Application de la stéréo PIV pour la mesures des écoulements bidirectionnels de fumée d'incendie traversant une trémie horizontale

International audienceL’écoulement des fumées à un passage d’une ouverture horizontale reliant deux compartiments superposés est étudié expérimentalement. Dans cette étude la technique de stéréo PIV est appliquée pour l’écoulement en convection mixte et en convection naturelle. Cet échange est gouverné à la fois par les effets de flottabilité dûs à une différence de température du fluide contenu dans les deux compartiments, et les effets d’une ventilation mécanique appliqué dans le compartiment inférieur. Une telle configuration entraîne un échange uni- ou bi-directionnel à travers l’orifice. Dans les expériences, la flottabilité est induite dans le compartiment inférieur grâce à une résistance électrique. Les résultats en convection naturelle montrent une forte similitude avec les résultats en convection mixte dans le régime bi-directionel

Experimental and numerical analysis of fire scenarios involving two mechanically ventilated compartments connected together with a horizontal vent

International audienceThis work deals with an experimental and numerical investigation of fire scenario involving two rooms mechanically ventilated and connected together with a horizontal vent. The objective is to investigate the effect of a horizontal opening on a fire scenario and especially on the burning rate. The study is based on the analysis of a set of large scale fire experiments performed in the framework of the OECD PRISME-2 project in the DIVA multi-room facility of the Institut de Ra-dioprotection et de Sûreté Nucléaire (IRSN), and of numerical simulations performed with the ISIS CFD code developed by IRSN. The fire scenario consists of two rooms, one above the other, mechanically ventilated, and connected to each other with a horizontal vent of 1 m2. The fire is heptane pool fire located in the lower room. The analysis focuses on the coupling between the burning rate, the flow at the vent and the configuration of the mechanical ventilation. Several regimes of combustion are encountered from well ventilated steady fire to under-ventilated un-steady and oscillatory fire. The results show that the burning rate is controlled by both the me-chanical ventilation and the downward flow from the vent. The numerical simulations highlight the specific pattern of the oxygen concentration field induced by the downward flow at the vent

Experimental and numerical study of fire event involving two simultaneous fire sources in confined and ventilated compartments

International audienceThe study deals with fire events involving two simultaneous fire sources in a confined and ventilated set of enclosures. Within the framework of fire safety assessment in nuclear facilities, it aims at investigating two configurations, one with two fire rooms separated by an adjacent empty room and a second one with two adjacent fire rooms. The objective is to analyse how a secondary fire may influence a primary fire, the flows at doorways and the overall smoke propagation. Some experimental results obtained for two large scale fire tests performed with gas burners are discussed. A specific analysis of the level of temperature in the fire rooms and in the adjacent room is presented. Numerical simulations with a zone approach are performed and satisfactory agreements are found between calculations and experimental data. The study shows that the distance between fires in case of multiple fire event is an important parameter. Close fires lead to an increase in aggression around the fire place whereas distance fire contributes to maximize the smoke temperature and therefore the aggression in the neighbouring area around fire places

Computational Fluid Dynamics Simulations of Water Spray Interaction with a Fire-Driven Flow in a Confined and a Mechanically Ventilated Enclosure

International audienceThe paper presents a detailed numerical study on the interaction between two water sprays and a fire-driven flow in a confined and mechanically ventilated enclosure of 4.9 m × 8.7 m × 3.9 m (height). The fire source is a 1.2 m × 1.2 m × 0.3 m propane burner, which is positioned in a corner and generates a well-controlled heat source of about 290 kW. The mechanical ventilation consists mainly of an inlet and an extraction ductdelivering each a flow rate of about 2500 m3/h. The two nozzles are mounted at 0.9 m below the ceiling and deliver a total flow rate of 107 l/min with a volume-median droplet diameter of about 470 μm. The CFD simulations carried out with the Fire Dynamics Simulator (FDS, 6.7.0) show a good agreement with the experimentally measured room pressure profile. More particularly, the under-pressure peak which occurs upon activation of the water spray system is well-predicted (i.e., -993 Pa in the experiments Vs -832 Pa in the simulations) provided that a sufficiently computational droplet injection rate is prescribed (Np = 80,000 droplets per second). As observed in the experiments, the CFD simulations show that the watersprays cause a relatively uniform species and temperature distribution from floor to ceiling. Thermal stratification is sustained though, especially in the region between the nozzles and the ceiling. Finally, an energy balance analysis shows that the experimentally estimated fraction of heat absorbed by the water (i.e., 65%) is higher than the one calculated in CFD (i.e., 40%)

