The numerical analysis of the effect of the different type of obstacles into the thermal stratifications in the hot water storage tank in solar energy system has presented. The geometrical details of the obstacle types are defined under the theoretical model caption. The present study is initial works that aimed to find the best obstacle type between all investigated cases to analyze in experimental setup later.

Nearly 30 different types of obstacles are considered in this work but, 12 of them is present here. The temperature difference distributions of the tank with and without obstacle, which is given in figure 3 and 4, are presented in figure 5.

Smooth tank is considered firstly to compare the thermal stratification inside the tank with obstacles. There is a little thermal stratification at the top of the smooth tank. So, the temperature difference between thermal stratifications is far from desired values in smooth tank. The temperature distribution of the smooth tank case is depicted in figure 4-13. The hot water (T2) and cold water (T4) has contact in all axial cross-sectional area when they enter the tank. The rotations of the hot and cold water velocity vectors occur. Because they hit each other towards to the tank wall. The hot water stratification has been destroyed by cold water in this condition. In order to keep higher thermal stratification, the axial contact area between cold and hot water must be decreased and cold water mustn’t be directed towards upper part of the tank. Therefore, the obstacles in different shapes as cylindrical, semi-cylindrical and conic are placed in the tank to get minimum axial contact area.

The thermal stratification area and thickness are higher in tank with obstacles rather than tank without obstacles. The thermal stratifications are shown in figure 4 for storage tank with 12 different type of obstacle and 1 smooth tank. Figure 5 represent the temperature difference according to the tank numbers to determine the best tank models in order to obtain higher thermal stratification. T3-T4 and ((T2-T1HT3 — T1)) must have higher and ((T2-T3) and (T1-T4)) must have lower value to obtain good thermal stratifications. With respect to all these argument, obstacle type 7 and 11 has supplied higher thermal stratification.

T3 and T1 temperature distributions according to the tank model are illustrated in figure 6. T3-T1 (temperature difference between water exit to the usage and water going to the storage tank) must be considered for choosing the tank model has better performance of thermal stratification. This temperature difference must be higher to obtain higher thermal stratification. Obstacle type 11 has higher value of T3-T1. It can be said that obstacle type 11 can supply highest thermal stratification between all investigated cases. The best indicators to evaluate the effect of the obstacles into the thermal stratifications are T3 and T1. T3 and T1 must be higher and lower as soon as possible, respectively. T3 and T1 values are very close in smooth tank.

1-052 EuroSun2004

Figure 4-3

1.5 ■ 1.4 ■ 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 °.5 0.4 0.3 0.2 0.1 0

1.5 1.4 1.3 1.2 a 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

SHAPE * MERGEFORMAT

Temperature distributions of obstacle type 11 and 7 for operation periods of 5, 10, 15, 20, 25 and 30 minutes are shown in figure 7 and 8 in z plane, respectively. These two different obstacles have different temperature for z=0.0, 0.1, 0.2 and 0.3 m during the 30 minutes operation period. Obstacle type 11 reach the maximum temperature in z=0.3 m for all operation period but, obstacle 7 can reach the maximum temperature in z= 0.6. The temperature values are nearly the same in between z=0.4 and z=1.5 as in figure 7 and 8. So, these two obstacle type nearly has the same temperature inside the tank in all points.

Figure 9 represent the T3 and T1 distributions in z plane versus operation time for obstacle types 11 and 7. These temperature presented here for 6 different time periods as t=5, 10, 15,

20, 25 a 30 minute as seen in figure 9. T3 values are nearly the same for these two obstacle type but, T-i has lower value in obstacle type 11 after the 10 minute of operation period. So, type 11 has higher temperature difference between T3 and T1 after 10 minute in obstacle typell rather than obstacle type 7. The case is desired for higher thermal stratification. In the

light of these parameters, obstacle type 11 has supplied the best thermal stratification for system has natural circulation inside between all investigated obstacles.

1. Conclusions

The numerical analysis of the effect of the using different kind of obstacles in the heat storage tank to obtain higher thermal stratification has been presented. The temperature distributions in the heat storage tank with different obstacles are also presented in z-r plane. Temperature distribution of the smooth tank is also presented.

