The Official Website of Dr. John H. Klote, PE - Updated June 22, 2015
Smoke is the major killer in building fires, and smoke control
provides significant protection from the threat of smoke.
At one time, zone fire modeling was extensively used for engineering analysis of various applications. These models are computer programs that provide a quick estimate of the smoke flow due to a room fire. Because of advances in computers and numerical methods, most computer fire modeling today is done by computational fluid dynamics (CFD). However, zone fire modeling is addressed here because it still has some useful applications, and it is recognized by NFPA 92 as a method of atrium smoke control analysis. Some zone fire models include routines for calculation of sprinkler activation time which can be of use in analyzing design fires for smoke control systems.
The idea of the zone fire model came from early observations of room fire experiments. In these fires, a smoke plume could be seen rising above the fire. When the plume reached the ceiling, it turned and flowed radially in what was called a ceiling jet. Then a smoke layer formed under the ceiling. There was a gradual transition zone at the bottom of the smoke layer. When smoke flows out of the burn room doorway, it formed what was called a doorjet. It was realized that the plume continued into the smoke layer even though it was not visible through the smoke. Research has shown that: (1) the temperature of the plume decrease with height above the fire, and (2) the mass flow of the plume increase with height above the fire.
Zone fire models consider a fire compartment to consist of an upper smoke layer and a lower nonsmoke layer. In the zone model, the mass flows of the smoke plume and the doorjet are calculated from empirical equations. For the zone model idealization, the temperature and concentrations of constituents are considered to be constant throughout each layer. These properties change only as a function of time. Zone models neglect the ceiling jet and the transition zone. Zone models consider that ceilings are flat and that rooms have uniform cross-sectional areas. The bottom of the smoke layer is considered to be a flat plane. Even with these simplifications, zone fire models were useful tools for many years, but they had to be used with care.
Early zone models had problems with numerical convergence, but today’s zone fire models have superior numerical routines. CFAST is a multiroom zone fire model that was developed at the U.S. National Institute of standards and Technology (NIST). CFAST can be downloaded from NIST at no cost, and it has many features. For this reason, it has become the de facto standard zone fire model in the U.S. For a general discussion of zone fire models with a CFAST example, see Chapter 18 of the Handbook of Smoke Control Engineering.
This sketch shows some complex flows that happen in room fires.
This figure is intended to give an idea of the simple nature of zone fire modeling.
|Scale Model Fire|
|Full Scale Fires|