Experimental investigation of the effects of a sidewall and cable arrangement on a horizontal cable tray fire in an open atmosphere

International audienceThe work deals with the influence of a sidewall and cable arrangement on the behavior of a fire involving horizontal cable trays in the framework of fire safety assessments in nuclear installations. The analysis is based on large-scale fire tests performed in open atmosphere in the frame of the OECD Nuclear Energy Agency (NEA) PRISME 3 project and on the corresponding simulations applying the FLASHCAT model for predicting the fire heat release rate. The fire configuration consists in five horizontal trays filled with PVC insulated power cables. The parameters investigated are the presence or absence of a sidewall and the cable arrangement (loose or tight bundles). The results show that the presence of a sidewall increases the fire HRR in comparison to a scenario without a sidewall. This effect is due to the increase of the flame spread velocity on the lower trays. Regarding cable arrangement, a tight configuration in bundles reduces the fire HRR in comparison to a scenario with a loose arrangement. This result is due to the reduction of the fire heat release rate per unit of area (HRRPUA) as well as the flame spread velocity. The performance of the FLASHCAT model in predicting the effects of the sidewall and the cable arrangement was also assessed on the basis of the fire tests and satisfactory agreements are reported. The presented analysis demonstrates that the fire scenario with horizontal cable trays against a sidewall and with a loose cable arrangement represents a conservative scenario for fire risk assessment. In addition, on the basis of these experiments, the effect of cable arrangement is more substantial than the sidewall effect

Multi-scale analysis of under-ventilated combustion regime in case of fire event in a confined and mechanically ventilated compartment

International audienceThis work deals with under-ventilated pool fires in confined and mechanically ventilated enclosures. The objective is to demonstrate the ability of reduced-scale experiments to investigate under-ventilated combustion regimes in case of mechanically ventilated enclosure by analyzing combustion regimes at two different scales. Fire scenario consists in a dodecane liquid pool fire located in a room mechanically ventilated. Reduced scale experiments concern a 0.031 m2 pool size in a 1.875 m3 enclosure and large scale experiments 0.4 m2 pool size in a 120 m3 enclosure. The varying parameter is the ventilation flow rate which allows the fire behavior to be investigated, from free burning to confined situations. At both scales, results show that the burning rate decreases with the ventilation flow rate and a linear relationship between oxygen concentration and fuel mass loss rate is emphasized. Comparison between the well-stirred reactor model and the experimental results shows good agreement for both scales. The work demonstrates the ability of reduced scale approach to reproduce the combustion process in a vitiated environment provided the use of appropriate dimensionless parameters: a reduced heat release rate, a thermal ratio parameter and the global equivalent ratio

Computational Fluid Dynamics Simulations of Water Spray Interaction with a Fire-Driven Flow in a Confined and a Mechanically Ventilated Enclosure

International audienceThe paper presents a detailed numerical study on the interaction between two water sprays and a fire-driven flow in a confined and mechanically ventilated enclosure of 4.9 m × 8.7 m × 3.9 m (height). The fire source is a 1.2 m × 1.2 m × 0.3 m propane burner, which is positioned in a corner and generates a well-controlled heat source of about 290 kW. The mechanical ventilation consists mainly of an inlet and an extraction ductdelivering each a flow rate of about 2500 m3/h. The two nozzles are mounted at 0.9 m below the ceiling and deliver a total flow rate of 107 l/min with a volume-median droplet diameter of about 470 μm. The CFD simulations carried out with the Fire Dynamics Simulator (FDS, 6.7.0) show a good agreement with the experimentally measured room pressure profile. More particularly, the under-pressure peak which occurs upon activation of the water spray system is well-predicted (i.e., -993 Pa in the experiments Vs -832 Pa in the simulations) provided that a sufficiently computational droplet injection rate is prescribed (Np = 80,000 droplets per second). As observed in the experiments, the CFD simulations show that the watersprays cause a relatively uniform species and temperature distribution from floor to ceiling. Thermal stratification is sustained though, especially in the region between the nozzles and the ceiling. Finally, an energy balance analysis shows that the experimentally estimated fraction of heat absorbed by the water (i.e., 65%) is higher than the one calculated in CFD (i.e., 40%)

