- Email: [email protected]
Fire modeling of an emerging fire suppression system
JCHAS 734 1–6
FEATURE
Fire modeling of an emerging fire suppression system Abstract. Self-contained fire extinguishers are a robust, reliable and minimally invasive means of fire suppression for gloveboxes. Test methodology has been developed (experiments and computations) to predict fire induced tube wall failure in small scale compartments such as gloveboxes. A small scale test apparatus has been developed to characterize tube wall temperature and breakage properties. Computational tools have been used to better understand experiments. The heat release rate and heat flux have been accurately predicted because the forward predicted temperatures closely matched the experimentally measured values. Data generated from computational modeling of fire phenomena helps to identify the limitations of self-contained fire extinguishers.
Q1
By Michael E. Cournoyer
INTRODUCTION
The glovebox, coupled with an adequate negative pressure gradient, provides primary confinement. This is the primary engineering control when working with plutonium in a nuclear research facility.1 Seismic events present real issues in designing gloveboxes. One of the most challenging accident scenarios is the post-seismic fire event. History has shown that glovebox fires can be extremely dangerous and may pose significant health hazards when the products of combustion include radioactive and toxic materials.2 Automatic fire suppression must be installed in gloveboxes to mitigate a fire event.3 These regulatory requirements stem from actual fire incidents. Water-based suppression and dry chemical systems are capable of providing fire suppression and are inherently reliable.4 Inertion is another approach to minimize the potential for fire by providing an oxygen deficient atmosphere that does not support combustion in the glovebox. The inertion systems are installed to support Michael E. Cournoyer is affiliated with the Los Alamos National Laboratory, Los Alamos, NM 87545, United States (Tel.: +1 505 665 7616; fax: +1 505 665 3657; e-mail: [email protected]).
1871-5532 http://dx.doi.org/10.1016/j.jchas.2014.05.011
process requirements typically involving pyrophoric metals. Fire mitigation is reliable up to the point at which the inert atmosphere can be maintained. Toppling of a glovebox during a seismic event eliminates fire suppression in the enclosure with these types of systems. As previously reported in this journal, self-contained fire extinguishers were demonstrated to be a robust, reliable and minimally invasive means of fire suppression for gloveboxes.5 See Fig. 1. They can be mounted to the interior of the glovebox. The self-contained fire extinguisher system employs a nylon tube that contains a jelled fire suppression media. The nylon tube is pressurized to 0.7 MPa at room temperature conditions, and the combination of thermally induced weakening of the nylon material associated with fire heat fluxes and expansion of the jelled fire suppression media causes the rupture and release of the fire suppressant. The overall system reliability is dependent on the tube failing at the same time for any given fire scenario. Even in the absence of radiation induced degradation of the nylon, there will be variability in the time of actuation associated with variations in the fire signature. One key design feature of the selfcontained fire extinguisher system is a release seam in the nylon tube wall. This failure seam is meant to reduce variability in the suppressant activation time. The manufacturer states that
the tube fails at a temperature of 150 8C, independent of the time history of the fire or thermal load that heats the tube to 150 8C. It is anticipated that there will be spatial variations in the gas temperature of a glovebox containing a fire. The heat transfer from the fire to the nylon tube will be by both convection and radiation. It is unlikely that the tube will ever be at a uniform temperature. Thus, it will be useful to characterize the temperature distribution on the tube to better understand how temperature variations on the tube might affect its rupture time. The manufacturer nylon tubes are guaranteed for an installation period of five years. Using Class A, B, and C combustibles and various combinations of combustibles, the self-contained fire extinguisher system has been demonstrated to successfully extinguish all three classes of fires in gloveboxes.5 The glovebox gloves were intact and pliable. The gloves did have some discoloration because of their proximity to the fire. The windows did not suffer any degradation or clouding and the gaskets showed no sign of fire damage. The design of the glovebox in question and the placement of the suppression tube will likely affect the heat transfer rates to the tube and thus the activation time. The effects of glovebox ventilation characteristics should be simulated and evaluated. In light of these issues, small-scale experiments of virgin and radiation
ß Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Inc.
