Fire development in compartments is often divided into phases depending on the dominant processes at any given stage of development. Ignition is dictated by the characteristics of the fuel item being ignited (i.e., ignition temperature, geometry, orientation, and thermophysical properties3) and the strength of the ignition source. Once the flames are sustained on a burning fuel item, a smoke plume develops, transporting mass and heat vertically as a result of the buoyancy of the smoke (see Figure 1-1). The plume will entrain air as it rises, thereby causing the smoke to cool and become diluted; as a result, the quantity of smoke being transported will increase with increasing elevation. After a smoke plume strikes the ceiling, the smoke travels horizontally under the ceiling in a relatively thin layer, referred to as a ceiling jet. As the ceiling jet travels, the smoke cools with increasing distance from the plume impingement point, in part because of air entrainment into the ceiling jet as well as heat losses from the ceiling jet to the solid ceiling boundary.
3 Thermophysical properties include thermal conductivity, specific heat, and density.
INTRODUCTION
In an ideal situation, once the ceiling jet reaches the enclosing walls, a Hot Gas Layer4 (HGL) develops. As a result of the continuing supply of smoke mass and heat via the plume, the HGL becomes deeper, and its temperature increases. Other properties of the smoke in the HGL also increase (including concentration of gas species and solid particulates).
Radiant heat from the HGL to other combustibles not involved in the fire increases their temperature. Similarly, the temperature of non-burning combustibles will also increase as a result of receiving thermal radiation from the burning item(s). As the other combustibles reach their respective ignition temperatures, they will also ignite. In some cases, the ignition of many other combustibles in the space caused by heating from the HGL occurs within a very short time span. This is commonly referred to as flashover.
Several aspects of fire behavior may be of interest when applying fire models, depending on the purpose of the modeling application. Analysts may seek to determine the effects associated with heating of targets submerged in smoke or receiving radiant heat from the flames, the response of ceiling-mounted detectors or sprinklers to the fire environment, or other phenomena.
Figure 1-1. Characteristics of compartment fires.
4 Hot Gas Layer or HGL is also called “smoke layer” or “hot upper layer” in other publications in fire protection engineering.
INTRODUCTION
The most common aspects of fire behavior that typically are of interest in such analyses include, but are not limited to:
Rate of smoke production
Rate of smoke filling
Properties of the ceiling jet
Properties of the HGL
Target response to incident heat flux via either thermal radiation or convection
A detailed review of each of these aspects is provided in texts on fire dynamics. A brief review of each is provided here.
Rate of smoke production
Smoke is defined as a combination of the gaseous and solid particulates resulting from the combustion process, plus the air that is entrained into the flame and/or smoke plume.
Consequently, the rate of smoke production at a particular height in the plume is the
combination of the generation rate of combustion products and air entrainment rate into the flame and/or smoke plume between the top of the fuel and the height of interest. In most cases, the air entrainment rate greatly exceeds the generation rate of fuel volatiles. Thus, the
correlations used to estimate the rate of smoke production are usually taken from experimental research on entrained air.
Rate of smoke filling
The rate of smoke filling is dependent on the rate of smoke production, the heat release rate (HRR), floor area, height and configuration of the space, and time from ignition. For a fire with a steady HRR, the rate of smoke filling in a compartment will decrease with time due to a
decrease in the smoke production rate, which decreases as the height available to entrain air decreases when the HGL deepens.
Properties of the ceiling jet
The ceiling jet transports smoke and heat horizontally away from the plume after it impacts with the ceiling. The response of ceiling-mounted fire detectors or sprinklers is governed primarily by their interaction with a ceiling jet. The temperature and concentration of smoke in a ceiling jet is principally dependent on the height and configuration of the space, distance to the ceiling impact point of the smoke plume, and the HRR of the fire.
Properties of the HGL
As smoke and heat are transported to the HGL via the smoke and fire plumes, the properties of the HGL will change. The principal properties of interest include the depth, temperature, and gas concentrations in the HGL. The magnitude of the properties depends on the HRR of the fire, geometry of the space, ventilation openings (permitting material from the HGL to leave the space, providing air to the fire, and/or causing a stirring action), yields of combustion products,
INTRODUCTION and the elapsed time after ignition. These changes can be tracked by considering the
conservation of energy, mass, and species relative to the HGL.
Target response to incident heat flux via either thermal radiation or convection
The targets’ temperature will increase as a result of receiving heat via either thermal radiation or convection. Radiation heat transfer is dependent on the intensity of thermal radiation emitted by a source, the size of the source, and the proximity of the target to the source. For this
application, the flame height, the portion of heat released from the fire as radiation, and the distance separating the target from the flame are the dominant parameters. Convective heating occurs whenever the target is submerged in the smoke plume or HGL.