Overview of the OECD PRISME 3 Project

International audienceFire hazard analyses and probabilistic fire safety analyses have demonstrated that fires may cause significant damages in nuclear power plants (NPPs). Fire modelling is nowadays applied by licensees or technical safety organisations to assess fire consequences in NPPs. Thereby, one important aspect is the availability of verified and validated fire models for such fire scenarios.Several members of the Organization for Economic Co-operation and Development (OECD) Nuclear Energy Agency (NEA) expressed their interest in participating in a joint international research project on the topic of fire events to be carried out under the auspices of the NEA. The PRISME (French acronym for “Fire Propagation in Elementary Multi-Room Scenarios”) Project was realized from 2006 to 2010, by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN, France) in its facilities specially designed for large-scale fire tests in confined environments. In the continuity of the PRISME project, PRISME 2 was launched in July 2011 ending in 2016. The main experimental results of these two projects have been summarized in OECD/NEA reports [1][2]. In parallel to the experimental campaigns, PRISME partners evaluated the capabilities of various fire simulation codes for modelling fire scenarios based on the PRISME results. Both PRISME 1 and PRISME 2 Projects highlighted the strong interaction between the fire dynamics and the mechanical ventilation. Indeed, the analysis of the tests greatly contributed to enhance the knowledge of under-ventilated fires including realistic and complex fires. An improvement in the validation process of different fire models was also noticed during these two experimental programs. From these experimental findings and modelling considerations, some grey zones have been highlighted and allowed to define the outlines of the PRISME 3 Project. Various recommendations have been provided for addressing some further phenomena not studied in the past Projects. These phenomena are smoke stratification and spread, fire propagation between electrical cabinets, and electrical cable tray fires in confined and ventilated conditions for new configurations. The ongoing PRISME 3 Project aims at addressing the above mentioned three phenomena and at providing answers to various issues of interest for nuclear safety. A total of eight countries have joined the PRISME 3 Project: Belgium (Bel V and Tractebel-ENGIE), Finland (Technical Research Centre VTT), France (IRSN as Operating Agent and Électricité de France – EDF), Germany (Gesellschaft für Anlagen- und Reaktorsicherheit – GRS), Japan (Nuclear Regulation Authority – NRA and Central Research Institute of Electric Power Industry – CRIEPI), Korea (Korea Institute for Nuclear Safety – KINS and Korea Atomic Energy Research Institute – KAERI), United Kingdom (Office for Nuclear Regulation, – ONR) and the United States of America (United States Nuclear Regulatory Commission – U.S. NRC).The objective of the first campaign, named S3 for Smoke Stratification and Spread, is to study new configurations of interest for smoke propagation in a mechanically ventilated multi-room facility with simple fire sources. This choice is relevant for a complete validation of fire models on smoke propagation. The first topic of interest is to combine vertical and horizontal smoke propagation coupled with a mechanical ventilation system. This experimental configuration allows highlighting multiple interaction mechanisms of propagation during the fire scenario. The second topic of interest concerns the