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Dr. Dr. h.c. Gun­ter Gerbeth

Director Institute of Fluid Dynamics
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Dr. Gerd Mutschke

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Juliane Kunze

Secretary Institute of Fluid Dynamics
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orangener PfeilCollegiate works and PhD theses

Buoyancy related mixing experiments

This work was carried out in the frame of the project "Coolant mixing in pressurized water reactors" (registration number: 1501216) funded from 01.04.1998 to 31.05.2002 by the German Federal Ministry of Economics and Labour.

Background

The mixing of slugs of water of different quality is very important for pre-stressed thermal shock situations. When the emergency core cooling system is activated during a loss-of-coolant accident, cold water is injected into the hot water in the cold leg and downcomer. Due to the large temperature differences, thermal shocks are induced at the reactor pressure vessel wall. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses. This temperature distribution is highly influenced by the mixing of the injected emergency core cooling water with the ambient water of higher temperature on the way through the primary circuit. The investigations of the processes of turbulent mixing under the influence of buoyancy forces caused by the temperature differences can contribute to the assessment of the thermal loading of the vessel wall.

Boundary conditions for the ROCOM experiments

The goal of the experiments was the generic investigation of the influence of density differences between the primary loop inventory and the emergency core cooling water on the mixing in the downcomer to find the conditions for transition between momentum controlled and buoyancy driven mixing. To separate the density effects from the influence of other parameters, a constant flow in the loop with the emergency core cooling injection nozzle was assumed in this study. The flow rate was varied in the different experiments between 0 and 15 % of the nominal flow rate, i.e. it was kept in the magnitude of natural circulation. The pumps in the other loops were switched off. The density difference between emergency core cooling and loop water has been varied between 0 and 10 %. The normalized density is defined as the ratio between emergency core cooling water density and density of fluid in the circuit. In all experiments, the volume flow rate of the emergency core cooling injection system was kept constant and all other boundary conditions are identical. Altogether 21 experiments have been carried out.

Due to the fact, that the test facility cannot be heated up, the necessary density differences were simulated by adding sugar (glucose) to the water that is injected into the cold leg. To observe the mixing of the emergency core cooling water, this water was tracered by small amounts of sodium chloride, as in previous experiments. Generating density differences by high salt concentrations is not possible, because the measurement system is very sensitive and would be saturated at high salt concentrations.

Results

Experiment without density difference

The experiments without density effects serve as reference experiments for the comparison. The left part of Fig. 1 visualises in an unwrapped view the time evolution of the tracer concentration measured at the two downcomer sensors. The downwards directed red arrow indicates the position of the loop with the running pump, in that case delivering 10 % of the nominal flow rate. At the upper downcomer sensor, the emergency core cooling water (injected in each experiment from t = 5 to t = 15 s) appears directly below the inlet nozzle. Due to the momentum created by the pump, the flow entering the downcomer is divided into two streams flowing right and left in a downwards directed helix around the core barrel. At the opposite side of the downcomer, the two streaks of the flow fuse together and move down through the measuring plane of the lower downcomer sensor into the lower plenum. Almost the whole quantity of emergency core cooling water passes the measuring plane of the lower downcomer sensor at the side opposite to the azimuthal position of the affected loop. The velocity field responsible for the observed tracer distribution is typical for single-loop operation. It has its maximum at the opposite side of the downcomer and a minimum at the azimuthal position of the running loop, which has been found in velocity measurements by means of a laser-Doppler anemometer(LINK) at the ROCOM test facility, too.

The maximum tracer concentration of the emergency core cooling water in the downcomer is 20.1 % of the injected water concentration at the upper sensor and 8.4 % at the lower sensor.

