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Research Papers: Nuclear Power

Three Dimensional Modeling of the Hydrodynamics of Oblique Droplet-Hot Wall Interactions During the Reflood Phase After a LOCA

[+] Author and Article Information
D. Chatzikyriakou

Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UKd.chatzikiriakou@imperial.ac.uk

S. P. Walker, B. Belhouachi

Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

C. Narayanan, D. Lakehal

 ASCOMP GmbH, Technoparkstrasse 1, 8005 Zurich, Switzerland

G. F. Hewitt

Department of Chemical Engineering and Chemical Technology, Imperial College, Prince Consort Road, London SW7 2BY, UK

J. Eng. Gas Turbines Power 132(10), 102914 (Jul 07, 2010) (6 pages) doi:10.1115/1.4000867 History: Received July 29, 2009; Revised August 04, 2009; Published July 07, 2010; Online July 07, 2010

During the reflood phase, following a loss-of-coolant-accident (LOCA), the main mechanism for the precursory cooling of the fuel is by convective heat transfer to the vapor, with the vapor being cooled by the evaporation of the entrained saturated droplets. However, it is believed that the droplets that reach the rod could have an effect on this cooling process. Despite the fact that those droplets do not actually wet the fuel rod due to the formation of a vapor film that sustains them and prevents them from touching the wall, the temperature drop caused by the impingement of such water droplets on a very hot solid surface (whose temperature is beyond the Leidenfrost temperature (1966, “A Track About Some Qualities of Common Water,” Int. J. Heat Mass Transfer, 9, pp. 1153–1166)) is of the order of 30150°C (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liquid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The associated heat flux is of the order of 105107W/m2 and the heat extracted is in the range of 0.05 J over the time period of the interaction (a few ms) (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liquid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The hydrodynamic behavior of the droplets upon impingement is reported to affect the heat transfer effectiveness of the droplets. In the dispersed flow regime the droplets are more likely to impinge on the hot surface at a very small angle sliding along the solid wall, still without actually touching it, and remaining in a close proximity for a much larger time period. This changes the heat transfer behavior of the droplet. Here, we investigate numerically the hydrodynamics of the impingement of such droplets on a hot solid surface at various incident angles and various velocities of approach. For our simulations, we use a computational fluid dynamics (CFD), finite-volume computational algorithm (TransAT© ). The level set method is used for the tracking of the interface. We present three-dimensional results of those impinging droplets. The validation of our simulation is done against experimental data already available in the literature. Then, we compare the findings of those results with previous correlations.

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Figures

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Figure 1

Qualitative comparison between experimental data (7) and our simulated results (TransAT© ). The droplet size is 140 μm and the droplet velocity is 3 m/s. The approach angle is 30 deg. The Weber number here is 7.

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Figure 2

Ratio of normal droplet velocity component after and before the impact against Weber number. Several droplet sizes and velocities were simulated. Good agreement between the predictions using the TransAT© code and the experimental measurements of Anders (7).

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Figure 3

2 mm drop, angle of approach is 20 deg, initial velocity 1.2 m/s

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Figure 4

200 μm drop, angle of approach is 20 deg, initial velocity 1.2 m/s

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Figure 5

Simulated results. Water droplets impinging at various angles on a surface beyond the Leidenfrost threshold. The vertical and the 60 deg impact are similar. When small angles of impingement are considered, then the behavior is different. Here, the Weber numbers are 4, 3, and 0.5 from top to bottom. The droplet diameter is 200 μm.

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Figure 6

Spreading of the droplet, time of proximity, and distance it travels close to the wall as a function of angle, for a 200 μm droplet with initial speed 1.2 m/s. These are simulations with TransAT© .

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Figure 7

Upper case: droplet with Weber number equal to 35; lower case: droplet with Weber number equal to 0.3

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Figure 8

Effect of angle of approach on the disintegration of the droplet. It is obvious that, for small angles of impingement, the droplet disintegrates at a lower Weber number. Good agreement between experiments and our simulated results.

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