conductivity of the obtained nanofluids at 25 °C, 35 °C, 45 °C, 55 °C and 65 °C were measured. Meanwhile, the sedimentation method was used to determine the stability of the prepared nanofluids. To investigate the heat transfer performance, water and nanofluids, containing Al2O3 and Tween 80 with the ratio of 0.3% and 0.2% respectively, were fed to the tube side of the heat exchanger at 4 different flow rates (2, 3, 4 and 5 L/min) and the total heat transfer coefficient was calculated. As a result of the study, it has been observed that the thermophysical properties of nanofluids prepared without the use of surfactants are substantially compatible with the literature. It was seen that densities of the nanofluids at all concentrations increased with the concentration of particles proportionally. Besides, the viscosity of the fluid increased with the addition of nanoparticles, but no regular increase was observed due to particle aggregation. It has been determined that the thermal conductivity values of nanofluids prepared at different rates are higher than the values of water. As the temperature increased, the thermal conductivity values of the fluids increased and the maximum thermal conductivity value obtained at 25 oC was achieved with nanofluid prepared using 1.1% Al2O3 by volumetric and 0.2% Tween 80 by weight ratio. SDS surfactant was found to have a negative effect on the stability of nanofluids, and the best stability was achieved with the Tween 80 surfactant. Besides, the use of nanofluid improved the total heat transfer coefficient and its maximum value was calculated as 11.54%. When the flow rate of fluids fed to the heat exchanger increased, the total heat transfer coefficient enhanced.