LAMINAR-FLOW EFFICIENCY OF STRATIFIED CHILLED-WATER STORAGE TANKS

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 stratif ied tank filling are compared with a one-dimensional model involving o nly conductive heat transfer across the thermocline separating the ent ering cold water and the exiting warm water. This one-dimensional mode l represents the minimum level of thermal mixing. The difference betwe en the one-dimensional and two-dimensional models are revealed by a ho rizontal average of the governing equation for the two-dimensional mod el. Comparison reveals that for inlet Reynolds numbers of approximatel y 100, the efficiency of the actual, two-dimensional filling is less t han 10% below the optimal efficiency of the model. Examination of an e ffective diffusivity, which can be associated with the mixing ignored in the one-dimensional model, reveals that the early and late stages o f the tank-filling process are responsible for most of the deviation b etween the actual and ideal performances. For the present Reynolds num ber range, the two-dimensional predictions for effective diffusivity a gree well with values derived from published experimental data. (C) 19 98 Elsevier Science Inc. All rights reserved.