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
1
JCHAS 734 1–6
Table 1. Nylon 6,6 stress behavior.
T (8C) 23 130 200
s (MPa) 90 30 10
Fig. 1. Self-contained fire extinguisher tube.
affected nylon need to be conducted. The resulting fluid dynamics modeling of heat transfer, and Finite Element Analysis (FEA) will better characterize the failure of self-contained fire extinguisher systems. Computational modeling of fire phenomena plays an increasingly important role in engineering and scientific endeavors.6 When experiments are performed, the correlation of simulation results with experimental data can be particularly useful in gaining insight into the underlying physics of the fire. The gas and glass temperatures can be modeled with the computational fluid dynamics code Fire Dynamics Simulator (FDS), which is maintained by the National Institute of Standards and Technology.7 If the glass temperatures in the FDS model match up with the actual glass temperatures, this lends further validity to the assumptions used in the combustion and heat transfer models. In the following report, computational analysis of fire induced of the failure of self-contained fire extinguisher tubes
is conducted. Calibrated and validated fire and mechanical modeling tools are used to understand the operating characteristics of self-contained fire extinguisher tubes for gloveboxes. Heat transfer by convection and radiation is used to characterize temperature distribution and variations affecting tube rupture time. Spatial variations in the gas temperature of a glovebox containing a fire are studied to determine how radiation exposure will degrade/modify the thermo-mechanical characteristics of the nylon and also the suppressant.
METHODOLOGY
At a high level of abstraction, the selfcontained fire extinguisher system analysis framework can be reduced (for the uncoupled and serially processed model) to the flow chart shown in Fig. 2. A fire model is used to calculate the gas temperatures and heat fluxes in the glovebox compartment. The fire sets
up a convective and radiative heat transfer field that heats the nylon tube and suppressant agent. An equation of state for the suppressant agent is used to determine the pressure time history of the agent. The tube ruptures at a time when the internal pressure provides a stress field in the tube that exceeds a critical stress. A limited number of experimental tests were also performed in a small-scale fire compartment. Property data has been compiled on the jelled fire suppression media and of the nylon tube material. A proprietary fire suppression agent – Envirogel is contained within notched nylon 6,6 tube. Components of the fire suppression agent include pentafluoro-ethane or hexafluoro-propane, sodium bicarbonate, and nitrogen charged to 0.7 MPa. Nylon 6,6 melts at about 250 8C. Relative humidity effect nylon 6,6 yield stress. Table 1 shows nylon 6,6 stress, as a function of temperature. Tests of tube heating and deformation have been run in a small-scale burn compartment to perform the thermal modeling and characterization of fire
Fig. 2. Self-contained fire extinguisher tube fire modeling framework.
2
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
JCHAS 734 1–6
heat and density do not vary with temperature. The pressure vessel model is illustrated in Fig. 6. The ideal gas law is assumed for the pressure versus time of working fluid inside the self-contained fire extinguisher tube. FEA tools consisted of SolidWorks Software and LibMesh, an opensource finite element solver. Variables in the simulations consisted of the following: size of glovebox, intensity of fire and location of the fire suppression system, vent hood and glass. Fig. 3. Schematic of small-scale FDS geometry setup. RESULTS
in the glovebox. The geometry of the small-scale experimental glovebox was modeled in FDS with inner dimensions of 35 cm (length) 15 cm (width) 45 cm (height). The fire source was input as a burner located at the bottom of the enclosure as a 3 kW fire to approximate the size of the Bunsen burner used in the small-scale experiments. The grid resolution was set to 2.5 cm cells, which results in a D*/dx ratio of 3.76 for a fire size of 3 kW. Fig. 3 shows a schematic of the model geometry. Hot gas layer temperatures can be simulated in a small-scale glovebox model in FDS. See Fig. 4. The fuel was methane with a heat of combustion of 50.0 MJ/kg and a soot yield of 0.01 g/g. The walls were assigned material properties of gypsum board, and the front of the enclosure contained a 25 cm 25 cm pane of glass on the front wall. The selfcontained fire extinguisher tube was modeled as a solid object with dimensions of 20 cm 5 cm 5 cm with a wall thickness of 1.65 mm and the material properties of nylon 6. More details on the temperature response of the tube will be discussed in the thermal modeling section. Vent holes were placed at the top and bottom of the geometry to provide a means of inflow and outflow. The area of the vent holes was determined by estimating an effective leakage area for the compartment based on the amount of oxygen available in the enclosure and by calibrating the model using the measured thermocouple temperatures. The self-contained fire extinguisher tube is modeled as a cylinder under
constant, uniform radiative heat flux. See Fig. 5. The heat transfer from the fire to the nylon tube was characterized by performing an energy balance using a lumped analysis approach for the tube wall and the working fluid. The following assumptions were made: forced convection with constant heat transfer coefficient and specific
Analytical (solid lines) vs. FDS (dashed lines) results for the convective and radiative heat fluxes on selfcontained fire extinguisher tube in the small-scale case are shown in Fig. 7. Simultaneously, a heat flux is applied on the surface of the self-contained fire extinguisher wall because of the fire. Analytical (solid lines) vs.
