E. ANDERSEN, U. JORDAN, L. J. SHAH, S. FURBO Department of Civil Engineering, Technical University of Denmark Building 118, DK-2800 Kgs. Lyngby, Denmark Tel.: +45 4525 1901, Fax: +45 4593 1755, ean@byg. dtu. dk, uj@byg. dtu. dk, ljs@byg. dtu. dk, sf@byg. dtu. dk

Introduction

Since the 1960’ties the influence of the thermal stratification in hot water tanks on the thermal performance of solar heating systems has been studied intensively. It was found, that the thermal performance of a solar heating system is increasing for increasing thermal stratification in the hot water tank.

The temperature of the storage water heated by the solar collector loop usually varies strongly during the day. In order to reach a good thermal stratification in the tank, different types of pipes, plates, diffusers and other devices have been investigated in the past (e. g. Loehrke, 1979). The aim pursued was to transport the heated water into the tank level of corresponding temperature.

Flexible stratification pipes (manifolds) have been further developed for example by (Gari et al., 1982). Furthermore, a wide variety of non flexible tubes with either open holes and perforated vertical plates inside the pipes (Davidson, 1992) or openings in form of balls (e. g. Leibfried, 2000) or flaps (e. g. described in Krause, 2001) have entered the market during the recent years.

In this paper an investigation of a stratification pipe with openings covered with flaps according to (Krause, 2001) is presented. The flaps are constructed with a soft material which allows the flap to close and open depending on the temperature and pressure differences inside and outside the pipe. Figure 1 shows schematic

illustrations of the pipe. The total height of the pipe is 328 mm, the outer diameter 60 mm, and the flaps are located with a distance of 292 mm in vertical direction (distance between the centre of each opening).

Preliminary laboratory tests by (Shah, 2002) with the same stratification pipe containing 5 openings showed that thermal stratification was well built up for a volume flow rate smaller than 8 l/min and larger than 4 l/min, regardless of the inlet temperature, the temperature level in the tank, and the thermal stratification in the tank. For volume flow rates larger than 8 l/min, however, the number of open flaps increased, so that
water entered the tank at different levels instantaneously. For volume flow rates smaller than 4 l/min laboratory tests indicated that cold water could be sucked in through an opening in a low level due to low pressure differences. The cold water that entered the pipe through these openings from the bottom of the store mixed with the heated water that flew through the pipe and thereby induced mixing in the tank during charging.

More detailed investigations of the flow structure close to the flaps of the stratification pipe are presented in the following for one set of operating conditions. Temperature measurements were carried out and an optical method called Particle Image Velocimetry (PIV) was used to visualize the flow around the flaps.

Experiments

Experimental Set-up

The experimental set-up is shown in Fig. 2. The set-up consists of a rectangular glass tank with side lengths of 400 x 400 x 900 mm3, a heating and a cooling unit, and standard PIV equipment (Dantec Dynamics). The PIV equipment consists of a laser, a camera and a processing system for analysing the pictures taken by the camera. Information about the PIV equipment is given in Table 1.

The camera is placed perpendicular to the laser that illuminates a thin slide in the flow.

The inlet consisting of three compound stratification inlet pipes placed in the centre of the tank. The inlet pipe is closed at the top. The outlet is placed in the bottom of the tank in the corner behind the inlet pipe. The temperature is measured in the middle of the pipe below each inlet and in 13 uniformly distributed levels in the tank. Also the in — and outlet temperatures are measured as well as the volume flow rate. The temperatures are measured with thermocouples type TT with an accuracy of 0.5 K. The volume flow rate is measured with an electro magnetic inductive flow meter, type HGQ1 from Brunata HG a/s. The flow meter has an accuracy of about ± 1 %.