Recordings of the velocity field were taken in a frame of 0.127 x 0.159 m2 at each inlet as shown in Fig. 4. The duration between two succeeding illuminations of the particle field varied between 8 and 100 ms depending on the flow around the inlets. The time delay between succeeding velocity vector recordings was about 250 ms. The volume flow rate was about 2 l/min. The initial tank temperature was about 20 °C. The outlet flow was lead through the heating unit where an electrical heating element of 3 kW heated the water

similar to the behaviour of a low-flow solar collector when the pump starts to circulate the flow. The laser was focused to the centre and to the position 20 mm from the centre of each of the three inlets and exactly the same experiment was repeated for each of the 6 laser positions. The duration of the experiments was 50 minutes. The flaps are referred to as flap 1, flap 2 and flap 3 counted from the bottom of the tank. Figure 5 shows the tank temperatures, the inlet temperature and the temperatures in the pipe of each experiment with a time interval of 5 minutes. From the figure it can be seen that the test conditions were practically the same in the experiments. Also the figure shows that the thermal stratification is build up in a good way.