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Enhancing Nuclear Safety


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Study of coalescence in spray headers: application to the spray system of PWRs

Christophe RABE, doctorate of University Pierre et Marie Curie, 229 p., defended on the 20th january 2009

Summary

During the course of a hypothetical severe accident in a Pressurized Water Reactor (PWR), spraying systems are used to preserve the reactor containment integrity. The efficiency of these sprays depends in particular on droplet characteristics (sizes and velocities), which may evolve during settling, in particular because of coalescence. The evolution of the droplet histogram thus depends on the outcome of binary droplet collision, which is the focus of this study. Our approach required building up an experimental setup which allowed to identify with precision the various collision outcomes:
coalescence, stretching or reflexive separation and bouncing. Physical conditions for these regimes to appear were mapped in terms of the three main parameters used in literature: the Weber number, the impact parameter and the diameters ratio. These experimental results were unified through a new, called “symmetric”, Weber number, defined as the ratio of the total kinetic energy of the two drops in the frame of the mass center to the their total surface energy. On the basis of this Weber number, three new models were then formulated in order to describe transitions between the main outcomes for drops with various sizes. These models are in good agreement with our experimental results. Our study focused then on the influence of ambient gas conditions on collision outcomes. In two different sets of experiments (under different pressures and with various helium concentrations), the bouncing outcome was identified. The influence of gas phase properties (density and viscosity) was identified and an empirical correlation describing the evolution of this regime with gaseous parameters was derived. All results were finally applied in a prospective analysis of the interaction between two sprays.

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Thesis report

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Jeanne Malet, the thesis supervisor

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