Water spray interaction with air-steam mixtures under containment spray conditions: comparison of heat and mass transfer modelling with the TOSQAN spray tests

Full text of publication follows: During the course of a hypothetical severe accident in a Pressurized Water Reactor (PWR), hydrogen can be produced by the reactor core oxidation and distributed into the reactor containment according to convection flows and water steam wall condensation. In order to mitigate the risk of detonation generated by a high local hydrogen concentration, spray systems are used in the containment. The TOSQAN programme has been created to simulate separate-effect tests representative of typical accidental thermal-hydraulic flow conditions in the reactor containment. The present work concerns the interaction of a water spray, used at the top of the containment in order to reduce the steam partial pressure, with air-steam mixtures. The main phenomena occurring when water spray is used are the mixing induced by spray entrainment and the condensation on droplets. In order to improve the latter phenomena, different levels of modelling can be used. The objective of this paper is to analyze experimental results obtained for water spray interaction with air-steam mixtures using different heat and mass transfer modelling. For this purpose, two modelling issues have been used: the first one is devoted for the determination of the gas thermodynamical properties, and the second one concerns the droplets characterization. In the first one, the gas thermodynamical analysis is performed using depressurization, gas temperature variation and humidity decrease during the spray injection. In this modelling, heat and mass transfer between the spray and the surrounding gas is treated in a global way by energy balance between the total amount of water and the gas. In the second one, droplets characterization is obtained by means of droplet size, temperature and velocities evolutions. In this modelling, the spray is considered as a single droplet falling with an initial velocity. Droplet interactions are neglected. Assessment of these two modelling is performed using experimental results obtained in the TOSQAN facility (7 m3 volume, 4 m high, 1.5 m i.d.): presentation of this facility, its instrumentation, the spray test scenario and the experimental results are presented in a companion paper. Experimental and theoretical results are presented in two steps corresponding to the two components of this two-phase flow: one concerning the steam-air mixture, and the other concerning the water spray. Concerning the gas phase, it is shown that the vessel depressurization (and the decrease of the connected variables, such as gas temperature and humidity) during the spray injection is lower than the theoretical results. Analysis of the experimental and theoretical results is performed in order to explain this difference: the effect of the convective heat transfer between the gas and the droplets, phenomenon not taken into account in the gas modelling, is to reduce the pressure loss in the experiment. Concerning the droplet phase, droplet temperature vertical profiles downward the nozzle are presented. It is shown that the theoretical and experimental profiles are qualitatively the same, indicating a good behaviour of the heat and mass transfer modelling for the droplets. However, in order to have a quantitative agreement, droplet size has to be well determined. Sensitivity of the theoretical results to this parameter is presented and shows the importance of droplet size determination in the heat and mass transfer characterization. (authors)