<p>The main significance of this research is to confirm the importance of key factors in improving heat transfer, neglecting these factors may cause significant impacts on the progress of nanofluid applications, namely thermal management and electronic cooling systems. The issue of heat transfer of mixed convection in rectangular chambers containing Al₂O₃/water nanofluids is numerically explored using the alternating direction implicit finite difference method. The vertical walls are subjected to heat fluxes, and the horizontal ones are adiabatic and mobile. The results showed the influence of different parameters: aspect ratio, 0.25 ≤ <i>A</i> ≤ 4, tilt angle, 0 ≤ <i>θ</i> ≤ 90, Peclet number, 0 ≤ <i>Pe</i> ≤ 100, and volume fraction, 0 ≤ <i>φ</i> ≤ 0.05, on heat transfer rate and flow. The outcomes showed that the use of nanofluids resulted in improvements in heat transfer, but this was only noticeable when the aspect ratio was less than a certain value. Beyond this threshold, pure water performed better in terms of heat transfer than the nanofluid. The critical value for the aspect ratio tends to vanish with the increase in Peclet number. This is due to the competition between the enhanced viscosity and conductivity where one can overcome another one depending on the convection mode. Additionally, tilt angle affects heat transfer in a parabolic way, with best angles varying with increasing Peclet number and nanoparticle volume fraction. These results indicate the necessity to determine the best operating conditions to achieve maximum thermal efficiency in heat transfer by means of nanofluids.</p>