Playing with Fire: Computational Fluid Dynamics (CFD) for Fire Protection

by Mike Lehner, P.E.

This article is part of Wood Harbinger’s newsletter series.

Fire is a chemical reaction that can take on a life of its own, growing and spreading wherever it can. It symbolizes different things in our lives, with its ability to provide warmth and light as well as its power to destroy. Its presence can change in an instant from positive to negative. Movies have capitalized on this, to make things more dramatic and, of course, provide another excuse to play with fire (instant action, just add explosion!).

Fortunately in real life, scientists, engineers, and others have dedicated years of study in order to devise more effective ways to harnessing fire’s unpredictable nature. Many equations have been developed that help anticipate and control the behavior of fire and its byproduct, smoke.

Quantifying Fire and Smoke Behavior

It’s no easy task. The number of different applications and scenarios of fire and smoke proliferation can be infinite and also difficult to predict. Through experience with controlled fires and fire testing, these mathematical equations have been verified in some scenarios, but not all. Because smoke behaves similarly to a gas, Computational Fluid Dynamics (CFD) modeling can be used in fire protection applications. It takes the formulas another step and can bring more accuracy to predictive models.

How CFD Modeling Works

CFD uses mathematical formulas and numerical analysis to determine fluid flows. It has numerous applications in engineering, including HVAC, aerospace, hydraulic systems, and water systems. The fire protection application of CFD modeling utilizes CFD to determine fire growth and propagation, smoke movement, and the effects of fire sprinklers.

CFD modeling involves a 3-dimensional model of the building, with a “fire” located inside. CFD breaks down the building and the fire into very small blocks that can be approximated into simple shapes, which fit these formulas. The number of blocks can range into the millions. We are fortunate in the current context that computing power is expanding and allowing us to study more and more complex fire scenarios, which results in a better understanding of fire’s behavior in many building types and space configurations.

CFD Modeling In Action

The value of CFD modeling is evident in many applications, though the tool earns its keep by helping engineers like myself validate smoke control system design. For example, CFD can show how much exhaust and supply air is required to provide a code-compliant, smoke-control system. This helps us make sure we’re on track to deliver solutions that work.

Wood Harbinger is currently part of the design team for the North Satellite Expansion project at Sea-Tac International Airport. This project includes atrium spaces with very high ceilings and challenging smoke- control requirements. We performed several CFD simulations to help the Port of Seattle Fire Department determine what level of exhaust will be needed in the space to assure that egress pathways are clear in the event of a fire. In other words, CDF modeling helped us design this space so that if there is a fire in the terminal, people can get out in the safest way possible.

The following images are screenshots from our CFD modeling efforts. Figure 1 simulates smoke propagating in a space and the HVAC system addressing the problem. As smoke levels increase, visibility decreases. The dark areas represent decreased visibility, and the lighter areas represents where visibility remains good.

CFD modeling - smoke visibility

Figure 1 – Visibility changes as smoke propagates in a space. With an effective smoke control system, high visibility is maintained.

This simulation shows a smoke control method that involves exhausting in the area, with make-up air (outside air) coming from adjacent areas. As the exhaust system pulls the smoke up and out of the space, greater visibility is maintained. The effectiveness of this smoke control method was validated with CFD modeling.

In another scenario, the CFD model shows the air-velocity profile, illustrating the speed and direction of airflows in a space (Figure 2). It shows many directional vector lines with different color levels to illustrate the velocity of the air. Note how the draft curtain at the top is causing an “eddy” to help redirect the flow to the right, where the exhaust fans are located.

CFD modeling - air velocity

Figure 2 – Smoke control strategies like a draft curtain can help move smoke toward an exhaust system. CFD modeling helps test the effectiveness of different strategies.

In comparison to Figure 1 and 2 above, Figure 3 below shows make-up air distributed throughout the room using the typical HVAC layout, which resulted in significantly reduced visibility. The CFD model verified that this method was not effective.

CFD modeling - smoke visibility, system not exhausting

Figure 3 – With typical HVAC system make-up air distribution, visibility decreases rapidly without an effective smoke control system.

Other Uses for CFD Modeling

As you can see, CFD modeling can be a useful tool in smoke control for fire protection applications. CFD is also used to determine the speed and intensity of a fire, depending on the fuel, configuration, and other factors. CFD has many uses throughout the mechanical engineering field. For example, Wood Harbinger has used CFD to help determine ventilation quantities for an indoor diesel exhaust application, with or without an exhaust collection system. We also performed CFD calculations to determine smoke-stack height to ensure that the smoke plume of an energy plant is dissipated correctly.

CFD is a proven and valuable tool in fire protection and mechanical design, helping engineers harness the behavior of unpredictable mediums to create environments that are safe for people and effective for the purposes they serve.

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