Deflagration

Deflagration (Lat: de + flagrare, 'to burn down') is subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it. Most 'fires' found in daily life, from flames to explosions such as that of black powder, are deflagrations. This differs from detonation, which propagates supersonically through shock waves, decomposing a substance extremely quickly. Deflagration (Lat: de + flagrare, 'to burn down') is subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it. Most 'fires' found in daily life, from flames to explosions such as that of black powder, are deflagrations. This differs from detonation, which propagates supersonically through shock waves, decomposing a substance extremely quickly. In engineering applications, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a firearm, or a piston in an internal combustion engine) with the force of the expanding gas. Typical examples of deflagrations are the combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine, and the rapid burning of gunpowder in a firearm or of pyrotechnic mixtures in fireworks.Deflagration systems and products can also be used in mining, demolition and stone quarrying via gas pressure blasting as a beneficial alternative to high explosives. Adding water to a burning hydrocarbon such as oil or wax produces a deflagration. The water boils rapidly and ejects the burning material as a fine spray of droplets. A deflagration then occurs as the fine mist of oil ignites and burns extremely rapidly. These are particularly common in chip pan fires, which are responsible for one in five household fires in Britain. The underlying flame physics can be understood with the help of an idealized model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel, separated by a thin transitional region of width δ {displaystyle delta ;} in which the burning occurs. The burning region is commonly referred to as the flame or flame front. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning. Two characteristic timescales are important here. The first is the thermal diffusion timescale τ d {displaystyle au _{d};} , which is approximately equal to where κ {displaystyle kappa ;} is the thermal diffusivity. The second is the burning timescale τ b {displaystyle au _{b}} that strongly decreases with temperature, typically as where Δ U {displaystyle Delta U;} is the activation barrier for the burning reaction and T f {displaystyle T_{f};} is the temperature developed as the result of burning; the value of this so-called 'flame temperature' can be determined from the laws of thermodynamics. For a stationary moving deflagration front, these two timescales must be equal: the heat generated by burning is equal to the heat carried away by heat transfer. This makes it possible to calculate the characteristic width δ {displaystyle delta ;} of the flame front: