ENGR403 Flashcards
Types of detection systems
- aspirating
- thermal detection
- linear beam detector
- photoelectric detector
Heat Transfer mechanisms
- Thermal radiation (waves and light)
- Thermal convection (movement molecules)
- Thermal Conduction (into solid materials})
Thermal Convection
- Thermal transfer between a solid material and in a fluid
- in a fires convection is usually the main heat transfer mechanism during pre-flashover fire
Thermal conduction
- Thermal transfer into soild material
Piloted Ignition
seperate ignition source us involved
autoignition
no pilot involved
more diffucult to achieve
temperature of surface for ignition much higher than piloted ignition
Thermally thin
Temperature increases uniformly over the throughhout the thickness
Thermally Thick q
Temperature profiles throughout the thickness changes considerably
Heat flux units
kW/m^2
Natural flow
flow of air is induced solely by the boundary of the flame
Types of fire spread
Natural Flow: flow of air is induced solely by the bouyancy of the flame
Forced Flow: Flow of air is caused by the ambient wind of a fan
Opposed flow: Refers to the flames spread opposite to the sirflow
Wind-aided Flow: refers to flame spread in the same direction as the airflow
natural flow/oposed flow
preheating region: 1 to 3 mm
fire propogation down and airflow up
Natural flow/Wind aided flow
preheating region: 0.2mm to 2m
air flow up and fire propogation up ^
Forced flow/Opposed Flow
preheating region 1 - 3mm
airflow right anf fire propogation left
Forced Flow.Winded Aidied
preheating region: 0.1 to 10mm
|»_space; fire propogation and air flow
Standard fire curve relation to compartment fire
- Equivalent fire severitiy: the equivalent time of exposure to the standard fire that would produce the same maximum effect in an element protected with insulating materials
e. g. maximum temperature or minimum capacity - this relationship can be used to work out minimum fire resistance rating for a structure exposed to a real fire but constructed with the elements that have been tested with a standard fire
Afflilifaton Theory
- Why do people use that exit and not that exit?
- why do people evacuate in groups?
The Affiliation Thoery, sime 1985, predicts that in a situation of potential entrapment people would move toward familiar persons and places
relationship between speed and density
the closer you are to people the slower you move
as you have more people around you you slow down
- transition from laminar flow to turbulent shuffling
- efficient through opening, high speed -> with an exit you keep focused and go through at a high speed
stairs are the least effieicnt because you are moving upward so hard work, effort moving body mass
Move slower in horizontal because you loose focus going down a corridor. You can’t walk as fast constanlt as you would through an exit
Stairs down, moving downstairs requires less effort then if your moving horizontally however, moving downstairs in high densities is tricky
Relationship between flow and density
Flow is the amount of people you can squeeze through an exit per unit of time
- The fact that we pack more poeple in means we can pack more people through the openeing. Then at a point you get a local maximum this is the most effieicnt possible. And then above that moving so slow so therefore it becomes less efficient \
Effective width
Reducing width when obstacles are encountered. Waist high obstacles don’t need to be reduced for becuase when your walking only your upper body sways. By decreasing the width when an obstacle is encountered it will reduce flow at the obstacle to account for the fact sway reduces density of people when there is an obstacle above the waist
Ionisation Smoke Detector
- Like a battery is has a circuit created with two plates on postive and one negative
- radioactive amarisian 241 (Am-241) added between the plates
- Am 241 sends out alpha particles and hits air molecules, oxygen and nitrogen
- hits an electron of course from 02 and N2
then get a positive ion and a negative electron which then go to oppositely charged plate. Ion collects and electron and becomes nitrogen again - This creates a current
- In fire get soot between two charged plates, soot has static energy
- The free electrons and ions will stick to aerosol particle
- Therefore, current reduces which is reduces below a threshold alarm goes off
Laminar, turbulent and stop go movement
- when density is low movement is very laminar, continuous with no impedence
- as density increases movement becomes more stop go and then eventually turn more turbulent shuffling as density becomes very high
Role Rule Model
The behaviour of a person in a specific situatio is affected by an implied set of rules connected to the everyday role of the person. The idea is that actions taken are related to the role the person has in the occupancy, rather than related to the person. Typical roles can be memebers of staff and their customers which will lead to different behaviour and action of the two groups
Thermal Detector
- Thermometers, threshold for actication of the alarm
- traditionally bi-metals expand differntly it would bend and close the circuit to set off alarm
- now thermocouple linked to a small computer and measures actual temperature of the air
- Activates at 54 degrees
- Computer look at a temperature but also now rate of rise
- measures over a 30 second period to see how much temp has increased in 30s.
- 4 degrees/ min the detector will activate
- Either or will set the detector off
- good in kitchen, nightclub toilet,
places wheresomke is often - can be a point or line detector - usually a wire gets heated
Aspirating detector
- Piping network in a room and then suck the air out and aanalyse it at a detector
- place where there are dust particles can blow it out and will work again
- cultural heritage buildingd more discrete doesnt decrease aesthetics
- Where there is no other good option, where smoke layer doesnt reach the ceiling
- High end detector
- > Photoelectric detector using laser instead of diode
- > Laser focuses at narrow point and has very sensitive single photon sensor, count number of particles going through and count the size
- > set detection range, ignores dust and only detects soot
Fire investigation
Investigation Analysis
- some evidence of specific fire
- use analysis to eliminate or sonfirm events
- Support investigation
For criminal cases, civil cases (insurance), coroner investigation, professional/regulatory - what we learnt
Was the fire deliberate - whats caused ignition?
Was there a failure in fire management process - how did the fire spread?
Was a specific product or material at fault - how did fire safety system perform
was there a failure in design? - how did poeple behave?
Are regulation deficient - how were casualties caused?
how did strucuture perform?
Zone Model
Zone model simpler than feild model divides fire compartment up into a few layers zones such as the plume and hot and smokey gas layer under the ceiling and relies on emperical relationships for the transfer of heat and smoke between zones
- Two homogenous zones hot upper layer and cool lower layer (temperature same through whole layer)
- Uniform Property assumption; emitting momentom and gas tranport effects
- Plume equations describing transport of smoke between lower and upper layer
- Solves for conservation of mass energy and specied missing momentum
upto 10 compartments
relatively short simulation time
Feild model
A feild model divides the compartment up into a large number of cells and redicts a different temprtature for each postion so results can be compared direxctly with the results of fire tests
- solve for conservation of mass momentum energy and species allwoing 3D spatially varying results
navier stokes eqns describe low speed, thermally driven flow
objects simulated as blocks
adv.
- resolve comlpex large geometry
- greater resolution
- more validation
disadv.
- Long simualtion time
- High computation time
- High level or experience
Water as supressant
- water is very effective at fire supression beause is consumes energy to get it to boil -> takes long time to boil
- very effecuve extinguishing medium
mainly cooling due to heast
heating of liquid water
heating of water vapour - effecitveness depends on size of drops-
small drops - cools flame
large drops - cools flame to some extent and reaches base of fire
Class A sprinklers - more for limiting fire
Class A and E water mist
- water mist on boats reduces chance of electrocution cos air betqeen water particles
- limit water weight on a boat
