Deflagraation

Deflagration, sometimes misspelled deflagraation, is a mode of combustion in which a flame front propagates through a flammable gas, vapor, or dust mixture at subsonic speeds relative to the local sound speed in the unburned mixture. The flame advances mainly by heat transfer and molecular diffusion, heating the adjacent mixture until it ignites. The pressure rise across a deflagration is typically modest compared with a detonation.

Deflagration contrasts with detonation, where a shock wave compresses and heats the mixture, causing an almost instantaneous reaction and a supersonic front. In deflagration, the reaction zone is driven by thermal conduction and diffusion rather than by a shock.

Two main regimes are recognized: laminar deflagration, with a smooth, stable flame front, and turbulent deflagration, where turbulence increases the burning rate and can accelerate the flame. The transition from laminar to turbulent burning depends on mixture properties, geometry, and flow conditions.

A hazard in many settings is the possibility of deflagration-to-detonation transition (DDT) in confined or obstructed spaces, where turbulence and reflections can drive the flame to much higher speeds and pressures. Prevention and safety measures include controlling confinement, avoiding large pockets of fuel–air mixtures, ensuring proper ventilation, controlling ignition sources, and, where appropriate, inerting or diluting the mixture.

Deflagration is the typical combustion mode in many practical engines and burners, where subsonic flame propagation enables controlled energy release.