Numerical Study on Mixing Characteristics of hot Water inside the Storage Tank of a Solar System with Different Inlet Velocities of the Supply Cold Water

To investigate the effects of hot water withdrawal from the storage tank of a solar system on the utilization percentage of solar heat gain, the mixing characteristics of hot water inside the storage tank with different inlet velocities of the supply cold water is studied by numerical simulations in this work. A parameter, called the entrainment factor, is introduced as an indication to quantify the water mixing in the tank. The useful thermal energy removal from the tank for different inlet velocities of the supply cold water is calculated and analyzed. The numerical simulation results show that an increase in the inlet velocity of the supply cold water will result in a larger entrainment factor and stronger water mixing in the tank. Hence the extent of water mixing is a key factor to determine the useful energy removal from the tank and the inlet velocity of the supply cold water is the main factor to determine the entrainment factor. The water discharging curves are obtained under different operation conditions and it is found that the temperature of the discharged water undergoes three stages: stable high-temperature stage, rapid decrease stage and stable low temperature stage, and the time durations of these stages are related with the inlet velocity. The useful energy removal from the tank under various operation conditions is calculated, and a relation to estimate such energy is obtained. Finally, the water mixing characteristics in the tank under different operation conditions are summarized, aiming at providing some useful information for solar engineers to optimize the design of solar systems.