Laminar flow efficiency of stratified chilled-water storage tanks

• Published in: The International journal of heat and fluid flow 1998-02, Vol.19 (1), p.69-78
• Main Authors: Homan, K.O., Soo, S.L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chilled-water storage; Diffusion in liquids; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Heat conduction; Laminar flow; Laminar unsteady mixing; Mathematical models; Reynolds number; Stratified flow; Temperature distribution; Thermal stratification; Transport and storage of energy; Unsteady flow
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/S0142-727X(97)10003-0

This paper presents results for the efficiency of a stratified chilled-water storage tank with one inlet and one outlet. Numerical solutions for the two-dimensional, unsteady, laminar flow during stably stratified tank filling are compared with a one-dimensional model involving only conductive heat transfer across the thermocline separating the entering cold water and the exiting warm water. This one-dimensional model represents the minimum level of thermal mixing. The difference between the one-dimensional and two-dimensional models are revealed by a horizontal average of the governing equation for the two-dimensional model. Comparison reveals that for inlet Reynolds numbers of approximately 100, the efficiency of the actual, two-dimensional filling is less than 10% below the optimal efficiency of the model. Examination of an effective diffusivity, which can be associated with the mixing ignored in the one-dimensional model, reveals that the early and late stages of the tank-filling process are responsible for most of the deviation between the actual and ideal performances. For the present Reynolds number range, the two-dimensional predictions for effective diffusivity agree well with values derived from published experimental data.