issue of multiple fire sources, simulating for example a seismically induced fire incident and its consequences on smoke propagation. The fire scenario involves two fire sources ignited simultaneously and located in two adjacent rooms or in two rooms separated by another one. The distance between fires is a key parameter determining different combustion regimes with or without interaction. The third topic of interest is smoke propagation induced by an elevated fire source. This configuration leads to a complex situation for the fire dynamics, since it evolves in a hot and vitiated environment. For the S3 campaign, six fire tests have been defined in the multi-compartments facility of IRSN, named DIVA, which is composed of a mechanical ventilation system. For this campaign, three or four rooms, of volume 120 m3 and 170 m3, have been implemented.The second campaign, named ECFS for Electrical Cabinet Fire Spread, aims to better understand the fire spread from an open-door cabinet to other adjacent or opposite cabinets, connected via cable trays. Four tests in the IRSN DIVA facility have been defined including the two configurations of interest: the adjacent one and the opposite one. For each configuration two tests involving HFR (halogenated flame retardant) or HFFR (halogen free flame retardant) insulated cables will be conducted. Two cable trays have been positioned above the cabinets and inside a false floor to diversify the potential paths of the fire propagation. Furthermore, for the adjacent configuration, the fire propagation through the walls and an air gap potentially separating the cabinets is also considered. In addition to these confined fire tests, four additional tests have been specified in open atmosphere. These tests are needed for characterizing the fire source or the fire spread from one electrical cabinet to another one in a reference configuration. The comparison of the two configurations will highlight the effect of the confinement on the fire spread.The purpose of the third campaign, named CFP for Cable Fire Propagation, is twofold: in a first step, the effect of the compartment geometry will be studied by conducting three cable tray fires in a corridor. The fire dynamics on a long cable tray will then be compared to those obtained in previous PRISME Project for shorter cable tray configurations. In a last step, additional scenarios involving the effects of under-ventilated conditions and of the cable tray configuration on cable fires will be investigated. In addition, an assessment of cable fire models used within simple or complex fire numerical tools will be conducted for these specific configurations. The cable type and the air renewal rate of the compartment will be considered. Consequences of such fire scenarios on the facility will be investigated through time sequences of gas pressure, temperature and concentrations inside the facility and in the ventilation network. The campaign is composed of six tests in the DIVA facility and two tests under the SATURNE calorimeter of IRSN which is composed of an extraction hood in an open domain on 20,000 m3. The main advances of PRISME 3 will make it possible, in a first step, to increase the predictive ability of models on smoke propagation problems for substantially complex situations. With regard to electric fires, especially electric cable fires, PRISME projects will also provide a fairly complete database and can therefore be used for model improvement. All the contributions of the project will allow the community to position itself on the scenarios of interest to study in the years to come

