Improving Thermal Performance in Building Heating, Ventilation, and Air Conditioning Systems: A Study of Natural Convection and Entropy in Plus-Shaped Cavity

Abstract

The impact of building design on energy efficiency has been widely studied, with cavity cooling emerging as an effective solution for indoor thermal comfort, where obstacles within the cavity can enhance fluid flow and improve natural convection heat transfer (HT). This research builds on the principles of cavity cooling for indoor thermal comfort, investigating entropy generation and HT behavior in a unique plus-shaped cavity containing a cold cylindrical element, analyzed through Computational Fluid Dynamics simulations. The Rayleigh number (Ra) ranges from 10³ to 10⁶, with a fixed Prandtl number (Pr) of 0.71, representing air as the working fluid, radius (r) of the cylinder ranges from 0 to 0.1, where r = 0 indicates no cylinder. The results indicate significant shifts in flow structure and temperature distribution across the cavity at varying Ra values, impacting the local and global entropy generation. High Rayleigh numbers lead to enhanced convective flows, intensifying entropy production near the cylinder surface due to steeper thermal gradients and vigorous recirculation zones. The increase in Ra from 10³ to 10⁶ leads to an increase in Nu_avg from 24.27 to 56.40 for the model without a cold object while from 39.62 to 123.83 for the model with a cold object. Moreover, the maximum enhancement in Nu_avg was 137.48% for Ra = 10⁵. Whereas, for the same value of Ra = 10⁵, the maximum increase in E_gen and Be was 474.92% and 33.16%, respectively. Also, a new correlation is proposed to calculate Nu_avg using response surface methodology. This study offers a comprehensive examination of heat flow characteristics and entropy generation rates in confined geometries, contributing valuable knowledge to thermal management and optimization in systems with internal obstacles.