Session: 02-02 Fluid Mechanics and Heat Transfer in Building Applications

Paper Number: 107270

107270 - Effects of an Annular Baffle on Heat Transfer to an Immersed Coil Heat Exchanger in Thermally Stratified Tanks of a Cylindrical Baffle on Heat Transfer to an Immersed Coil Heat Exchanger in Thermally Stratified Tanks 

Direct use of solar energy to heat our homes and hot water remains an important use of solar energy. One of the challenges facing solar thermal applications is storage of the thermal energy. A number of domestic solar hot water systems and many combined space and water heating systems employ heat exchangers immersed in the storage fluid to charge and/or discharge the tank.  In unmodified tanks, operation of the heat exchangers produces buoyancy driven flows that result in circulation patterns that keep the tank isothermal. Further, tank recirculation creates a mixed convective flow across the heat exchanger. Prior studies have investigated cylindrical baffles that create an annular region with the tank wall in which a coiled heat exchanger is situated [1-6]. The coiled heat exchanger is placed at the top of the annular region and used to discharge the tank energy. These studies determined that baffles improve heat transfer to the immersed heat exchanger, and those benefits are derived primarily by increasing the velocity of the flow over the heat exchanger. The baffle also generates some thermal stratification in the tank, which can improve heat transfer by providing a higher driving temperature difference between the heat exchanger wall and the approaching fluid [1-6]. We have investigated baffle shape [3,4], annular region width [5], and heat exchanger pitch [6], identifying designs that optimize heat transfer to the immersed heat exchanger. Moreover, in each study, the way the designs affected the velocity of fluid around the heat exchanger and surrounding fluid temperature were characterized and used to explain the physics behind the observed improvements. In general, there is often a trade-off between fluid velocity and temperature, and more benefit is observed from improvements in velocity. 

Most of this past work focused on a baffle in an initially isothermal tank, though one study early did investigate an initially thermally stratified tank, and found that the baffle helped maintain thermal stratification and thus high temperatures approaching the heat exchanger for 15 minutes, however we did not compare heat transfer or other metrics of tank performance [1]. In the present study, we investigate tanks with three different initial stratifications: “baseline” (60% at 60°C, 40% at 40°C), “smaller cold band” (80% at 60°C, 20% at 40°C), and “smaller temperature difference” (60% at 60°C, 40% at 50°C). For each initial stratification, we conducted four types of experiments: experiments with the stratified tank with and without the baffle as well as comparison experiments in isothermal tanks with the same initial energies, again with and without the baffle. The most useful performance measure for comparison is the rate of heat transfer to the immersed heat exchanger. In all stratification cases, with the baffle in place, the stratified tank slightly outperforms the isothermal tank with equivalent energy. In addition, in the stratified tanks, the baffle improves heat transfer relative to the same tank without a baffle. In stratified tanks, the baffle results in lower velocities over the heat exchanger, in contrast to results in isothermal tanks. The cooled plumes descending from the heat exchanger are impeded from falling freely through the baffle by the cold layer below. However, the baffle maintains higher surrounding fluid temperatures, which has enough influence on heat transfer to overcome the impacts of lower velocities and explains the improved performance due to the baffle in thermally stratified tanks. 

[1] Haltiwanger &. Davidson (2009), Solar Energy, 83, pp.193-210. 

[2] Su & Davidson (2008), J. Sol. Energy Eng., 130, 021016.

[3] Nicodemus, et al. (2017), Solar Energy, 157, pp. 911-919.

[4] Nicodemus, et al. (2018), Solar Energy, 182, pp. 304-315.

[5] Nicodemus, et al. (2020), J. En. Res. Tech., 142, 050901. 

[6] Nicodemus, et al. (2022), J. Sol. Energy Eng.,144, 061010.

Presenting Author: Julia Nicodemus Lafayette College

Presenting Author Biography: Julia Nicodemus is an associate professor at Lafayette College, an undergraduate liberal arts college with an engineering program. She works with undergraduates on heat transfer research to improve storage in solar thermal tanks.