Effect of baffle and shroud designs on discharge of a thermal storage tank using an immersed heat exchanger. (15th November 2017) - Nicodemus, Julia Haltiwanger
Highlights: Heat transfer to an immersed heat exchanger was improved by all 3 baffle-shrouds. Heat transfer was primarily improved via increased storage fluid velocity. Thermal stratification had a secondary but significant role in observed benefits. Baffle width creates a trade-off between improved stratification & fluid velocity. The simplest design performed best, improving nondimensional heat transfer by 27%. Abstract: Heat exchangers immersed in thermal storage tanks are an increasingly popular way to charge and/or discharge energy from the tank. The present experimental study investigates the effects of different baffle and shroud configurations on heat transfer to an immersed copper coil heat exchanger during discharge of a thermal storage tank. The baffle-shrouds create annular regions with the tank wall within which the heat exchanger is located. Negatively buoyant plumes form from the heat exchanger and are directed by the shroud, which surrounds the heat exchanger, into the annular baffle below and through the baffle to the bottom of the tank, while water from the top of the tank is drawn into the top of the shroud to flow over the heat exchanger. Experiments are conducted in 300 L unpressurized storage tank filled with initially isothermal water at 60 °C. Water flows through the coiled copper tube heat exchanger placed at the top of the tank at a rate of 0.1 kg/s and with an inlet temperature of 20 °C. Transient heat transfer rate, produced water temperature, fractional energy discharge, temperature distributions in the tank, and Nu D – Ra D correlations are used to assess how the baffle-shrouds affect tank performance. The three baffle-shroud configurations represent different degrees of fidelity to numerical optimization studies in the literature. However, the simplest design and the one with the least fidelity to those studies—a straight baffle-shroud with a constant width of twice the heat exchanger diameter—performs best by all measures considered. The straight baffle-shroud increases the storage-side convective heat transfer by 27% in the first 90 min of discharge relative to a control experiment with no baffle-shroud. The improved heat transfer is attributed primarily to increased velocity over the heat exchanger, though the ∼2 °C thermal stratification generated by the baffle-shroud also contributes. The other baffle-shrouds have increasingly narrow baffle regions, which results in lower velocities but slightly higher thermal stratification. The benefit of the slight improvements in thermal stratification is outweighed by the cost of the decreased velocity over the heat exchanger.