deflagrations

Deflagration is a rapid combustion process in which a flame front propagates through unburned material at subsonic speeds, driven mainly by heat transfer and mass diffusion from the reaction zone to adjacent reactants. It contrasts with detonation, where the reaction front is driven by a shock wave and travels at supersonic speeds. The term covers flames moving through gases, vapors, and certain solid fuels or powders.

In gaseous mixtures, the flame front advances as the hot combustion products behind the front preheat the unburned gas ahead, initiating further reaction. The propagation speed, or laminar flame speed, depends on the fuel–oxidizer ratio, temperature, pressure, and geometry. Turbulence, obstacles, and confinement can markedly increase the effective flame speed by enhancing mixing and heat transfer. In solids, deflagration refers to combustion that proceeds through the material by heat transfer to adjacent regions, rather than by a shock-driven process.

Hazards arise most clearly in confined or congested spaces where a deflagration can produce significant pressure rise. Under certain conditions, a deflagration can transition to detonation, a phenomenon known as deflagration-to-detonation transition (DDT). This transition is influenced by factors such as confinement, geometrical features, and turbulence, and it is a central safety concern in many industrial contexts.

Deflagrations occur with a variety of fuels, including hydrocarbon-air and hydrogen-air mixtures, as well as combustible dusts such as coal, aluminum, or sugar powders. Understanding deflagration mechanisms and controlling confinement and mixing are important for risk assessment in chemical, mining, and processing industries.