Beskriv orden/händelsen Flashcards
Describe from an enclosure perspective: ignition
In the early stages, the enclosure has no effect on the fire, which then is fuel controlled. Besides releasing energy, a variety of toxic and nontoxic gases/solids are produced.
Describe from an enclosure perspective: plume
The hot gases in the flame are surrounded by cold gasa, and the hotter, less dense mass will rise upward du to density difference, or else called buoyancy. The buoyant flow, inclueds any flame, is referred to as a fire plume.
As the hot gases rises, cold air will be entrained into the plume. This mixture of combustion products and air will impinge on the ceiling of the fire compartment and cause a layer of hot gases to be formed.
Describe from an enclosure perspective: Ceiling jet
Then the plume flow impinge on the ceiling, and the gases spread across it as a momentum-driven circular jet. The velocity and temperature of this jet is of importance, since quantitative knowledge of these variables will allow estimates to be made on the response of any smoke or heat detectors (and sprinkler links in the vicinty of the ceiling).
The ceiling jet eventually reaches the walls of the enclosure and is forced to move downward along the wall. The gases in the jet ate still warmer than the surrounding ambinet air, and flow will turn upwards due to buoyancy. A layer of hot gases will thus be formed under the ceiling.
Describe from an enclosure perspective: Gas temperatures
Experiment has shown, for a wide range of compartment fires, that it is reasonable to assume that the room becomes divided into two district layers:
A hot upper layer consisting of a mixture of combustion products and entrained air
A lower layser of cold air consisting of air.
The properties in each layer change with time but are assumed to be uniform throughout each layer.
Describe from an enclosure perspective: The hot layer
The plume continues to entrain air from the lower layser and transport it toward the ceiling. The hot layer therefore grows in volume, and the layer interface descends toward the floor.
Describe from an enclosure perspective: Heat transfer
As the hot layer descends and increases in temperature, the heat transfer processes are augmented. Heat is transferred by radiation and convection from the hot gas layer to the ceiling and walls that are in contact with the gases. heat from the hot layer is also radiated toward the floor and lower walls, some heat is absorbed by the cold layer.
Describe from an enclosure perspective: Vent flows
If there is a opening to the adjacent room or out to the atmosphere, the smoke will flow out through it as the hot layer reaches the top of the opening. Often, the increasing heat in the enclosure will cause the breakage of windows and thereby create a opening.
Describe from an enclosure perspective: Flashover
The fire may continue growing by:
- increased burning rate
- flame spread over the first ignited item
- ignition of the secondary fuel package
The upper hot layer increase in temperature and may become very hot. As a result of radiation from the hot layer towards other combustible material in the enclosure, there may be a stage where all the combustible material is ignited, with a very rapid increase in ERR. The very fast and sudden transition from a growing fire to a fully developed fire is called a flashover
Describe from an enclosure perspective: the fully developed fire
At the fully developed stage, flame extends out through the opening and all the combustible material in the enclosure is involved in the fire. The fully developed fire can burn for a numbers of hours, as long as there is sufficient fuel and oxygen available for combustion.
Describe from an enclosure perspective: Oxygen starvation
In the case of no openings, the hot layer will soon descend towards the flame region and eventually cover the flame. The air entrained into the combustion zone now contains little oxygen and the fire may die due to oxygen starvation.
Even if the ERR decreases, the pyrolysis may continue at a relatively high rate, causing the accumulation of unburn gases in the enclosure. If a window or door opend/cracks at this point, cold air will flow in through its lower part. This may diminish the thermal load in the enclosure, but the fresh air may cause an increase in ERR. The fire may then grow towards a flashover.
Describe from an enclosure perspective: backdraft
The inflowning air may mix up with the unburnt pyrolysis products from oxygen-starved fire. Any ignition source, such as a golwing ember, can be ignited the flamble mixure. This leads to an explosive or very rapid burning of the gases out thought the openings this pheneomenin is called backdraft.
Usually a backdraft will last for a very short time then be followed to flashover sence the unburned material will ignite.
Describe from an enclosure perspective: smoke gas explosion
When unburnt gases from an underventilated fire flow through leakage into an closed spce connected to the fire room, thr gases there can mux very well with air to from a combustible gas mixture. A small spark is then enough to cause a smoke explotion, which can have very serious consequences.This phenomenon is, however, very seldom oberved in enclosure fires