Fig. 4. Hot gas layer temperatures in small-scale glovebox model in FDS.
Fig. 5. Schematic of radiative and convective heat transfer to nylon tube.
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
3
JCHAS 734 1–6
Fig. 6. Pressure vessel model.
Fig. 7. Convective and radiative heat fluxes comparison.
Fig. 8. Tube wall and working fluid (oil) temperature vs. time comparison.
FDS (dashed lines) results of the temperature on the surface of the self-contained fire extinguisher wall and working fluid (oil) are shown in Fig. 8. Based upon the heating history, a simple ideal gas thermodynamic model for the pressurization agent (e.g., nitrogen) coupled to the simplest stress analysis model indicates that the selfcontained fire extinguisher tube fails at relatively low internal pressures 1 MPa. Computational Fluid Dynamics 4
predicts heat flux and wall temperature of self-contained fire extinguisher tube in the small-scale case. See Figs. 9 and 10.
DISCUSSION
Using a wide range of modeling approximations, the framework shown in Fig. 2 was exercised. At the computationally intensive boundary of the
analysis, detailed computer engineering analysis tools were used to analyze the fire physics and the stress analysis. Computational and analytical models have been identified for the fire physics, heat transfer processes, thermodynamic analysis of the working fluid, and stress analysis of self-contained fire extinguisher tubes. The results of the gas phase modeling have been a success. There is good correlation between actual and simulated radiation and convection heat flux. See Fig. 6. The actual heat fluxes were smoother because the thermocouples dampen out the high frequency temperature fluctuation.8 Of primary interest has been characterizing wall temperature evolution on the self-contained fire extinguisher tube for various fire heat release rate within the glovebox. The heat release rate has been adjusted in FDS to match the actual gas temperatures. The simulation data modeled the rate at which the fire reaches steady state. Using the FDS modeling, the heat release rate is predicted to be approximately 3 kW. There is reasonable agreement between the experimental and FDS thermocouple temperatures, which indicates that the fire size, hot gas layer stratification, and convective and radiative effects were reasonably modeled in FDS. Concerning the solid phase modeling, Fig. 7 shows the results of how the temperatures for self-contained fire extinguisher tube wall and working fluid evolve over time. The actual wall and working fluid temperatures correlate well with simulated gas temperatures. The FDS model follows the same trend line as the experiments. The black strip of tape on the selfcontained fire extinguisher tubes facilitates alignment of the tube for optimal discharge during installation. Design of the glovebox and the placement of the suppression tube affect the heat transfer rates to the tube and its activation time. Those characteristics have been simulated and evaluated, see Fig. 8. Simple constant mass and constant volume ideal gas heating models coupled to a fire-tube heat transfer model are able to predict this pressurization and failure path. Embedded boundary/ volume-of-fluid methods in FDS allow complex geometry simulation, see
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
JCHAS 734 1–6
Fig. 9. Time sequence of net heat flux of self-contained fire extinguisher tube.
