Transient Natural Convection and Entropy Generation Analysis in a Stratified Square Enclosure

Articles in Press

Document Type : Research Article

Authors

1 Department of Chemical Engineering, Z. H. Sikder University of Science and Technology, Shariatpur 8024, Bangladesh

2 Department of Computer Science and Engineering, Z. H. Sikder University of Science and Technology, Shariatpur 8024, Bangladesh

3 Department of Mathematics, Jagannath University, Dhaka 1100, Bangladesh

Abstract

This study examines transient natural convection (NC) heat transfer (HT) and entropy generation (Egen) in a square enclosure containing thermally stratified water. The model considers uniform heating at the bottom wall, stratified vertical walls, and a cooled top boundary. Governing equations are addressed through the finite volume (FV) method, with simulations performed across a range of Rayleigh numbers (Ra) from 100 to 5 × 10⁶, a fixed Prandtl number (Pr) of 7.01, for water. Various physical quantities were analyzed across the spectrum of Ra to capture the complex dynamics of convective flow. This encompasses streamline and isotherm plots, the temporal evolution of temperature, phase-space representations via limit cycles and limit points, along with spectral analysis, including the average Nusselt number (Nu), local entropy generation (El), and local Bejan number (Bel). The results reveal successive bifurcations with increasing Ra: a pitchfork bifurcation (Ra = 8 × 10³–10⁴) leads to symmetry breaking, a Hopf bifurcation (Ra = 2 × 10⁵–3 × 10⁵) induces periodic oscillations, and chaotic flow emerges at Ra = 10⁶–2 × 10⁶. The average Nusselt number at the bottom wall increases from 5.84 for Ra = 2 × 105 to 15.87 for Ra = 2 × 106, corresponding to a 171.75% enhancement in the HT. The numerical framework was validated by comparing it with existing numerical literature, confirming the results’ reliability.

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AUT Journal of Mechanical Engineering

Articles in Press, Accepted Manuscript
Available Online from 07 October 2025

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