Unsteady Natural Convection and Entropy Generation in Thermally Stratified Trapezoidal Cavities: A Comparative Study

This study numerically investigates unsteady natural convection (NC) heat transfer (HT) and entropy generation (Egen) in trapezoidal cavities filled with two thermally stratified fluids. Both air-filled and water-filled configurations are analyzed to evaluate and compare their thermal performance under varying conditions. The cavities are characterized by a heated base, thermally stratified sloped walls, and a cooled top wall. The governing equations are numerically solved using the finite volume (FV) approach. The study considers a Prandtl number (Pr) of 0.71 for air and 7.01 for water, Rayleigh numbers (Ra) ranging from 103 to 5 × 107, and an aspect ratio (AR) of 0.5. Flow behavior is examined through various parameters, including temperature time series (TTS), average Nusselt number (Nu), average entropy generation (Eavg), average Bejan number (Beavg), and ecological coefficient of performance (ECOP). Three bifurcations are identified during the transition from steady to chaotic flow for both fluids. The first is a pitchfork bifurcation, occurring between Ra = 105 and 2 × 105 for air, and between Ra = 9 × 104 and 105 for water. The second, a Hopf bifurcation, is observed between Ra = 4.7 × 105 and 4.8 × 105 for air, and between Ra = 105 and 2 × 105 for water. The third bifurcation marks the onset of chaotic flow, occurring between Ra = 3 × 107 and 4 × 107 for air, and between Ra = 4 × 105 and 5 × 105 for water. At Ra = 106, the average HT in the air-filled cavity is 85.35% higher than in the water-filled cavity, while Eavg is 94.54% greater in the air-filled cavity compared to water-filled cavity. At Ra = 106, the thermal performance of the cavity filled with water is 4.96% better than that of the air-filled cavity. These findings provide valuable insights for optimizing thermal systems using trapezoidal cavities and varying working fluids.