Fig. 10. Time sequence of wall temperature of self-contained fire extinguisher tube.
Figs. 9 and 10. Next, models can be scale up to a full-scale glovebox to predict gas temperatures, self-contained fire extinguisher tube temperatures and activation. Radiation damage and chemical exposure from harsh environmental conditions are known to degrade the
lifetime of engineered components, especially polymer materials.9 The existing nylon tube material is highly challenged by inorganic acids; additionally the material absorbs water increasing its overall moisture content. Both of these factors may limit the potential number of gloveboxes the
tube could be installed in. The reliable operation of the self-contained fire extinguisher tubes in nuclear glovebox environment can be enhanced by addressing these issues. It is unclear how radiation exposure will degrade/modify the thermo-mechanical characteristics of the nylon
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
5
JCHAS 734 1–6
and the suppressant. An important aspect of the rupture process is the thermodynamic state of the jelled fire suppression media. It is assumed that radiation exposure does not markedly modify the chemistry of these gases as this may affect the molecular weight and the critical temperature when the internal pressure exceeds the nylon failure pressure. In summary, self-contained fire extinguisher tubes remain functional and provide an active means of fire suppression in the glovebox. Test methodology has been developed (experiments and computations) to predict fire induced wall failure in small scale compartments such as gloveboxes. A small scale test apparatus has been developed to characterize tube wall temperature and breakage properties. Computational tools have been used to better understand experiments. In this experimental campaign, the thermal environment that causes a tube wall failure has been determined. The heat release rate and heat flux has been accurately predicted because the forward predicted temperatures closely matched the experimentally measured values. This experiment serves as a proof of concept upon which additional experiments will be based.
6
CONCLUSIONS
FDS models allow results from simulations on small-scale tests of tube heating and deformation be applied to a large scale glovebox fire suppression systems. Small scale fires are simulated more accurately by adjusting the heat release rate manually. The heat flux data obtained from FDS could be used in a FEA program to model the heat transfer and stresses in the self-contained fire extinguisher tube wall. Data generated from computational modeling of fire phenomena help identify the limitations of self-contained fire extinguishers. This increases technical knowledge and augments operational safety. ACKNOWLEDGEMENTS The author would like to acknowledge the U.S. Department of Energy and LANL’s Plutonium Science & Manufacturing Directorate for support of this work. The author would also like to acknowledge Ofodike A. Ezekoye and his team at the University of Texas for collecting and interpreting the data.
REFERENCES 1. Rael, D.; et al. Retrofit of an engineered gloveport to a Los Alamos National
2.
3. 4.
5.
6.
7.
8.
9.
Laboratory’s Plutonium Facility glovebox. Proceeding from WM’08. Phoenix, Arizona, February 24–28, J. Am. Soc. Mech. Eng. 2008. http://www.lm.doe.gov/land/sites/co/ rocky_flats/closure/references/068-RPs %20Report-2003-0011.pdf, accessed 5/23/2014. DOE Standard 1066-99, DOE Standard: Fire Protection Design Criteria. NFPA No. USS14, ‘‘U.S. Experience with sprinklers and other automatic fire extinguishing equipment,’’ February 2010. Rosenberger, M. S.; et al. Glovebox issues: Controlling fire hazards after an earthquake. J. Chem. Health Safety, 2012, 19(4), 1–6. Ezekoye, O. A.; et al. Effects of leakage in simulations of positive pressure ventilation. Fire Technol. 2009, 45, 257–286. McGrattan, K., et al., National Institute of Standards and Technology Special Publication 1019-5, Gaithersburg, MD, (2010). Weinschenk, C.; Ezekoye, O. A. Analysis of thermocouple responses to turbulent radiating environment. Proceeding from AJTEC 2011. March 15, 2011. Cournoyer, M. E.; et al. Minimizing glovebox glove failures. Part III: deriving service lifetimes. Proceeding from WM’06. Tucson, Arizona, February 26–Mar 2, J. Am. Soc. Mech. Eng. 2006.
