Hydraulic and heat transfer study of SiO2/water nanofluids in horizontal tubes with imposed wall temperature boundary conditions

The convective heat transfer of SiO2/water colloidal suspensions (5-34 wt.%) is investigated experimentally in a flow loop with a horizontal tube test section whose wall temperature is imposed. Experiments were performed at different inlet temperatures (20, 50, 70 degrees C) in cooling and/or heating conditions at various flow rates (200 < Re < 10,000). The Reynolds and Nusselt numbers were deduced by using thermal conductivity and viscosity values measured with the same temperature conditions as those in the tests. Results indicate that the heat transfer coefficient values are increased from 10% to 60% compared to those of pure water. They also show that the general trend of standard correlations is respected. The problem of suspension stability at the highest temperatures is discussed. In order to evaluate the benefits provided by the enhanced properties of the nanofluids studied, an energetic performance evaluation criterion (PEC) is defined. This PEC decreases as the nanoparticle concentration is increased. This process is also discussed in this paper. (c) 2011 Elsevier Inc. All rights reserved.