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Comparative study of the influences of different water tank shapes on thermal energy storage capacity and thermal stratification
Author: Yang Zheng | Print | Close | Text Size: A A A | 2017-11-28

The influences of different water tank shapes on thermal energy storage capacity and thermal stratification in the static mode of operation is investigated in this study under laminar natural convection. A new experimental apparatus is built, and a numerical model is developed to simulate the flow and heat transfer in the water tank. Computational results agree with the experimental data. Among the 10 different water tank shapes studied, the sphere and barrel water tanks are ideal for thermal energy storage capacity, whereas the cylinder water tank is the least favorable. The thermal energy storage capacity is closely related to the surface area of the water tank. According to the characteristics of the velocity and temperature fields, these shapes can be divided into three categories: shapes with sharp corners, those with hemispheres, and those with horizontal plane surface. Shapes with sharp corners have the highest degree of thermal stratification, whereas the shapes with horizontal plane surface possess the lowest. That of the shapes with hemispheres lies in between these two degrees. The thermal stratification of different shapes is determined by the flow at the bottom of the water tank and the heat transfer from the fluid to the environment.

Conclusions

This study investigates the influence of 10 different water tank shapes on thermal energy storage capacity and thermal stratification in the static mode of operation. Based on the experimental and numerical results, the following conclusions are drawn:

1. The thermal energy storage capacities of the sphere and barrel are the best, while the capacity of the cylinder is the worst. The capacities of the remaining shapes are almost equal. Thermal energy storage capacity is determined by the ratio of surface area to volume.

2. According to the velocity field pattern of the water tanks, the tank shapes can be divided into three categories: tank shapes with sharp corners, those with hemispheres, and those with horizontal plane surfaces. In the tank with sharp corners, no flow is observed in the thermal stratification region, and circulation occurs in the isothermal region and in the boundary layer. A vortex is not generated in the velocity field. In the tank with hemispheres, the flow in the thermal stratification region is weak, as is the circulation in the isothermal region and in the boundary layer. Moreover, a simple vortex is produced in the top region of the water tank. In the tank with horizontal plane surface, circulation is observed in the central region and in the boundary layer. In addition, a complicated vortex system is generated in the top region of the tank. The temperature fields of the water tanks are almost similar. Isothermal regions are established in the upper and middle parts of the water tank, and thermal stratification regions are created in the lower part. Furthermore, the temperature boundary layer is attached to the lateral wall.

3. The tank with sharp corners has the highest degree of thermal stratification, whereas the tank with horizontal plane surface has the lowest. The tank with hemispheres displays a moderate degree of thermal stratification compared with the other two categories. The phenomenon is attributed to the following reasons. The fluid at the bottom of the water tank with sharp corners is static, and the heat loss of the fluid in this region to the environment results in a sustained drop in temperature. The flow is obvious at the bottom of the water tank with horizontal plane surface, and thermal stratification is caused by the heat loss during downward flow in the boundary layer of the lateral wall. The flow at the bottom of the water tank with hemispheres is very slow; therefore, thermal stratification is induced by the heat loss during downward flow in the boundary layer and during the slow flow in the bottom region.

The next stage of the work will focus on quantitatively investigating the influence of the variation in the angle of the sharp corner and of the curvature variation in the hemisphere on thermal energy storage capacity and thermal stratification.

The results have been published on Renewable Energy 85 (2016) 31-44.

Fig. 1. Schematic diagram of the experimental equipment

 (1. Tank wall; 2. thermal insulation material; 3. Teflon insulation pad; 4. wooden support).

 
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