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
Q2
FEATURE
Fire modeling of an emerging fire suppression system Abstract. Self-contained fire extinguishers are a robust, reliable and minimally invasive means of fire suppression for gloveboxes. Test methodology has been developed (experiments and computations) to predict fire induced tube wall failure in small scale compartments such as gloveboxes. A small scale test apparatus has been developed to characterize tube wall temperature and breakage properties. Computational tools have been used to better understand experiments. The heat release rate and heat flux have been accurately predicted because the forward predicted temperatures closely matched the experimentally measured values. Data generated from computational modeling of fire phenomena helps to identify the limitations of self-contained fire extinguishers.
Q1
By Michael E. Cournoyer
INTRODUCTION
The glovebox, coupled with an adequate negative pressure gradient, provides primary confinement. This is the primary engineering control when working with plutonium in a nuclear research facility.1 Seismic events present real issues in designing gloveboxes. One of the most challenging accident scenarios is the post-seismic fire event. History has shown that glovebox fires can be extremely dangerous and may pose significant health hazards when the products of combustion include radioactive and toxic materials.2 Automatic fire suppression must be installed in gloveboxes to mitigate a fire event.3 These regulatory requirements stem from actual fire incidents. Water-based suppression and dry chemical systems are capable of providing fire suppression and are inherently reliable.4 Inertion is another approach to minimize the potential for fire by providing an oxygen deficient atmosphere that does not support combustion in the glovebox. The inertion systems are installed to support Michael E. Cournoyer is affiliated with the Los Alamos National Laboratory, Los Alamos, NM 87545, United States (Tel.: +1 505 665 7616; fax: +1 505 665 3657; e-mail: [email protected]).
1871-5532 http://dx.doi.org/10.1016/j.jchas.2014.05.011
process requirements typically involving pyrophoric metals. Fire mitigation is reliable up to the point at which the inert atmosphere can be maintained. Toppling of a glovebox during a seismic event eliminates fire suppression in the enclosure with these types of systems. As previously reported in this journal, self-contained fire extinguishers were demonstrated to be a robust, reliable and minimally invasive means of fire suppression for gloveboxes.5 See Fig. 1. They can be mounted to the interior of the glovebox. The self-contained fire extinguisher system employs a nylon tube that contains a jelled fire suppression media. The nylon tube is pressurized to 0.7 MPa at room temperature conditions, and the combination of thermally induced weakening of the nylon material associated with fire heat fluxes and expansion of the jelled fire suppression media causes the rupture and release of the fire suppressant. The overall system reliability is dependent on the tube failing at the same time for any given fire scenario. Even in the absence of radiation induced degradation of the nylon, there will be variability in the time of actuation associated with variations in the fire signature. One key design feature of the selfcontained fire extinguisher system is a release seam in the nylon tube wall. This failure seam is meant to reduce variability in the suppressant activation time. The manufacturer states that
the tube fails at a temperature of 150 8C, independent of the time history of the fire or thermal load that heats the tube to 150 8C. It is anticipated that there will be spatial variations in the gas temperature of a glovebox containing a fire. The heat transfer from the fire to the nylon tube will be by both convection and radiation. It is unlikely that the tube will ever be at a uniform temperature. Thus, it will be useful to characterize the temperature distribution on the tube to better understand how temperature variations on the tube might affect its rupture time. The manufacturer nylon tubes are guaranteed for an installation period of five years. Using Class A, B, and C combustibles and various combinations of combustibles, the self-contained fire extinguisher system has been demonstrated to successfully extinguish all three classes of fires in gloveboxes.5 The glovebox gloves were intact and pliable. The gloves did have some discoloration because of their proximity to the fire. The windows did not suffer any degradation or clouding and the gaskets showed no sign of fire damage. The design of the glovebox in question and the placement of the suppression tube will likely affect the heat transfer rates to the tube and thus the activation time. The effects of glovebox ventilation characteristics should be simulated and evaluated. In light of these issues, small-scale experiments of virgin and radiation
ß Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Inc.
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
1
JCHAS 734 1–6
Table 1. Nylon 6,6 stress behavior.
