Fire dynamics Flashcards
Andres
Define ‘what is a fire’
A hot gas that releases energy in the form of heat. It requires a fuel and an oxidiser
Define heat release rate
The rate in which heat energy is release from the flame. It is also known as the fire size.
What is the fire tetrahedron
Oxygen, fuel, heat, chemical reaction
What are the four stages of a fire and explain each of them briefly.
Incipient - heat flux provided by ignition source to the fuel. Localised, small fire.
Growth - fire rapidly growing in size and intensity. Compartment begins to fill with smoke and temperature is rapidly increasing.
Burning - entire compartment is in flames, fire at maximum intensity. Fire is consuming all available oxygen and fuel at a high rate. Critical life safety point.
Decay - available fuel and oxygen decreasing. Fire intensity weakens as rate of combustion slows. Extinguishes naturally or by intervention.
Why does the max heat release rate not account for the materials in the compartment?
Flashover has already occurred so the type of material has no effect on the HRR.
Explain fire as a thermochemical reaction.
It is an exothermic reaction (heat energy is released). The energy is transferred to surrounding combustable objects. The reaction can only occur when the mixture of fuel and oxygen is adequate.
It can be stoichiometric (everything that reacts burns) or sooty combustion (C is produced as well as H20 and CO2).
What are examples of stored heat, produced heat, and transferred heat?
Heat of combustion, heat release rate, heat flux.
What are the numbers specific to flashover conditions? At what heat flux do humans feel immediate pain?
T = 600C (smoke layer)
Heat flux = 20-25 kW/m2
Pain at 15 kW/m2
Explain thermal radiation. Which formulas are relevant for it?
Heat is transferred through electromagnetic waves (infrared or visible spectrum).
q = EsigmaT4 (assume E=1), XQ/4piR2, and one for between two surfaces with shape factor
Shape factor accounts for the fraction of energy transferred due to geometric shapes.
Explain thermal convection. Which formula is relevant for it?
Heat is transferred between a solid and a fluid in motion. It is the main transfer mechanism in post flashover fires. Newtons law of cooling - q=h(Tf-Ts). For a compartment, if qc is positive, the heat flux is leaving the compartment and cooling it.
Explain thermal conduction. Which formula is relevant for it?
Heat is transferred into the solid materials, providing the molecules with kinetic energy. These molecules vibrate more, causing the ones next to them to do the same. Results in heating through the solid.
Explain what pyrolysis is
The initial stage of combustion. Thermochemical decomposition of organic material at elevated temps without the presence of O2. The compounds produced are usually flammable (leads to ignition).
Explain autoignition and piloted ignition
Driven by temperature only. Fuel undergoes exothermic reaction that cannot be sufficiently cooled.
External energy is applied by an ignition source e.g match, spark
How do you find q.”_crit for different experiments (graphs)
if q” vs t_ig, use assmptote
if 1/sqrt(t_ig) vs q” use x intercept
What are the pyrolysis and pre-heating regions for surface fire spread?
- where gas fuel is released from the surface to feed the flame
- where heat flux from the flame heats up the material
