The tank is divided into three parts, where the upper part, middle part and lower part correspond to flow through from flap 3, flap 2 and flap 1 respectively. The power supply to each part of the tank and the temperatures measured in the stratification inlet pipe is shown in Fig. 7. The inlet temperature is shown in Fig. 5. The power supply to each part of the tank is a result of the water flowing through the inlet, the downward flow in the tank, the tank heat loss and the downward thermal conduction. From the figure it can be seen that the power supply to the upper part, flow through flap 3, is high in the first 10 minutes after which flap 2 opens and the power supply in the middle part of the tank is then highest. Through the whole experiment water is sucked in through flap 1 and the power supply to the lower part of the tank is first of all caused by the downward flow in the tank towards the outlet at the bottom of the tank. The temperature in the inlet pipe is higher at the bottom of the pipe than in the middle and the top of the pipe and only slightly higher in the middle than in the top. This also indicates that cold water enters the lower inlet.

An additional experiment was conducted in order to verify that water was sucked in through flap 1. The test conditions were the same as in the previous experiments but flow

through the middle and lower inlets, flap 1 and 2 were physically prevented. The flow was only allowed through the upper opening, flap 3. The inlet temperature and the temperatures in the pipe with and without flap 1 and flap 2 closed is shown in Fig. 8. From the figure it is obvious that the temperature was practically the same in the whole pipe when flow through flap 1 and flap 2 was physically prevented.

In contrary, two temperature levels were measured throughout the whole experiment when all three flaps were in operation.

The power supply to the tank and the temperatures in the inlet pipe in the two experiments are shown in Fig. 9. The figure shows that the power supply to the upper part of the tank is almost the same in both experiments while the power supply to the remaining part of the tank differs in the experiments. The most significant difference between the experiments is the power supply to the lower part of the tank after about 26 minutes. At this time only a third of the tank volume has been replaced. After 34 min, nearly the total heat supply is moved to the bottom part of the store for the case that all flaps are in operation, whereas the heat is almost moved completely to the middle storage part, if the flow through the two lower flaps is prevented. Further the figure shows that the inlet temperature from the upper and middle flaps is several degrees lower when all three flaps can move freely.

Conclusion

A marketed stratification inlet pipe was investigated by means of Particle Image Velocimetry (PIV), a non-intrusive optical method and by temperature measurements inside and outside the inlet pipe. The pipe consisting of three compound pipes was built into the centre of a glass tank with a volume of about 140 litres. The functioning of the pipe was investigated for a typical operation condition where the tank was heated from about 20 °C to about 40 °C with a volume flow rate of about 2 l/min. The volume flow rate used in the experiment is close to a typical volume flow rate that develops from mixed (natural — forced) convection in the pipe when a compact heat exchanger is integrated below the stratification inlet device.

It was fount that thermal stratification was built up in a good way with the used test conditions, but also that effects of inertia influence the flow trough the stratification pipe and thereby the thermal stratification. If the inlet temperature was low compared to the tank temperature and slowly increased to the set temperature level, more cold water was sucked into the pipe from the lower flap at the beginning of the measurement.

Finally it was found that small amounts of cold water were sucked in through the flap at the bottom of the tank during the whole experiment that lasted for 50 minutes. This lead to a several degrees reduced inlet temperature at the top of the tank.

Further detailed investigations are needed, before the function of the stratification inlet pipe is full elucidated.

Reference

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Gari H. N., Loehrke R. I. (1982). A controlled buoyant jet for enhancing stratification in a liquid storage tank. Journal of Fluids Engineering, Vol. 104, pp. 475-481.

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Shah L. J., Morrisin G. L., Behnia M. (1999). Characteristics of Vertical Mantle heat Exchangers for Solar Water Heaters. Solar Energy, Vol. 67, No 1-3, pp 79-91.

Shah L. J. (2001). Heat Transfer Correlations for Vertical Mantle Heat Exchangers. Solar Energy, Vol. 69, No. 1-6, pp 157-171.

Knudsen S. (2003). Analysis of the flow structure and heat transfer in a vertical mantle heat exchanger. iSES International Solar World Congress, Gothenburg, Sweden, CD — ROM P6 55

Jordan U., Furbo S. (2004). Impact of inlet devices on the thermal stratification of a storage tank. EuroSun European Solar Energy Conference, Freiburg, Germany. Paper. Raffel M., Willert C., Kompenhans J. (1998). Particle Image Velocimetry. A Practical Guide. Springer Berlin. ISBN 3-540-63683-8.

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