Evaluation of Structural Ventilation Techniques
The objective of this USFA/National Institute of Standards and Technology (NIST) research partnership was to improve firefighter safety by enabling a better understanding of structural ventilation techniques, including positive pressure venting (PPV) and natural venting. In addition, by examining structural fire ventilation using full-scale fire experiments with and without PPV using the NIST Fire Dynamics Simulator (FDS), a technical basis for improved training in the effects of ventilation on fire behavior was provided.
Characterizing Positive Pressure Ventilation using Computational Fluid Dynamics
Full-scale experiments were conducted to characterize a Positive Pressure Ventilation (PPV) fan, in terms of velocity. Experiments were performed in an open atmosphere and in a simple room geometry. The results of the experiments were compared with Fire Dynamic Simulator (FDS) output. The measurements of both sets of experiments compare favorably with the FDS model results. With the correct geometry, vent placement, and boundary location FDS predicted velocities that were within 10 percent for the open atmosphere and 20 percent for the simple room geometry. The Smokeview visualization of the FDS results of the PPV fan's flow pattern, and the flow out of the window also correlated well with those measured experimentally.
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The photos above show the comparison between the visualization of the PPV fan flow on the left and the Smokeview visualization of the simulated PPV flow pattern on the right.
Effect of Positive Pressure Ventilation on a Room Fire
Fire departments may use ventilation blowers or fans to pressurize a structure prior to suppressing a fire. This pressurization or positive pressure ventilation (PPV) tactic can assist in the venting of smoke and high temperature combustion products and make attacking the fire easier than without PPV. However, this tactic also provides additional oxygen to the fire and can increase the rate of heat and energy being released. PPV has not been characterized carefully enough to establish specific guidelines for optimum use.
This study examined gas temperatures, gas velocities and total heat release rate in a series of fires in a furnished room. The use of the PPV fan created slightly lower gas temperatures in the fire room and significantly lower gas temperatures in the adjacent corridor. The gas velocities at the window plane were much higher in the PPV case than in the naturally ventilated scenario. This higher velocity improved visibility significantly. PPV caused an increase in heat release rate for 200 seconds following initiation of ventilation but the heat release rate then declined at a faster rate than that of the naturally ventilated experiment.
Photographs of the burn room doorway on right with PPV and the flames exiting the burn room window during the same experiment on the left.
Photographs of the burn room doorway on right with natural ventilation and the flames exiting the burn room window during the same experiment on the left.
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Effect of Positive Pressure Ventilation on a Room Fire (PDF, 1.7 Mb)
Evaluation of the Ability of Fire Dynamics Simulator to Stimulate Positive Pressure Ventilation in the Laboratory and Practical Scenarios
This report, which incorporates information from Characterizing Positive Pressure Ventilation using Computational Fluid Dynamics and Effect of Positive Pressure Ventilation on a Room Fire reports, also compares data from three full-scale Positive Pressure Ventilation (PPV) experiments with simulations completed using the Fire Dynamics Simulator (FDS). All experiments qualify and quantify the comparison of the experimental results with the FDS results. A concluding scenario is modeled utilizing the calibration of the full-scale experiments to examine the effects of PPV on a fire in a two-story, colonial style house.
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Full-Scale Evaluation of Positive Pressure Ventilation in a Firefighter Training Building
A series of full-scale experiments was conducted in a three-story firefighter training burn building to compare natural ventilation with positive pressure ventilation (PPV). A wood pallet and dry hay fire were allowed to burn in the structure with all doors and windows closed until the fire reached an oxygen-limited state. A door and window were then opened. The structure was ventilated naturally or with a positive pressure fan placed at the front door. Fourteen different configurations of fire room and vent locations were examined, each with both natural and positive pressure ventilation. Gas temperatures, air velocities, fire room oxygen concentrations and differential pressures were recorded and compared for the different configurations and ventilation techniques.
The data indicate that, with both natural and positive pressure ventilation techniques, using correct ventilation scenarios resulted in lower temperatures within the structure at the 0.61 m (2 ft) height, where victims may have been located, and at the 1.22 m (4 ft) height, where firefighters may have been operating. There were only limited ventilation configurations where the temperatures in rooms other than the fire room exceeded the victim or firefighter threshold temperatures with either ventilation technique. The use of positive pressure ventilation resulted in visibility improving more rapidly and, in many cases, cooled rooms surrounding the fire room. However, the use of positive pressure ventilation also caused the fire to grow more quickly, and in some cases, created higher temperatures at the lower elevations within the structure. Overall, this limited series of experiments suggests that PPV can assist in making the environment in the structure more conducive for firefighting operations.
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Evaluating Positive Pressure Ventilation in Large Structures: School Pressure and Fire Experiments
Related to the USFA/NIST sponsored PPV research, the Department of Homeland Security’s Science and Technology Directorate sponsored, along with NIST, a series of experiments run in a masonry educational building examining the ability of fire service PPV fans to limit smoke spread or to remove smoke from areas where potential occupants may be located. The PPV fans are able to accomplish this by creating pressures higher than that of the fire to manage where the smoke and hot gases flowed in the building. Preliminary experiments examined the pressure increase created by portable fans and mounted fans in different configurations and locations. The two main fire scenarios included a long hallway with classrooms and a gymnasium. Both scenarios included fires that produced a large amount of smoke and hot gases, and instrumentation was placed to assess tenability criteria and how PPV tactics can either increase or decrease tenability. Measurements included temperature, pressure, thermal imaging, and video views. In the limited series of experiments in the long hallways of this masonry educational building, the use of PPV to increase pressure to reduce temperatures, limit smoke spread, and increase visibility was effective. This series of experiments demonstrated that fire service PPV fans can be used successfully in large structures to increase tenability of potential victims and improve conditions for firefighting crews.
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NIST Fire Dynamics Simulator (FDS) and Smokeview
FDS is a computational fluid dynamics (CFD) model of fire-driven fluid flow. The software solves numerically a form of the Navier-Stokes equations appropriate for lowspeed, thermally-driven flow with an emphasis on smoke and heat transport from fires. Smokeview is a visualization program that is used to display the results of an FDS simulation. Further information on this may be found on the NIST Web site.
Last Reviewed: August 6, 2008
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