Combustion

Fire triangle

fire tetrahedron

1.        Fuel – material that burns

2.        oxygen

3.        Heat – ignition source

4.        Chemical chain reaction – sustains combustion

Open vs closed systems

·        Open system:

o   Oxygen is freely available from the air.

o   Pressure doesn’t build up significantly.

o   Common in fires and deflagrations (e.g., burning wood, gasoline fire).

·        Closed system:

o   Limited oxygen supply.

o   Pressure can rise rapidly.

o   Common in confined explosions or detonations (e.g., bomb casing).

Deflagration definition:

·        Rapid combustion where the flame front moves slower than the speed of sound

detonation definition:

Extremely rapid combustion where the reaction front moves faster than the speed of sound

Analytical scheme for deflagration

-            Collect → Extract/Desorb → Analyze → Interpret

o   Collect: debris + ACS (activated carbon strips)

o   Extract/Desorb: solvent extraction, SPME(solid phase micro extraction), thermal desorption

o   Analyze: GC‑MS

o   Interpret: match to accelerant patterns

-            Note: it is important to understand the background because the polymeric sample may look like an ignitable liquid

Physical evidence from detonation (shrapnel)

pieces of casing, metal, or objects propelled by the blast