Overview of the OECD PRISME 3 Project

International audienceFire hazard analyses and probabilistic fire safety analyses have demonstrated that fires may cause significant damages in nuclear power plants (NPPs). Fire modelling is nowadays applied by licensees or technical safety organisations to assess fire consequences in NPPs. Thereby, one important aspect is the availability of verified and validated fire models for such fire scenarios.Several members of the Organization for Economic Co-operation and Development (OECD) Nuclear Energy Agency (NEA) expressed their interest in participating in a joint international research project on the topic of fire events to be carried out under the auspices of the NEA. The PRISME (French acronym for “Fire Propagation in Elementary Multi-Room Scenarios”) Project was realized from 2006 to 2010, by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN, France) in its facilities specially designed for large-scale fire tests in confined environments. In the continuity of the PRISME project, PRISME 2 was launched in July 2011 ending in 2016. The main experimental results of these two projects have been summarized in OECD/NEA reports [1][2]. In parallel to the experimental campaigns, PRISME partners evaluated the capabilities of various fire simulation codes for modelling fire scenarios based on the PRISME results. Both PRISME 1 and PRISME 2 Projects highlighted the strong interaction between the fire dynamics and the mechanical ventilation. Indeed, the analysis of the tests greatly contributed to enhance the knowledge of under-ventilated fires including realistic and complex fires. An improvement in the validation process of different fire models was also noticed during these two experimental programs. From these experimental findings and modelling considerations, some grey zones have been highlighted and allowed to define the outlines of the PRISME 3 Project. Various recommendations have been provided for addressing some further phenomena not studied in the past Projects. These phenomena are smoke stratification and spread, fire propagation between electrical cabinets, and electrical cable tray fires in confined and ventilated conditions for new configurations. The ongoing PRISME 3 Project aims at addressing the above mentioned three phenomena and at providing answers to various issues of interest for nuclear safety. A total of eight countries have joined the PRISME 3 Project: Belgium (Bel V and Tractebel-ENGIE), Finland (Technical Research Centre VTT), France (IRSN as Operating Agent and Électricité de France – EDF), Germany (Gesellschaft für Anlagen- und Reaktorsicherheit – GRS), Japan (Nuclear Regulation Authority – NRA and Central Research Institute of Electric Power Industry – CRIEPI), Korea (Korea Institute for Nuclear Safety – KINS and Korea Atomic Energy Research Institute – KAERI), United Kingdom (Office for Nuclear Regulation, – ONR) and the United States of America (United States Nuclear Regulatory Commission – U.S. NRC).The objective of the first campaign, named S3 for Smoke Stratification and Spread, is to study new configurations of interest for smoke propagation in a mechanically ventilated multi-room facility with simple fire sources. This choice is relevant for a complete validation of fire models on smoke propagation. The first topic of interest is to combine vertical and horizontal smoke propagation coupled with a mechanical ventilation system. This experimental configuration allows highlighting multiple interaction mechanisms of propagation during the fire scenario. The second topic of interest concerns the issue of multiple fire sources, simulating for example a seismically induced fire incident and its consequences on smoke propagation. The fire scenario involves two fire sources ignited simultaneously and located in two adjacent rooms or in two rooms separated by another one. The distance between fires is a key parameter determining different combustion regimes with or without interaction. The third topic of interest is smoke propagation induced by an elevated fire source. This configuration leads to a complex situation for the fire dynamics, since it evolves in a hot and vitiated environment. For the S3 campaign, six fire tests have been defined in the multi-compartments facility of IRSN, named DIVA, which is composed of a mechanical ventilation system. For this campaign, three or four rooms, of volume 120 m3 and 170 m3, have been implemented.The second campaign, named ECFS for Electrical Cabinet Fire Spread, aims to better understand the fire spread from an open-door cabinet to other adjacent or opposite cabinets, connected via cable trays. Four tests in the IRSN DIVA facility have been defined including the two configurations of interest: the adjacent one and the opposite one. For each configuration two tests involving HFR (halogenated flame retardant) or HFFR (halogen free flame retardant) insulated cables will be conducted. Two cable trays have been positioned above the cabinets and inside a false floor to diversify the potential paths of the fire propagation. Furthermore, for the adjacent configuration, the fire propagation through the walls and an air gap potentially separating the cabinets is also considered. In addition to these confined fire tests, four additional tests have been specified in open atmosphere. These tests are needed for characterizing the fire source or the fire spread from one electrical cabinet to another one in a reference configuration. The comparison of the two configurations will highlight the effect of the confinement on the fire spread.The purpose of the third campaign, named CFP for Cable Fire Propagation, is twofold: in a first step, the effect of the compartment geometry will be studied by conducting three cable tray fires in a corridor. The fire dynamics on a long cable tray will then be compared to those obtained in previous PRISME Project for shorter cable tray configurations. In a last step, additional scenarios involving the effects of under-ventilated conditions and of the cable tray configuration on cable fires will be investigated. In addition, an assessment of cable fire models used within simple or complex fire numerical tools will be conducted for these specific configurations. The cable type and the air renewal rate of the compartment will be considered. Consequences of such fire scenarios on the facility will be investigated through time sequences of gas pressure, temperature and concentrations inside the facility and in the ventilation network. The campaign is composed of six tests in the DIVA facility and two tests under the SATURNE calorimeter of IRSN which is composed of an extraction hood in an open domain on 20,000 m3. The main advances of PRISME 3 will make it possible, in a first step, to increase the predictive ability of models on smoke propagation problems for substantially complex situations. With regard to electric fires, especially electric cable fires, PRISME projects will also provide a fairly complete database and can therefore be used for model improvement. All the contributions of the project will allow the community to position itself on the scenarios of interest to study in the years to come