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Smoke Modeling with CFD
CFD simulations of flow and thermal fields surrounding a smoke source, provide a visualization of smoke egress that can be leveraged to guide the design of smoke control systems required to meet smoke and safety regulations.
This article reviews building design considerations for smoke control systems that are enhanced by CFD smoke modeling.
Physically testing the operation of a smoke control system prior to its final installation is often impractical; so mitigating smoke must be done right the first time!
CFD simulations of smoke egress in a virtual environment, as opposed to physically tracing smoke, allows system performance to be evaluated against regulations and "what if" scenarios by visualizing smoke plooms, egress, and visibility.
CODES, STANDARDS, AND REGULATIONS
While specific codes/standards depend on site location, smoke management systems are required by the Internal Building Code (IBC) for atriums, hospitals, detention facilities, underground/windowless facilities, warehouses, high-rise stairs, and smoke-protected assembly seating to name a few. The IBC even requires submission of smoke analyses (Section 909.4) to support the smoke control system and its method of operation.
The Nation Fire Protection Association (NFPA) provides guidelines for design and analysis techniques (92, 92A, 92B, 96, 97, 99, 99B) along with the NFPA Life Safety Code 8.6.7:
“For other than existing, previously approved atriums, an engineering analysis is performed that demonstrates that the building is designed to keep the smoke layer interface above the highest unprotected opening to adjoining spaces, or 6ft above the highest floor level of exit access open to the atrium, for a period equal to 1.5x the calculated egress time or 20 minutes, whichever is greater.”
The meteorological definition of visibility is: “The max distance at which a standard object can be seen and identified with the unaided eye”
While the National Research Council (NRC) defines visibility as: “The furthest distance at which an object can be perceived”
By any definition, smoke is not good for visibility. Below, images of a space before and after smoke impacts visibility are shown alongside a simulation image of smoke beginning to obscure an exit sign.
Left and center images are from M.J. Ouellette , “Visibility of Exit Signs,” National Research Council Canada. On the right, a Sim Specialists LLC. CFD model predicts smoke moving from the neighboring space.
CFD relates the distance of visibility to a mass concentration of particulate and an extinction coefficient. The National Institute of Standards and Technology (NIST) has published several papers, two (2) of those great resources are found here: Smoke Production and Properties and Smoke Extinction Coefficient of Flame Generated smoke.
In CFD, visibility can be shown on planes, surfaces, traced throughout the model, and extracted for plots to guide design decisions.
Above, a smoke source in a neighboring space causes visibility to decrease in the adjoining hallway.
Below, a CFD model predicts the concentration and egress of smoke caused from a subway train fire. The visual insight and analysis data leads to ventilation sizing and placement modifications to reduce smoke egress and the impact of smoke on visibility.
Smoke is a major building design consideration regulated for safety. Understanding smoke egress and system performance compared to standards is often impractical with physical testing.
Smoke modeling with CFD provides insight that drives design decisions and increases confidence in a smoke control systems ability to perform.