Effects of an Annular Baffle on Heat Transfer to an Immersed Coil Heat Exchanger in Thermally Stratified Tanks

Abstract

The effect of a cylindrical baffle on heat transfer to an immersed heat exchanger is investigated in initially thermally stratified tanks. The heat exchanger is located in the annular region created by the baffle and the tank wall. Three cases of initial thermal stratification are explored, and in each case experiments are conducted with and without the baffle in the stratified tank and in a comparable isothermal tank with the same initial energy. The baffle maintains the high initial temperature of the upper zone of the stratified tanks for 10-16 minutes, as cool plumes that form on the heat exchanger are confined to the annular baffle region until they exit at the bottom of the tank. Regardless of stratification, the baffle always improves heat transfer to the immersed heat exchanger. In the isothermal tanks, the baffle increases total energy extracted in the first 30 minutes of discharge by over 20%. In stratified tanks, the baffle increases total energy extracted in 30 minutes of discharge by 9% to 16%. Initially, the improvement in heat transfer is due to the higher driving temperature differences around the heat exchanger. After all the water from the hot zone has entered and flowed through the baffle, the tank is basically isothermal, and velocity increases as the fluid temperature drops, maintaining rates of heat transfer higher than that in the tank without the baffle. In tanks with the baffle, stratification has only a modest positive effect on heat transfer to the immersed heat exchanger.