A COEFFICIENT TO CHARACTERIZE MIXING IN SOLAR WATER STORAGE TANKS

A dimensionless coefficient is developed to characterize the level of mixing in solar water storage tanks. The MIX number, based on the heig ht weighted energy, or moment of energy, in the tank, ranges from 0 to 1, with 0 representing a perfectly stratified (unmixed) tank and 1 re presenting a fully mixed tank. Limiting values are based on theoretica l determinations of the maximum and minimum values of the moment of en ergy in a tank without mixing and a tank with complete mixing, respect ively. Use of the new MIX number is illustrated by experimental data o btained in a 372-liter storage tank operated with both a conventional drop-tube inlet and a rigid, porous stratification manifold. The initi al tank temperature profile, the temperature of the water entering the tank, and test duration are varied in three testing schemes. Fluid mi xing is quantified by measured vertical temperature profiles and the d imensionless MIX number.