T (8C) 23 130 200
s (MPa) 90 30 10
Fig. 1. Self-contained fire extinguisher tube.
affected nylon need to be conducted. The resulting fluid dynamics modeling of heat transfer, and Finite Element Analysis (FEA) will better characterize the failure of self-contained fire extinguisher systems. Computational modeling of fire phenomena plays an increasingly important role in engineering and scientific endeavors.6 When experiments are performed, the correlation of simulation results with experimental data can be particularly useful in gaining insight into the underlying physics of the fire. The gas and glass temperatures can be modeled with the computational fluid dynamics code Fire Dynamics Simulator (FDS), which is maintained by the National Institute of Standards and Technology.7 If the glass temperatures in the FDS model match up with the actual glass temperatures, this lends further validity to the assumptions used in the combustion and heat transfer models. In the following report, computational analysis of fire induced of the failure of self-contained fire extinguisher tubes
is conducted. Calibrated and validated fire and mechanical modeling tools are used to understand the operating characteristics of self-contained fire extinguisher tubes for gloveboxes. Heat transfer by convection and radiation is used to characterize temperature distribution and variations affecting tube rupture time. Spatial variations in the gas temperature of a glovebox containing a fire are studied to determine how radiation exposure will degrade/modify the thermo-mechanical characteristics of the nylon and also the suppressant.
METHODOLOGY
At a high level of abstraction, the selfcontained fire extinguisher system analysis framework can be reduced (for the uncoupled and serially processed model) to the flow chart shown in Fig. 2. A fire model is used to calculate the gas temperatures and heat fluxes in the glovebox compartment. The fire sets
up a convective and radiative heat transfer field that heats the nylon tube and suppressant agent. An equation of state for the suppressant agent is used to determine the pressure time history of the agent. The tube ruptures at a time when the internal pressure provides a stress field in the tube that exceeds a critical stress. A limited number of experimental tests were also performed in a small-scale fire compartment. Property data has been compiled on the jelled fire suppression media and of the nylon tube material. A proprietary fire suppression agent – Envirogel is contained within notched nylon 6,6 tube. Components of the fire suppression agent include pentafluoro-ethane or hexafluoro-propane, sodium bicarbonate, and nitrogen charged to 0.7 MPa. Nylon 6,6 melts at about 250 8C. Relative humidity effect nylon 6,6 yield stress. Table 1 shows nylon 6,6 stress, as a function of temperature. Tests of tube heating and deformation have been run in a small-scale burn compartment to perform the thermal modeling and characterization of fire
Fig. 2. Self-contained fire extinguisher tube fire modeling framework.
2
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
JCHAS 734 1–6
heat and density do not vary with temperature. The pressure vessel model is illustrated in Fig. 6. The ideal gas law is assumed for the pressure versus time of working fluid inside the self-contained fire extinguisher tube. FEA tools consisted of SolidWorks Software and LibMesh, an opensource finite element solver. Variables in the simulations consisted of the following: size of glovebox, intensity of fire and location of the fire suppression system, vent hood and glass. Fig. 3. Schematic of small-scale FDS geometry setup. RESULTS
in the glovebox. The geometry of the small-scale experimental glovebox was modeled in FDS with inner dimensions of 35 cm (length) 15 cm (width) 45 cm (height). The fire source was input as a burner located at the bottom of the enclosure as a 3 kW fire to approximate the size of the Bunsen burner used in the small-scale experiments. The grid resolution was set to 2.5 cm cells, which results in a D*/dx ratio of 3.76 for a fire size of 3 kW. Fig. 3 shows a schematic of the model geometry. Hot gas layer temperatures can be simulated in a small-scale glovebox model in FDS. See Fig. 4. The fuel was methane with a heat of combustion of 50.0 MJ/kg and a soot yield of 0.01 g/g. The walls were assigned material properties of gypsum board, and the front of the enclosure contained a 25 cm 25 cm pane of glass on the front wall. The selfcontained fire extinguisher tube was modeled as a solid object with dimensions of 20 cm 5 cm 5 cm with a wall thickness of 1.65 mm and the material properties of nylon 6. More details on the temperature response of the tube will be discussed in the thermal modeling section. Vent holes were placed at the top and bottom of the geometry to provide a means of inflow and outflow. The area of the vent holes was determined by estimating an effective leakage area for the compartment based on the amount of oxygen available in the enclosure and by calibrating the model using the measured thermocouple temperatures. The self-contained fire extinguisher tube is modeled as a cylinder under
constant, uniform radiative heat flux. See Fig. 5. The heat transfer from the fire to the nylon tube was characterized by performing an energy balance using a lumped analysis approach for the tube wall and the working fluid. The following assumptions were made: forced convection with constant heat transfer coefficient and specific
Analytical (solid lines) vs. FDS (dashed lines) results for the convective and radiative heat fluxes on selfcontained fire extinguisher tube in the small-scale case are shown in Fig. 7. Simultaneously, a heat flux is applied on the surface of the self-contained fire extinguisher wall because of the fire. Analytical (solid lines) vs.
