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Fires and Explosions > Fires 5 > Flashcards

Fires 5 Flashcards

1
Q

Define pyrolysis

A

Decomposition of molecules via heat (oxygen not required)
* if enough heat is applied to any material it will eventually break down

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2
Q

Define a glowing/smouldering combustion

A

No flame, hot materials on the surface of which combustion (oxidation) is occuring
* not a gas phase reaction
* Thermal decomposition of organic substances to produce compounds of lower molecular mass, without using oxygen
* Produces radials
* Products formed can be both flammable and volatile ?????

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3
Q

Define a flame

A

The region where chemical interactions between gases occur, the result being the evolution of heat and light

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4
Q

Key points about pyrolysis

A
  • solids and liquids cannot be on fire
  • only gases produce flames
  • The solid undergoes a pyrolytic decomposition, this then forms a flammable gas, the flammable gas can then burn
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5
Q

What process(es) are happening when a cigarette is burned?

A

Smouldering combustion and pyrolysis
* Have glowing embers which are reacting with air - smouldering combustion
* Paper is degraded and changing colour - pyrolysis

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6
Q

What things do we need for a flame?

A
  • Oxygen, heat and fuel
  • Need the correct ratio of fuel and oxygen
  • Needs to be gaseous
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7
Q

What is the zone called when there is the correct ratio of fuel and oxygen?

A

Combustion zone

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8
Q

Height in related to flames

A
  • The observed height of the flame is variable
  • The ratio of oxygen to fuel usually changes as the distance from the fuel surface changes
  • This is why a candle flame tends to be a little bit above the actual wax, it is not the wax which is burning, it is the vapours produced which ae released as you get further away from that
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9
Q

What is happening when a solid fuel is burning?

A
  • Pyrolysis is happening at the surface because of the heat of the fire
  • This makes really volatile and flammable small radicals which move upwards
  • As these vapours move up they will get far enough away from the fuel, to a point where the ratio of fuel and oxygen is correct and then you will get the flames
  • These vapours will then get gradually oxidised into carbon monoxide and carbon dioxide
  • As these molecules move up, they will draw in new air at the bottom which feeds the newly formed pyrolysis products
  • Will produce fully oxidised (CO2) and partially oxidised (CO, C) products
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10
Q

What is the type of flame that a candle produces?

A

Laminar flame
* idealised type of flame (optimal shape) because it has a very linear structure
* Got loads of area where oxygen can come in
* Most of the soot will form fully oxidised products because this is an idealised combustion

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11
Q

How does a candle burn (flaming combustion)?

A
  • The heat melts the wax to form a liquid
  • This liquid is taken up by the wick and evaporates and while it is still in a gas state, it will set on fire
  • Clear delineation of temperatures
  • Fuel is used up at the top of the flame (fast reaction, plentiful oxygen supply
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12
Q

Why is the temperature closes to the fuel one of the lower ones in a candle?

A

The energy is being taken up by evaporation and there is not as much oxygen in the air, as you get further away from the wick the temperature will increase

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13
Q

How do flames persist in a more forensic setting, when the flames wont be tidy like with a candle?

A
  • Will get turbulent flames, this is where the airflow, which is being drawn in by the heat, is going up and its being drawn in at such a rate that it is no longer even
  • Creates a wall of vapours
  • Only really one area where the oxygen can come in (ratio of surface area to oxygen is less) therefore the distribution of oxygen is uneven (less optimal)
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14
Q

What is a turbulent flame?

A

Where the airflow, which is being drawn in by the heat, is going up, it is being drawn in at such a rate that it is no longer even

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15
Q

What are diffusion flames?

A

The gases or vapours supporting the flame diffuse upwards or outwards from the surface of the fuel, oxygen diffuses towards the fuel from the surrounding air
* Most flames are diffusion flames

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16
Q

What is significant about fires that are produced from a pool of liquid fuel?

A
  • Underneath where the fuel was, the floor will be less badly burnt than the surrounding area, this is because the fire is actually cooler underneath compared to the surrounding areas so the fuel can insulate the floor
  • Fuel source provides a layer of protection to the area underneath it - can spot where a fire has occurs, depends on what material the fuel is on (concreate will absorb a lot of the fuel compared to a glossy floor)
17
Q

Why are flames different colours?

Theoretically

A
  • Flame colour is determined by the wavelength of light emitted which depend on the flame contents
  • Can be used to identify elemental compositions of a flame
  • Hydrocarbons burn blue in plentiful oxygen supply
  • Elements have different colours based on emission
18
Q

Why are flames different colours?

In an actual fire investigation

A
  • Classic yellow flames are from incomplete combustion
  • Soot or other liquid/solid products raised to incadescent temperatures
  • Temperature detemines the colour of the flame
19
Q

What is incandescent temperatures?

