ICHS, September 2007 On The Use Of Spray Systems: An Example Of R&D Work In Hydrogen Safety For Nuclear Applications C. Joseph-Auguste 1, H. Cheikhravat 2, N. Djebaïli-Chaumeix 3 and E. Deri 1 1 CEA 2 IRSN 3 CNRS-ICARE

2 Spray systems in French PWR

3 oArgument against: ◦ may generate flammable mixtures or enhance flame propagation through turbulence induced by sprays oArguments for: ◦ reduce overpressures in the containment ◦ remove any scattered radioactive aerosols ◦ could cause heat sinks and mixtures homogenization Experimental set-ups and numerical modelling

4 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

5 Spherical bomb

6 Modelling strategy ( ) 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

7 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

8 LP analysis: hypotheses o conservation of mass and energy (impermeable and adiabatic walls) o ideal gases o constant volume o 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 T gas (t 0 ) = 413K o T liq (t 0 ) = 298K Before After

9 LP analysis: computation  Calculation of amount of energy released due to complete combustion E 1 (T fin )  Calculation of amount of energy necessary to heat and evaporate liquid water and heat steam E 2 (T fin )  Final thermal equilibrium: solving of the equation E 1 (T fin ) = E 2 (T fin ) T fin  Calculation of the final density  Calculation of the final pressure (ideal gas relation)

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

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

12 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 water 2) 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

13 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