Fig. 4. Hot gas layer temperatures in small-scale glovebox model in FDS.
Fig. 5. Schematic of radiative and convective heat transfer to nylon tube.
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
3
JCHAS 734 1–6
Fig. 6. Pressure vessel model.
Fig. 7. Convective and radiative heat fluxes comparison.
Fig. 8. Tube wall and working fluid (oil) temperature vs. time comparison.
FDS (dashed lines) results of the temperature on the surface of the self-contained fire extinguisher wall and working fluid (oil) are shown in Fig. 8. Based upon the heating history, a simple ideal gas thermodynamic model for the pressurization agent (e.g., nitrogen) coupled to the simplest stress analysis model indicates that the selfcontained fire extinguisher tube fails at relatively low internal pressures 1 MPa. Computational Fluid Dynamics 4
predicts heat flux and wall temperature of self-contained fire extinguisher tube in the small-scale case. See Figs. 9 and 10.
DISCUSSION
Using a wide range of modeling approximations, the framework shown in Fig. 2 was exercised. At the computationally intensive boundary of the
analysis, detailed computer engineering analysis tools were used to analyze the fire physics and the stress analysis. Computational and analytical models have been identified for the fire physics, heat transfer processes, thermodynamic analysis of the working fluid, and stress analysis of self-contained fire extinguisher tubes. The results of the gas phase modeling have been a success. There is good correlation between actual and simulated radiation and convection heat flux. See Fig. 6. The actual heat fluxes were smoother because the thermocouples dampen out the high frequency temperature fluctuation.8 Of primary interest has been characterizing wall temperature evolution on the self-contained fire extinguisher tube for various fire heat release rate within the glovebox. The heat release rate has been adjusted in FDS to match the actual gas temperatures. The simulation data modeled the rate at which the fire reaches steady state. Using the FDS modeling, the heat release rate is predicted to be approximately 3 kW. There is reasonable agreement between the experimental and FDS thermocouple temperatures, which indicates that the fire size, hot gas layer stratification, and convective and radiative effects were reasonably modeled in FDS. Concerning the solid phase modeling, Fig. 7 shows the results of how the temperatures for self-contained fire extinguisher tube wall and working fluid evolve over time. The actual wall and working fluid temperatures correlate well with simulated gas temperatures. The FDS model follows the same trend line as the experiments. The black strip of tape on the selfcontained fire extinguisher tubes facilitates alignment of the tube for optimal discharge during installation. Design of the glovebox and the placement of the suppression tube affect the heat transfer rates to the tube and its activation time. Those characteristics have been simulated and evaluated, see Fig. 8. Simple constant mass and constant volume ideal gas heating models coupled to a fire-tube heat transfer model are able to predict this pressurization and failure path. Embedded boundary/ volume-of-fluid methods in FDS allow complex geometry simulation, see
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
JCHAS 734 1–6
Fig. 9. Time sequence of net heat flux of self-contained fire extinguisher tube.
Fig. 10. Time sequence of wall temperature of self-contained fire extinguisher tube.