Fig. 1: Time record of the perturbation at the sensors in the downcomer (unwrapped view, the red arrow indicates the azimuthal position of the loop with emergency core cooling water injection) in the experiments with variation of density difference of the injected water

Experiment with a density difference of 10 %

The right part of Fig. 1 shows the experiment, carried out at the same flow conditions, but the density difference between the injected emergency core cooling water and the primary loop coolant is now 10 %. In that case a streak formation of the water with higher density is observed. At the upper sensor, the emergency core cooling water covers a much smaller azimuthal sector. The density difference partly suppresses the propagation of the emergency core cooling water in horizontal direction. The emergency core cooling water falls down in an almost straight streamline and reaches the lower downcomer sensor directly below the affected inlet nozzle. Only later, coolant containing emergency core cooling water appears at the opposite side of the downcomer. The maximum concentration values observed at the two downcomer sensors are in the same range as in the case without density differences, i.e. 20.1 % and 9.7 % from the initial concentration in the emergency core cooling water tank.

The visualizations of the behaviour of the emergency core cooling water in the downcomer reveals that in case of momentum driven flow, the emergency core cooling water covers nearly the whole perimeter of the upper sensor and passes the measuring plane of the lower sensor mainly at the opposite side of the downcomer. When the density effects are dominating, the sector at the upper measuring device covered by the emergency core cooling water is very small. The emergency core cooling water falls down straightly and passes the sensor in the lower part of the downcomer below the inlet nozzle of the working loop.

Experiment with a density difference of 4 %

Fig. 2 shows an experiment with a density difference of 4 %, while the flow rate was again 10 % of the nominal. At the upper sensor, the width of the azimuthal sector covered by the emergency core cooling water is in-between the two cases with 0 % respectively 10 % density difference. Near the lower sensor, the emergency core cooling water reaches the opposite side of the downcomer and the region below the inlet nozzle position almost at the same time. That means, that one part of the emergency core cooling water follows the stream lines of the external momentum driven flow field and another part directly falls down due to the internal momentum created by density differences. We consider this as an intermediate state between momentum and density driven flow. The experiment shown in Fig. 2 was therefore assigned to the transition region between the two flow regimes.

Based on the described observations, the set of conducted experiments, was divided into three groups: (1) buoyancy dominated flow, (2) momentum dominated flow and the (3) transition region. The conditions at the inlet into the downcomer were used to calculate Froude-numbers of the experiments according to the following formula:

Fig. 2: Time record of the perturbation at the sensors in the downcomer (unwrapped view, the red arrow indicates the azimuthal position of the loop with emergency core cooling water injection) in the experiment with a density difference of 4 %

  

Fig. 3: Matrix of carried out experiments and isolines of the Froude-Numbers

  

Lines of constant Froude-numbers calculated by means of this formula are shown in Fig. 3. All experiments, identified as density dominated are located in the region left of the isoline Fr = 0.85 and all momentum dominated points are found right of the isoline Fr = 1.5. These two numbers are critical Froude numbers separating the two flow regimes for the ROCOM test facility. Therefore, the critical Froude number for transition between momentum controlled and buoyancy driven mixing is about 1.0.

Publications

S. Kliem, H.-M. Prasser, G. Grunwald, U. Rohde, T. Höhne, F.-P. Weiss
ROCOM experiments on the influence of density differences on the coolant mixing inside the reactor pressure vessel of a PWR
Proc. Annual Meeting on Nuclear Technology ‘02, pp. 65-69, INFORUM GmbH, Bonn (2002)

G. Grunwald, T. Höhne, S. Kliem, H.-M. Prasser, K.-H. Richter, U. Rohde, F.-P. Weiss
Kühlmittelvermischung in Druckwasserreaktoren - Teil 2, Experimentelle Ausrüstung und Simulation der Vermischung
Report FZD-367, ISSN 1437-322X, 164p., Rossendorf (2003)

H.-M. Prasser, G. Grunwald, T. Höhne, S. Kliem, U. Rohde, F.-P. Weiss
Coolant mixing in a PWR - deboration transients, steam line breaks and emergency core cooling injection - experiments and analyses
Nuclear Technology, vol. 143 (1), pp. 37-56 (2003)

T. Höhne, S. Kliem, U. Bieder
Modeling of a buoyancy-driven flow experiment at the ROCOM test facility using the CFD-codes CFX-5 and TRIO_U
Nucl. Eng. Design, vol. 236, pp.1309-1325 (2006)

 

Contact

S. Kliem