Numerical and experimental study on the design of a stratified thermal storage system

The purpose of this study is to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions. To this end, a computer program is developed to investigate the fluid flow in a tank, using Patankars SIMPLE algorithm. The validation of program is made successfully by the comparison against experimental data measured with a bench scale facility. Further a systematic investigation has been made in terms of important design and operational parameters such as storage tank size (commercial-scale and bench-scale), loading time, shape of diffuser, turbulence model and inlet velocity or Fr No. Considering the thermal efficiency of storage tank is critically impaired by the effect of flow recirculation and mixing by turbulence, a novel model with minimum mixing between hot and cold water is proposed for the evaluation of the performance of storage system by the assumption of the uniform plug-type flow. This is made by solving only the following governing equation of temperature which has no convective mixing with constant axial velocity, that is, partial derivative(rhoT)/partial derivativet + upartial derivative(rhoT)/partial derivativex = del (.) (k/c(p)delT) Based on a series of parametric investigation, a number of useful results can be drawn. In general the large scale system shows better storage performance than the small system. As the increase of loading time, the degree of stratification lowered due to the increased effect of heat transfer by convection and diffusion via thermal stratification region. And the curved type diffuser shows better performance of thermal stratification compared to the diffuser of flat type. As for the case of large-scale tank, the effect of Fr No., or inlet velocity does not show any significant effect on the thermal stratification compared to the small size one. The calculated results show that the model of ideal type-plug flow can be used as a possible tool for the evaluation of performance of storage system by giving the reference condition of best thermal stratification or least mixing condition. But in the case of full scale system, the difference between ideal and actual cases not significant compared to the case of bench scale. It is considered possibly due to the relatively decreased heat transfer effect by the increased effect of water flow in the full scale system. Two turbulence models, say standard k-epsilon and RNG k-epsilon models, show no visible difference in this study. (C) 2003 Published by Elsevier Ltd.