Key words: Stratified fluid layers; Salt–gradient pond; Hydrodynamic, heat and mass transfer; Numerical simulation; Finite–volume method; Transient regime.

Abstract: In this paper, we studied numerically the effect of thermal Rayleigh number and the buoyancy ratio on the transient fluid flow, heat and mass transfer in a salt–gradient pond (SGP) wherein the layers of fluid are stratified. The objective of this numerical work is to give a fine knowledge of the hydrodynamic, thermal and solute characteristics during the storage of thermal energy in a SGP. The pond is filled with a mixture of salt and water to constitute three zones with different salinities: Upper Convective Zone (UCZ), Non–Convective Zone (NCZ) and Lower Convective Zone (LCZ). Water is heated by a heat serpentine covered the bottom of the pond. The transport equations for continuity, momentum, thermal energy and mass transfer are solved by a finite–volume method to provide
the fields of temperature, concentration and velocities in a SGP in transient regime. Simulations are performed for several values of both the thermal Rayleigh number in the range between 104 and 107, and the buoyancy ratio in the range between 0 and 10, whose influence on the temporal evolution of velocities, temperature and concentration distributions in a SGP are analyzed and discussed. The flow structure shows the generation of convective cells in the lower and upper zones of the pond, and permits to explain the slight increase of temperature in the UCZ and the important rise of temperature in the LCZ. In addition, this work shows the importance of the buoyancy ratio to preserve the high temperature in the bottom of the pond and to reduce the phenomena of heat and mass transfer across the NCZ.