The results from these analyses as;

The tank with obstacles has better thermal stratifications rather than smooth tank.

The obstacle types have gap in the center, have better thermal stratification than obstacle types that have gap near the tank wall.

Tank models 7 and 11 have higher thermal stratification. So, these obstacles have supplied hot water in higher degree to use and also have water to the heater tank in lower degree. This is desirable case for thermal stratifications. It is seen that other obstacle have little effect into the thermal stratifications.

In z=0.2 plane, the average temperature values in radial direction is being lower in obstacle type 11 compared to the obstacle type 7 while the time period increases. Because tank model 11 has cylindrical shaped obstacle in z=0.2. This obstacle prevents the destroying effect of the cold water into the thermal stratification while the operation period of time. So, the temperature of the water going to the heater would decrease while the time period increases in tank 11. This is also desirable condition for heat storage tank.

T3 (water temperature for usage) has increased in tank 11 and T1 (water is going to the heater) has decreased while the increase of operation times. So, obstacle type 11 has the highest temperature difference between T3 and T1. This is the desired criteria for thermal stratifications. In the light of all these considerations, the tank, has obstacle type 11, is the best tank type for thermal stratification in between investigated cases.

Using obstacle has improved the solar collector efficiency as well as thermal stratifications. Because, T1 (water return to the solar collector) would decrease when the obstacle is used. So,

the solar collector efficiency would also increase.

Acknowledgement

The authors thank to the Erciyes University for FLUENT 6.1.22 code. References

1. Alizadeh S., An Experimental and Numerical Study of Thermal Stratification in a Horizontal Cylindrical Solar Storage Tank, Solar Energy, Vol.66, No.6, pp.409-421, 1999.

2. Al-Nimr M. A., Temperature Distribution inside Electrical Hot Water Storage Tanks, Applied Energy, Vol.48, pp.353-362, 1994.

3. Misra R. S., Thermal Stratification with Thermo siphon Effects in Solar Water Heating Systems, Energy Conversion Management, Vol.35, No.3, pp.193-203, 1994.

4. Helwa N. H., Mobarak A. M., Effect of Hot Water Consumption on Temperature Distribution in a Horizontal Solar Water Storage Tank, Applied Energy, Vol.52, pp.185-194, 1995.

5. Hariharan K., Badrinarayana K., Temperature Stratification in Hot Water Storage Tanks, Energy, Vol.16, No.7, pp.977-982, 1991.

6. Hahne E., Chen Y., Numerical Study of Flow and Heat Transfer Characteristics in Hot Water Stores, Solar Energy, Vol.64, No.1-3, pp.9-18, 1998.

7. Mo Y., Miyatake O., Numerical Analysis of the Transient Turbulent Flow Field in a Thermally Stratified Thermal Storage Water Tank, Numerical Heat Transfer, Part A, Vol.30, pp.649­667, 1996.

8. Eames P. C., Norton B., The Effect of Tank Geometry on Thermally Stratified Sensible Heat Storage Subject to Low Reynolds Number Flows, Int. J. of Heat Transfer, Vol.41, No.14, pp.2131-2142, 1998

9. P. V. Suhas, Numerical Heat Transfer and Fluid Flow, pp. 79-109, Hemisphere Pub. Co., New York, 1980.

10. FLUENT 6.1.22 user’s guide. Fluent Incorporated, Centerra Resource Park, 10, Cavendish Court, Lebanon, NH 03766, USA, 2001.

Nomenclatures

Vk Hot water velocity from tank to tank

V§ Cold water velocity from mainlines to tank

D Tank diameter

H Tank height

f1 The distances between cold water enter and exit point and bottom of the tank

d Pipe diameter

Si The distance between the point hot water enter and tank ceiling

d1 The gap diameter in the center of the obstacle

l Channel length

T Temperature

T3 Usage water temperature

T 1 Temperature of the water going to the heater

T 2 Temperature of the water coming from the heater

T4 Temperature of the water coming from the mainlines.

t Time