A

The things which are making the light in flames are solid or liquid particles which get super hot, and so hot that they emit light

20
Q

Glowing/smouldering combustion

A
  • Surface oxidation (not in gas phase)
  • Absence of flame
  • Presence of very hot materials on surface of which combustion is occuring
  • Occurs in substances that can form a char by pyrolysis
  • Because it doesnt take place in the gas phase, can occur at very low O2 concentration, especially if there is already oxygen in the fuel (carbohydrates like cellulose)
  • Char can slow a fire because it acts as a physical barrier to flaming combustion - prevents oxygen from getting to the fuel source
21
Q

What is a backdraft?

A
  • If a room runs out of oxygen, then the fire will die down to a smouldering combustion, firefighters open the door, they let in a lot of oxygen, and suddenly you get an enormous amount of flames forming
  • Also called ventilation induced flashover
  • Major hazard in firefighting because the flames can rise up quickly
22
Q

Characteristics of a smouldering combustion

A
  • Small associated amount of direct damage associated with them
  • Can produce large amounts of poisonous CO
  • Usually the first and/or the last stage of a fire incident
  • A slow smouldering fire can persist for prolonged periods of time before producing flames
23
Q

The application of heat in a fire

A
  • Every method by which fires are ignited involve application of heat
  • Under the right circumstances, the addition of heat is enough to cause a fire to start (generation of radicals)
  • Heat not only is the driving force but also accelerates them
  • Heat spreads fire
  • Heat causes damage
  • First limiting factor of fires
24
Q

What is the order in which limits a fire?

A
  1. Heat - it is lost rapidly to the surroundings
  2. Oxygen - a closed room will deplete oxygen and result in a smouldering fire
  3. Fuel - until the entire structure is destroyed
25
Q

What are the three routes of heat transfer?

A
  1. Conduction
  2. Convection
  3. Radiation
26
Q

What is conduction?

A

The tranfer of heat through a material by direct atomic or molecular contact
* the heat energy is atomic vibration
* Most important in solids where the atoms are in direct contact with each other

27
Q

How does conduction spread fires through solid material?

A
  • The required activation energy is conducted into adjacent unreacted areas of fuel
  • Materials with a low thermal conductivity (insulators) heat up the most quickly - they will reach the required temperature for pyrolysis (and therefore ignition) quickly
28
Q

How do materials like metal spread fires?

A
  • They have high thermal conductivity - dissipates heat (gets conducted away)
  • Metals can transfer large quantities of heat energy to other locations
29
Q

What influence does specific heat capacity have on fires?

A
  • It is how much energy is required to raise a substances temperature by one degree
  • Materials with low specific heat capacity will get hotter faster and is more likely to start to get to pyrolysis temperatures
  • Also explains why water isnt flammable because it has a very high specific heat capacity
30
Q

What is convection?

A

Transfer of heat in a gas or liquid by the circulation of molecules, which is caused by temperature differences

31
Q

How does convection affect the spread of fire?

A
  • Regions of high temperature are less dense, and therefore rise upwards
  • Atoms have more relative motion, therefore need to spread out more because they bump into each other more
  • This makes them less dense and they tend to go to the top of the room
  • Once you’ve got that movement of gases this will set up currents which will make the heat dissipate around the room, which can heat other things in the room
  • These currents also draw more oxygen into the base of the fire, increasing ventilation
  • Because of convection walls and ceilings might ignite because the gases cause them to become hot enough
32
Q

What is radiation?

A

The emission of heat as electromagnetic radiation (infra-red region)
* Doesnt involve the transfer of heat through molecules
* The intensity of raditation increases rapidly as the temperature get hotter
* It falls off rapidly as you get away from the source

33
Q

How does radiation affect the spread of fire?

A
  • Massive role in the spread of fire
  • Can start fires nearby just by radiating the heat of one flame onto it
34
Q

What is flameover?

A

When flames spread horizontally at great speed

35
Q

What is radiation iduced flashover?

A

Downward spread of fire because the flames are at the top and they radiate downwards
* Always happens after flashover

35
Q

What is direct flame impingement?

A
  • When you hold one flame against something and that thing sets alight
  • Combination of both convective and radiative mechanisms
  • Plume of hot gases rises by buoyant flow and transfers heat to new fuels by convective transfer and radiation
  • New fuel pyrolyzes and generates flammable gases, which ar eignited by flames of plume
36
Q

What are the different colours produced from temperatures bands in hydrocarbon fires?

A

dull red (first visible glow) = 500-600
dull red = 600-800
bright cherry red = 800-1000
orange = 1000-1200
bright yellow = 1200-1400
white = 1400-1600

Fires and Explosions (20 decks)

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