A Simulation Study of Forced, Free, and Mixed Convection of Al2O3-Water Nanofluid Flow Inside a Cavity

This study aims to investigate the heat transfer and fluid flow characteristics of alumina (Al₂O₃) based nanofluids under various convective conditions, with a particular focus on understanding the interplay between thermal performance and hydrodynamic costs. The primary goal is to elucidate the innovative aspect of using nanofluids in different flow regimes, addressing a fundamental question regarding their efficiency.
A numerical simulation approach was employed to analyze the behavior of the nanofluid. The simulations were conducted under specific boundary conditions, including a cold, rightward-moving top plate and a hot, leftward-moving bottom plate. The side walls were treated as adiabatic. The nanofluid, containing 5% by volume of Al₂O₃ nanoparticles, was modeled considering both forced convection due to the moving plates and natural convection driven by temperature differences. The Richardson number, representing the ratio of natural to forced convection, was systematically varied to cover a wide range of flow regimes.
This study numerically investigates the thermofluid performance of Al₂O₃–water nanofluid (5% vol.) in a cavity across forced (Ri=0.1), mixed (Ri=1), and natural (Ri=10) convection regimes. While heat transfer shows modest gains (Nu increases from 13.67 to 13.77 at Ri=0.1, and 14.78 to 16.23 at Ri=1), the friction factor significantly rises (e.g., from 1.31 to 2.39 at Ri=0.1). The Thermally Enhanced Performance (TEP) index consistently remains below unity (approx. 0.82-0.84), indicating that increased viscous resistance outweighs thermal benefits. Consequently, the Al₂O₃–water nanofluid does not enhance overall system energy efficiency under the studied conditions, highlighting the need for holistic performance assessment in engineering applications.