Effect of Buoyancy Ratio on Double-Diffusive Natural Convection Flow in a Rectangular Enclosure

Abstract

Double diffusive natural convection driven by Simultaneous temperature and concentration gradients are fundamental to many engineering and environmental processes. The buoyancy ratio N , defined as the ratio of solutal to thermal buoyancy forces, critically determines whether thermal or solutal effects dominate the flow structure, heat transfer, and species transport. While most existing studies rely on two-dimensional simplifications and alternative numerical methods, this paper presents a three-dimensional numerical investigation of the effect of the buoyancy ratio on steady, laminar double-diffusive natural convection in a rectangular enclosure with an aspect ratio of 0.5. The continuity, momentum, energy, and concentration equations governing the flow are solved using the finite-volume method with the SIMPLE algorithm on a staggered grid. Parametric simulations are performed for buoyancy ratios N = 1.0, 2.5, 5.0, and 10.0, with fixed Prandtl number Pr = 7.0, Lewis number Le = 2.5, and Rayleigh number Ra = 50000. Results show that increasing the buoyancy ratio from 1.0 to 10.0 significantly enhances the uniformity of the temperature field and enhances vertical heat transfer. Concentration profiles become sharper and more localized as the buoyancy ratio is increased. At a high buoyancy ratio, solutal buoyancy dominates, producing a thin solutal boundary layer and strong stratification, thereby promoting salt fingering. Velocity profiles become stronger, more organized, and more stable as the buoyancy ratio increases. The 3-dimensional nature of the flow reveals that the effect of the buoyancy ratio in the enclosure is isotropic; the primary circulation plane along the z - y plane shows the greatest changes, while the secondary planes ( x - y and x - z ) show only a significant change. The finite volume method proves to be an effective tool for 3-dimensional parametric studies. The results provide a valuable benchmark for engineering applications where controlling the balance between thermal and solutal buoyancy is crucial, such as chemical reactors, drying systems, and thermal management devices.