ICHS, 11-13 September 2007

On The Use Of Spray Systems: An Example Of R&D Work In Hydrogen Safety For Nuclear

Applications

C. Joseph-Auguste1, H. Cheikhravat2, N. Djebaïli-Chaumeix3 and E. Deri1

1 CEA 2 IRSN

3 CNRS-ICARE

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Spray systems in French PWR

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Spray systems in French PWRo Argument against: ◦ may generate flammable mixtures or enhance flame

propagation through turbulence induced by sprays

o Arguments for: ◦ reduce overpressures in the containment ◦ remove any scattered radioactive aerosols ◦ could cause heat sinks and mixtures homogenization

Experimental set-ups and numerical modelling

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Experimental set-up: spherical bomb measure of flame velocity

stainless steel sphere (i.d. 500 mm) equipped with 4 opposites quartz windows (100 mm optical diameter, 40 mm thick)

black polished surface in order to suppress multiple diffusion

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Spherical bomb

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Modelling strategy (2007-2009) Main experimental objective: study of the

effect of droplets on a hydrogen-air flame in a humid atmosphere by measuring flame velocity

Use of the measured flame velocity to obtain more detailed CFD models but study of thermodynamics aspects is also needed

Lumped-Parameter analysis

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Principle of the Lumped-Parameter (LP) modelling

Based on heat and mass transfers

Assymptotic analysis: final state

Preliminary work for a future CFD modelling the CFD results must correspond to the LP ones after vaporization

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LP analysis: hypotheseso conservation of mass and

energy (impermeable and adiabatic walls)

o ideal gaseso constant volumeo complete combustion o the whole energy liberated

by the combustion vaporizes the liquid water

o air is considered as a binary mixture (N2-O2)

o hydrogen combustion is a single-step reaction

o Tgas(t0) = 413K o Tliq(t0) = 298K

Before After

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LP analysis: computation

Calculation of amount of energy released due to complete combustion E1(Tfin)

Calculation of amount of energy necessary to heat and evaporate liquid water and heat steam E2(Tfin)

Final thermal equilibrium: solving of the equation E1(Tfin) = E2(Tfin) Tfin

Calculation of the final density Calculation of the final pressure (ideal

gas relation)

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Final temperature vs initial H2 mole fraction for ≠ initial volume fractions of

liquid water

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Final pressure vs initial H2 mole fraction for ≠ initial volume fractions of liquid water

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Conclusions

o First results of the experimental study devoted to characterize the spray in terms of size distribution

o LP analysis: 1) heat sink as expected in the presence of liquid

water2) the steam due to vaporization becomes

important as the amount of initial hydrogen reaches a certain value (between 12% and 15% with our data) so that the final pressures are higher than the AICC pressure

3) this first study is a preliminary work for the full CFD modelling taking into account the reaction rate and a polydisperse spray

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Future work

o Experimental: study of the influence of the spray on a hydrogen-air flame

o CFD: current coupling of 1D spherical combustion model with a two-phase flow model

Appendice: work in progress Several two-phase flow models exist

=> which one?

Current study of drop residence time and evaporation time to choose the best one