Chapter 6 Flashcards
During fires within a compartment, the characteristics of the initial fuel package, as well as all other fuels present, will influence the rate of fire spread and growth within the space.
6
During the development of an incipient fire, the rate of flame spread, and heat release rate is greatly dependent on the configuration and characteristics of the fuels involved.
6
As radiant heat from the fire warms nearby fuels, it continues the progress of further pyrolysis allowing the flames to continue to spread and involve more fuel surfaces causing the fire’s HRR to increase as the fire moves into the growth stage.
6
As the fire burns, the gaseous products of combustion move upwards due to differences in temperature, density, and pressure between the room temperature air and the gases generated and heated by the fire, creating a thermal/plume.
6
When the plume reaches the ceiling, the flow is diverted horizontally under the ceiling as a ceiling jet and flows in all directions until the gases strike the wall of the compartment.
6
The rate of air entrainment to the fire is influenced by the rate of outflowing gases.
6
The height at which the flow changes direction is known as the neutral plane.
6
As the fire continues to grow, the ceiling layer gas temperature and the intensity of the radiation on the exposed combustible contents in the room increases.
6
Flame over, which describes the condition where flames propagate through or across the ceiling layer only and do not involve the surfaces of target fuels, may be present.
6
The high radiant heat flux present causes the surface temperature of the combustible fuels within the compartment to rise, and pyrolysis gases produced.
6
When the hot gas layer temperature reaches approximately 590*C, a heat flux from the hot gas layer of approximately 20 kW/m2 at floor level is often present.
6
Flashover, which is a rapid transition of a growth phase fire to a fully developed fire, is a dangerous phenomenon and has claimed the lives of countless firefighters.
6
Time to flashover from ignition was as little as 3 to 5 minutes.
6
Flashover may occur multiple times in a structure as the fire progresses from one area to another with each event having a potential to impact other compartments.
6
In a fully developed fire, the air flow into the compartment is not sufficient to burn all of the combustibles being pyrolyzed by the fire, and the fire will shift from fuel limited to ventilation limited where the HRR is limited by the amount of oxygen available.
6
Although pyrolysis can continue throughout the compartment, flaming combustion will only occur where there is sufficient oxygen present.
6
Fully developed fires are ventilation limited.
6
During the development of an incipient fire, the rate of flame spread and HRR is greatly dependent on the configuration and characteristics of the fuels involved.
6
As radiant heat from the fire warms nearby fuels, it continues the progress of further pyrolysis, allowing the flames to continue to spread and involve more fuel surfaces, causing the fire’s HRR to increase as the fire moves into the growth stage.
6
As the fire burns, the gaseous products of combustion move upwards due to differences in temperature, density, and pressure between the room temperature air and the gases generated and heated by the fire, creating a fire plume.
6
As the fire compartment’s oxygen concentration decreases below what is needed for combustion, the fire will go into early decay.
6
As the fire becomes re-established with oxygen, flashover and full development are now possible.
6
As a fire develops within a compartment that is interconnected to other spaces in the structure, fluid flows develop due to the pressure differentials created by the fire.
6
As the fire gases move out of their original compartment, they will transfer thermal energy through conduction, convection, and radiation at a rate that is influenced by many factors, including, but not limited to, temperature and velocity.
6
Hot gas flows have claimed the lives of many fire fighters and can be extremely dangerous when crews are positioned in the exhaust portion of the flow path, which is between the fire and the exhaust vent.
6
Exhaust portions of the flow path exist above the neutral plane and allow fire gases to exit the structure.
6
Intake portions of the flow path exist below the neutral plane and allow fresh air into the structure.
6
The safest position from which to mount an interior fire control is to place firefighters on the intake side of the flow path.
6
A bidirectional flow can often be seen at an opening positioned at the same level as the fire.
6
A unidirectional flow can often be seen when two openings exist at different elevations allowing the whole inlet or exhaust openings to be positioned completely below or above the neutral plane.
6
The most dangerous place for firefighters to be located is in the exhaust portion of the flow path.
6
Exhaust portions of the flow path have had gas speeds recorded up to 20mph.
6
Modern firefighter PPE can only protect firefighters a few seconds in high-temperature and high-velocity flows.
6
Dynamic flow is a condition where unidirectional or bidirectional flow of smoke/air presents irregular stratification and shape or alternates in direction.
6
A unidirectional or bidirectional flow of smoke/air that presents irregular stratification and shape or alternates in direction is identified as dynamic flow.
6
Dynamic flows may be caused by oscillations in the combustion cycle or as the result of being impacted by wind.
6/w
Wind speeds as low as 9mph can have a significant impact on the flow behavior of the fire gases and increase the risk of fire extension and threat to human life.
6
Under the right conditions, the fuel to air mixture in the vicinity of the smoldering source can become ignited, and a smoke explosion occurs as a deflagration moves quickly through the premixed flammable mixture.
6
With the recent addition of rigid foam board to exteriors of some buildings to increase the insulative performance, significantly faster flame spread has been observed.
6
