Results

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All following results are from the transient calculation and are time averaged over two minutes, like in the experiment. The calculated temperature field is shown in Fig. 5. Despite a careful and detailed 3d modeling of the geometrical and thermal characteristics of the SUCOS-3D ex­periment, the calculated pool temperature (Tp. cai = 29.2°C) is lower than the measured one Tp. exp 32.6 C. the corresponding deviation is (Tp_i-Тр_иЧр) (Тр_иЧр-"I/0oi ) = 34%.

In order to find possible reasons for this deviation one should compare the calculated flow field to the experimental one. The calculated flow field for SUCOS-3D is very similar to the ex­perimentally found and calculated one for SUCOS-2D. A stable natural convection loop devel­ops, Fig. 6: the heated fluid rises from the copper plate through the chimney to the covered water level; here the warm flow turns right to the horizontal side area and flows on without an intensive contact to the horizontal cooler; the water is mainly cooled in the vertical side area from where it returns to the pool where it is heated again; part of the cold water from the hori­zontal coolers moves from time to time in form of cold plumes against the mean flow down­wards through the chimney and mixes with the rising heated water. These non-stationary plumes cause the strong time dependence of the heat flux on the copper plate, Fig 4. Accord­ing to this flow field, the temperatures in the horizontal side area of SUCOS-2D are higher than the ones in the pool under the tilted roof, similar to the temperatures in Fig. 5.

The flow field in the experiment SUCOS-3D must be reconstructed from the measured tem­peratures because no velocity measurements were performed. Other than in SUCOS-2D here we find in the experiment the highest temperatures not in the horizontal side area, but below the tilted roof, Fig. 7. Therefore, a different behavior of the natural convection has to be de­duced: We have at least to expect stronger mixing between cold counter-current downward flow with hot rising fluid in the chimney.

Подпись:

Подпись: FIG. 7. Distributions of measured temperatures in SUCOS-3D.

—► 0.01 m/s

Possible reasons for the disagreement in the pool temperature and in the natural convection loop were investigated. Since SUCOS-2D calculations showed a high sensitivity of the natural convection on thermal disturbances, the thermocouple support structure installed in the chim­ney was additionally modeled as a thermally interacting structure. Unfortunately, the exact po­sition of the movable probe is not known. Nevertheless, the new results with probe support structure are better than the previous ones and bring the calculated pool temperature to the right direction but not yet enough to achieve a satisfactory agreement.

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So far, the cooling performance of the vertical coolers was overestimated by all previous calcu­lations. Therefore, a further numerical study was performed changing the kind of the thermal boundary conditions for the active vertical coolers: values for the wall heat fluxes deduced from the experiment were pre-set instead of using surface temperatures. Then, the calculated pool temperature Tp, cai= 32.8 °С agrees well with the experimental one, Fig. 8: the deviation is reduced from 34% to only 2%. Despite of this positive result, qualitatively the same natural convection loop is obtained like in the previous calculations. This means, the calculated flow field still shows no agreement with the reconstructed one in the experiment.

The differences between experiment and calculation in the behavior of the flow field were fur­ther analyzed by means of vertical temperature distributions, which were measured in the chimney and in the vertical side area (Carteciano et al. 2000). The amount of water flowing down from the horizontal cooler to the vertical one was much less in the experiment than cal­culated, which causes reduced heat fluxes at the outermost section of the vertical coolers. An other significant difference between calculation and experiment can also be found in the chim­ney: recirculation of cold water flowing back from the horizontal cooler to the chimney is cal­culated, while cold water was registered in the experiment in the plane of measurements only under the tilted roof. The origin of the cold water under the roof in the experiment was recon­structed by analyzing the cooling performance of the horizontal coolers which are divided into two big and two small ones. The cooling performance of one small cooler is in the experiment as high as the one of a big cooler despite the cooling surface ratio of about 1:2. Therefore, a stronger water flow was obviously present over the small cooler. This cold flow returns to the corners of the chimney (Fig. 9) and is recorded only when it reaches the thermocouples at posi­tion R below the chimney.

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A decisive conclusion on the disturbance responsible for the experimental flow behavior was not found in all previous calculations and studies. Further deviations in the modeling or bound­ary conditions used in the simulations from those in the experiments have to be considered. One example, which would explain the unexpected results from the SUCOS-3D experiment, is to postulate that one of the horizontal coolers was slightly tilted against the main flow direc­tion.

Additional calculations with a 2d slab model similar to SUCOS-2D were performed with dif­ferent inclinations of the horizontal coolers from 0 to 4 mm, corresponding respectively to 0 and 1.04 % slope. The mass flow of the cold water going back to the chimney from the hori­zontal coolers increases due to this measure by about 70% and the corresponding heat removal by this flow increases by about 55%! Therefore, the mixing between the cold water and the heated water rising through the chimney is strongly increased like in the experiment. These re­sults show that a very small slope of the horizontal coolers can influence the flow behavior in a drastic way and would explain the experimental flow field, but a final check is not possible be­cause the experimental facility is already disassembled.