Figs. 9 and 10. Next, models can be scale up to a full-scale glovebox to predict gas temperatures, self-contained fire extinguisher tube temperatures and activation. Radiation damage and chemical exposure from harsh environmental conditions are known to degrade the
lifetime of engineered components, especially polymer materials.9 The existing nylon tube material is highly challenged by inorganic acids; additionally the material absorbs water increasing its overall moisture content. Both of these factors may limit the potential number of gloveboxes the
tube could be installed in. The reliable operation of the self-contained fire extinguisher tubes in nuclear glovebox environment can be enhanced by addressing these issues. It is unclear how radiation exposure will degrade/modify the thermo-mechanical characteristics of the nylon
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
5
JCHAS 734 1–6
and the suppressant. An important aspect of the rupture process is the thermodynamic state of the jelled fire suppression media. It is assumed that radiation exposure does not markedly modify the chemistry of these gases as this may affect the molecular weight and the critical temperature when the internal pressure exceeds the nylon failure pressure. In summary, self-contained fire extinguisher tubes remain functional and provide an active means of fire suppression in the glovebox. Test methodology has been developed (experiments and computations) to predict fire induced wall failure in small scale compartments such as gloveboxes. A small scale test apparatus has been developed to characterize tube wall temperature and breakage properties. Computational tools have been used to better understand experiments. In this experimental campaign, the thermal environment that causes a tube wall failure has been determined. The heat release rate and heat flux has been accurately predicted because the forward predicted temperatures closely matched the experimentally measured values. This experiment serves as a proof of concept upon which additional experiments will be based.
6
CONCLUSIONS
FDS models allow results from simulations on small-scale tests of tube heating and deformation be applied to a large scale glovebox fire suppression systems. Small scale fires are simulated more accurately by adjusting the heat release rate manually. The heat flux data obtained from FDS could be used in a FEA program to model the heat transfer and stresses in the self-contained fire extinguisher tube wall. Data generated from computational modeling of fire phenomena help identify the limitations of self-contained fire extinguishers. This increases technical knowledge and augments operational safety. ACKNOWLEDGEMENTS The author would like to acknowledge the U.S. Department of Energy and LANL’s Plutonium Science & Manufacturing Directorate for support of this work. The author would also like to acknowledge Ofodike A. Ezekoye and his team at the University of Texas for collecting and interpreting the data.
REFERENCES 1. Rael, D.; et al. Retrofit of an engineered gloveport to a Los Alamos National
2.
3. 4.
5.
6.
7.
8.
9.
Laboratory’s Plutonium Facility glovebox. Proceeding from WM’08. Phoenix, Arizona, February 24–28, J. Am. Soc. Mech. Eng. 2008. http://www.lm.doe.gov/land/sites/co/ rocky_flats/closure/references/068-RPs %20Report-2003-0011.pdf, accessed 5/23/2014. DOE Standard 1066-99, DOE Standard: Fire Protection Design Criteria. NFPA No. USS14, ‘‘U.S. Experience with sprinklers and other automatic fire extinguishing equipment,’’ February 2010. Rosenberger, M. S.; et al. Glovebox issues: Controlling fire hazards after an earthquake. J. Chem. Health Safety, 2012, 19(4), 1–6. Ezekoye, O. A.; et al. Effects of leakage in simulations of positive pressure ventilation. Fire Technol. 2009, 45, 257–286. McGrattan, K., et al., National Institute of Standards and Technology Special Publication 1019-5, Gaithersburg, MD, (2010). Weinschenk, C.; Ezekoye, O. A. Analysis of thermocouple responses to turbulent radiating environment. Proceeding from AJTEC 2011. March 15, 2011. Cournoyer, M. E.; et al. Minimizing glovebox glove failures. Part III: deriving service lifetimes. Proceeding from WM’06. Tucson, Arizona, February 26–Mar 2, J. Am. Soc. Mech. Eng. 2006.
Journal of Chemical Health & Safety, July/August 2014
Please cite this article in press as: Cournoyer, M.E., Fire modeling of an emerging fire suppression system, J. Chem. Health Safety (2014), doi:http://dx.doi.org/10.1016/j.jchas.2014.05.011
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