Aspect ratio's critical role in enhancing natural convective heat transfer with temperature-dependent nanofluids within rectangular enclosures

Significant research has been performed on natural convection involving nanofluids inside rectangular enclosures since they are extensively used in thermal engineering. However, a significant gap remains in comprehending the conflicting results in nanofluid behavior observed between experimental and numerical studies. This study seeks to bridge this divide by examining natural convective heat transfer within vertical and horizontal enclosures filled with Al2O3/Water nanofluid. Using temperature-dependent viscosity and thermal conductivity, we aim to elucidate the key parameters influencing nanofluid enhancement. The results obtained through numerical simulations, using the finite difference method, showed the impact of different parameters namely nanoparticle volume fraction, 0 ≤ φ ≤ 0.05, nanoparticle size, dnp = 13, 29, and 45 nm, and aspect ratio, 0.25 ≤ A ≤ 4, on various factors, including heat transfer rates, maximal stream function, and temperature and stream function contours. The outcomes revealed that the use of nanofluids leads to improvements in heat transfer, but only when the aspect ratio is below certain critical values, Acr = 0.56, 0.48, and 0.53 for φ = 0.01, 0.03, and 0.05 respectively. Beyond these values, pure water performs better than the nanofluids in terms of heat transfer where the enhancement drops to -8%, -23%, and -22% when φ = 0.01, 0.03, and 0.05 respectively. Additionally, this critical value relies on the nanoparticle volume fraction and it rises as the nanoparticle diameter augments. This effect is believed to be caused by the high enhancement in viscosity compared to thermal conductivity when the convective mode dominates the conductive one. Furthermore, a larger aspect ratio leads to better heat transfer and the smaller nanoparticles are favorable for better heat transfer for a particular nanoparticle volume fraction. These findings carry significant implications for the field of nanofluid engineering, emphasizing the indispensable consideration of aspect ratios in optimizing thermal performance. © 2023 The Author(s)