The influence of nanofluid ph on natural convection

Abstract

The vast majority of experimental studies of nanofluids under natural convection have shown that the heat transfer rate decreases in contrast to observations of increased heat transfer rate for forced convection and boiling heat transfer. This surprising result has not been fully understood and the purpose of this study is to shed light on the physics behind the decrease of heat transfer in Al2O3 – deionised (DI) H2O nanofluids under natural convection. A classical Rayleigh-Benard configuration has been employed, where the test medium is heated from the bottom and cooled from the top of an optically accessible chamber, while the sidewalls are insulated. Al2O3 – H2O nanofluids with nanoparticle concentration within the range of 0.03 to 0.12 vol. % are used and tested under turbulent natural convection, Rayleigh number Ra ~ 109, until steady state conditions are reached. For the synthesis of the nanofluid, pure DI water and high purity nanopowder, supplied by two different vendors, are involved with and without adopting the electrostatic stabilization method. The temperature measurements at different locations around the chamber allow the quantification of the natural convection heat transfer coefficient and the corresponding Nusselt and Rayleigh numbers. All the measured quantities are compared with those for DI water that serves as a benchmark in this study. It is found that the presence of nanoparticles systematically decreases the heat transfer performance of the base fluid under natural convection. An explanation for the reported degradation can be attributed to the buoyant and gravitational forces acting in the system that appear to be inadequate to ensure or maintain good nanofluid mixing. The results also show that as the nanoparticle concentration increases, the temperature of the heating plate increases, suggesting the presence of an additional thermal barrier imposed at the hot plate of the chamber. This can be attributed to the formation of a stationary thin layer structure of nanoparticles and liquid close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. The addition of a small amount of acetic acid to control the pH value of the nanofluid reduces the thickness of the thin layer structure close to the hot plate, leading to reduction of the rate of heat transfer decrease. A similar behaviour is observed when a different nanopowder that forms an acidic suspension is used. This behaviour is credited to the significantly increased nanofluid stability attained through the electrostatic stabilization method. Such a method takes advantage of the repulsive forces imposed due to the electric double layers that surround individual nanoparticles. The understanding of the influence of the nanofluid pH on the stability of nanosuspensions and its impact on heat transfer rate can lead to future guidelines for the effective use of nanofluids.