Deflagration

Deflagration is a mode of combustion in which a flame front propagates through a gas or solid at subsonic speeds, driven mainly by heat transfer and mass diffusion from the burned to the unburned material. In contrast to detonation, deflagration occurs with the burning rate below the local sound speed in the unburned mixture, and pressure changes are relatively gradual. As a result, the flame is sustained by thermal conductivity and species diffusion rather than a shock wave.

In gaseous mixtures, the flame front advances as heat and reactive species diffuse into the unburned region, igniting it at the flame surface. The laminar burning velocity depends on the fuel–oxidizer pair, initial temperature and pressure, and mixture composition; turbulent motion and confinement can greatly increase the effective burning rate. In solids, deflagration refers to a slow-to-moderate-burning front that propagates by heat transfer to adjacent material, often accompanied by surface gasification or pyrolysis.

Deflagration is common in everyday combustion, such as flames from candles, stoves, and engines, and is a central concern in safety engineering for dusts, gases, and porous materials. It contrasts with detonation, in which a Chapman–Jouguet or Zeldovich–von Neumann–Döring wave propagates the reaction at supersonic speed behind a shock. In energetic materials and confined systems, deflagration can sometimes transition to detonation (deflagration-to-detonation transition), posing particular risks and requiring mitigation.