Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppression with reduced quantities of water [1]. Its use can also limit properties losses by limiting the damage to structures and objects consequent to large water release. However the mechanisms of extinguishment are very complex because of the strong interaction of the aerosol particles with the fluid dynamics and fire structures. This poses several issues for a proper dimensioning of the system. In this work, a simple zone model [2] is derived to assess some of the minimum requirements that a proper dimensioning of the system has to satisfy. Based on simplifying assumptions valid for UFWM, i.e. when water droplets have a diameter less than 50 microns, balance equations of energy, air, water vapor and liquid water particles are derived for the volumes formed by the partitioning of a confined ambient interested by a fire into three zones: fire column (zone 1), ceiling zone (zone 2) and the remaining of the ambient (zone 3). Being the aim to verify that a proper amount of UFWM release is determined, the logic is to assign the expected fire and its extinction rate and then estimate if the assumed water release is commensurate. This assessment is obtained by verifying several indicators during the model evolution: temperature reached in the fire zone, depth of the hot smokes ceiling layer, amount of liquid water mist in zone 3, amount of air (oxygen) in the fire zone. Validation and limits of the model are derived comparing results with a full scale fire test data of a workstation (tables, books, paper files, chair and computer) completed by a CDF numerical simulation with the FDS software by NIST. The fire test proved the effectiveness of the UFWM system. The zone model is able to reproduce a coherent evolution of the variables by adopting the UFWM flow rate used in the successful extinction test. Despite its simplicity, the model appear useful for a first estimate of the UFWM flow rate required to extinguish a given fire, taking into account the interaction between the different zones of the compartment and the possibility for the WM to reach the fire zone. Furthermore, several extinction mechanisms can be estimated: fire cooling by UFWM evaporation, oxygen depletion by water vapor substitution, effectiveness of the thermal radiation shield provided by the UFWM. The work is at an early stage and the validity of results needs to be confirmed with respect to several parameters ranging from the dimensions of the compartment, to the fire power. Acknowledgements. Data about full scale fire tests have been provided by Norsim srl. Dr. Roberto Caruso has contributed to the FDS simulation setup and to the collection and analysis of test and model results. [1] Z. Liu and A. K. Kim, A Review of Water Mist Fire Suppression Systems-- Fundamental Studies, J. Fire Protection Eng., 10:32 (1999). [2] D. Alexander and E. Li, Water Mist Fire Suppression Modeling, DRDC Atlantic CR 2009-132, June 2009.
A zone model for fast verification of release of Ultrafine Water Mist for fire extinction in compartments
Francesco Saverio Marra
2015
Abstract
Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppression with reduced quantities of water [1]. Its use can also limit properties losses by limiting the damage to structures and objects consequent to large water release. However the mechanisms of extinguishment are very complex because of the strong interaction of the aerosol particles with the fluid dynamics and fire structures. This poses several issues for a proper dimensioning of the system. In this work, a simple zone model [2] is derived to assess some of the minimum requirements that a proper dimensioning of the system has to satisfy. Based on simplifying assumptions valid for UFWM, i.e. when water droplets have a diameter less than 50 microns, balance equations of energy, air, water vapor and liquid water particles are derived for the volumes formed by the partitioning of a confined ambient interested by a fire into three zones: fire column (zone 1), ceiling zone (zone 2) and the remaining of the ambient (zone 3). Being the aim to verify that a proper amount of UFWM release is determined, the logic is to assign the expected fire and its extinction rate and then estimate if the assumed water release is commensurate. This assessment is obtained by verifying several indicators during the model evolution: temperature reached in the fire zone, depth of the hot smokes ceiling layer, amount of liquid water mist in zone 3, amount of air (oxygen) in the fire zone. Validation and limits of the model are derived comparing results with a full scale fire test data of a workstation (tables, books, paper files, chair and computer) completed by a CDF numerical simulation with the FDS software by NIST. The fire test proved the effectiveness of the UFWM system. The zone model is able to reproduce a coherent evolution of the variables by adopting the UFWM flow rate used in the successful extinction test. Despite its simplicity, the model appear useful for a first estimate of the UFWM flow rate required to extinguish a given fire, taking into account the interaction between the different zones of the compartment and the possibility for the WM to reach the fire zone. Furthermore, several extinction mechanisms can be estimated: fire cooling by UFWM evaporation, oxygen depletion by water vapor substitution, effectiveness of the thermal radiation shield provided by the UFWM. The work is at an early stage and the validity of results needs to be confirmed with respect to several parameters ranging from the dimensions of the compartment, to the fire power. Acknowledgements. Data about full scale fire tests have been provided by Norsim srl. Dr. Roberto Caruso has contributed to the FDS simulation setup and to the collection and analysis of test and model results. [1] Z. Liu and A. K. Kim, A Review of Water Mist Fire Suppression Systems-- Fundamental Studies, J. Fire Protection Eng., 10:32 (1999). [2] D. Alexander and E. Li, Water Mist Fire Suppression Modeling, DRDC Atlantic CR 2009-132, June 2009.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
