ASHRAE Journal - June 2012 - 40

Ceiling Jet

B
Uniform Smoke Layer Plume Smoke Layer Interface Plume Smoke Exhaust Not Shown Fire Fire

Exhaust

Transition Zone

Atrium Fire

Zone Model Idealization of an Atrium Fire

Figure 3: Sketch of an idealized zone model representation of an atrium fire. predetermined height mentioned previously for the time it takes the occupants to safely evacuate. It also is recommended that unsteady natural venting systems be analyzed with the aid of a CFD model. The considerations about calculation evacuation time for natural smoke filling systems also apply here. into and out of it. In a real fire, the temperature and concentration of contaminants vary throughout the smoke layer with the highest values tending to be near top of the smoke layer. In real fires, there is also a gradual transition zone between the smoke layer and the lower layer as shown in Figure 3A. In a zone fire model, the smoke layer has a uniform temperature and uniform concentrations. This means that the temperature at any place in the smoke layer is the same as everywhere else in the smoke layer, and the same can be said about the concentration of each contaminant. Zone fire models do not simulate the transition zone, but the bottom of the smoke layer is simulated as a horizontal plane called the smoke layer interface as shown in Figure 3B. The zone model considers the air a fraction of an inch (or centimeter) below the smoke layer interface to be as free of smoke as the rest of the lower layer. Occupants in the lower layer near the smoke layer interface will actually be in the transition zone exposed to some smoke. Unfortunately, neither zone fire models nor the algebraic equations of Chapter 16 can be used to evaluate this smoke exposure. It is believed that in many situations, conditions in the transition zone may be tenable. CFD modeling can be used to evaluate tenability at this location. Zone fire models do not simulate the time it takes for the plume to reach the ceiling, which is small in a normal size room but larger in an atrium. Empirical equations for this lag time are discussed later in this Chapter. Zone fire models do not simulate plume flow, but they use empirical equations to calculate plume temperature and the mass flow. Even with the previous limitations, zone fire models have proven to be very useful tools for many applications, but they must be used with care. Chapter 18 has more detailed information about zone fire models.

Methods of Analysis
The methods that can be used for analysis of atrium smoke control systems are algebraic equations, zone fire modeling, CFD modeling and scale modeling.

Algebraic Equations
Atrium smoke control makes use of many algebraic equations. Some of these are based on the fundamental principles of engineering, and others are empirical correlations based on experimental data. Equations for smoke filling, natural venting and the airflow velocity to prevent smoke backflow are discussed later in the chapter. Chapter 16 addresses the algebraic equations for steady mechanical smoke exhaust, and these equations are based on the zone fire model concepts discussed in the next section. In the following section on zone fire modeling, the discussion about smoke exposure in the transition zone also applies to systems designed with the algebraic equations of Chapter 16. When another method of analysis is used, algebraic equations are often used to determine starting points for the analysis.

Zone Fire Modeling
In an atrium fire, smoke flows upward in a plume that entrains air as it rises. When the plume reaches the ceiling, it turns and becomes a ceiling jet that flows under the ceiling (Figure 3A). Figure 3B shows an idealized zone model representation of an atrium fire. Zone fire models are simple models that consider a fire compartment to be divided into two zones: (1) a smoke layer and (2) a lower layer that is free or nearly free of combustion products. The smoke layer can change in size based on the mass flowing
40 ASHRAE Journal

CFD Modeling
CFD consists of dividing a space of interest such as an atrium into a large number of cells, and using a computer program to solve the governing equations for each cell. CFD is
ashrae.org June 2012

ASHRAE Journal - June 2012

Table of Contents for the Digital Edition of ASHRAE Journal - June 2012