The test was started by opening the valve in the outlet line, see Fig. 1.

The outlet line has an inner diameter of 65 mm. The velocity of the fluid entering the pressure vessel (inner diameter 448 mm) is about 0.2-0.3 m/s at the beginning. Analysing the thermocouple readings a mixing zone of about 70-100 mm in the lower part of the vessel can be evaluated; this is plausible. From the temperature distribution at the highest level (T2.02) it can be concluded that the length of the mixing zone did not change along the height of the vessel.

The two special grids have been installed to evaluate the possibilities of plumes. Fig. 5 and 6 show the temperature deviations from the mean temperature for the lower and upper grid, resp. With the exception of time period during which the mixing zone passes the grid the deviations are about < 0.2 K.

In Fig. 7 the temperature curves measured with the thermocouples are given. In Fig. 8 the time when the mixture zone passes a level is shown. Differentiating the curve (spline — approximation) given in Fig. 8 with time results in the velocity inside the pressure vessel, see Fig. 9. Multiplying the velocity with the density (p = 981 kg/m3 ) and the cross section area of the pressure vessel (0.158 m2) result in the mass flow M.

The vertical temperature profiles are given in Fig. 10.

FIG. 5. Temperature deviations from the FIG. 6. Temperature deviations from the mean temperature for the lower grid. mean temperature for the upper grid.

FIG. 7. Temperature profile in the pressure FIG. 8. Time to pass for the mixture zone vessel. through the pressure vessel.

The energy transferred from the vessel to the condenser part can be calculated from the mass flow M and the difference between the inlet and outlet enthalpies of the condenser

Li = M(hin-hout).

In Fig. 11 the transferred energy is shown. At the beginning of the natural circulation the transferred power is about 180 kW.