Heat transfer study of water and air-based nanofluids with Al₂O₃ nanoparticles in a circular pipe using a multiphase approach

Abstract

The current work investigates the thermal characteristics of nanofluid flow (water and air as base fluids with Al₂O₃ nanoparticles) in a circular pipe at constant heat flux. Numerical simulations were performed using the Eulerian-Euleian two-phase model with an RNG $k-\epsilon$ turbulent model with enhanced wall function. Results showed that the two-phase approach reduced the error by about 3–5% in the prediction of the average heat transfer coefficient. Within the range of a volumetric fraction of nanoparticles from 1 to 5% and inlet velocity from 10 to 25 m/s, heat transfer performance increased significantly compared to pure fluid flow. For water, the particle motion from the wall to the center of the pipe was empowered by thermophoresis in addition to higher turbulent kinetic energy leading to a profit index of about 3.5, while for air; the Brownian motion of particles increased the viscosity and thermal conductivity near the wall leading to profit index of about 240 indicating high amount of heat transfer. For both fluids, the heat transfer effectiveness ratio increased at a higher volume fraction of nanoparticles (3.5 for water-based nanofluid and 140 for air-based nanofluid). These values indicate promising effects of nanoparticle addition, especially for air, where the volume fraction had a much more significant effect than water.

Keywords:

• Nanofluid
• Al₂O₃ nanoparticles
• Eulerian
• heat transfer
• Nusselt number

Disclosure statement

No potential conflict of interest was reported by the author(s).