Faster Flame Propagation Research Articles

An experiment of the combustion characteristics with laser-induced spark ignition

The combustion characteristics and minimum ignition energies using laser-induced spark ignition were demonstrated for quiescent methane-air mixtures in an optically-accessible, constant volume combustion chamber. Initial pressure and equivalence ratio as well as spark energy were varied in order to explore the flame behavior with laser-induced spark ignition. Shadowgraphs for the early stages of combustion process showed that the flame kernel becomes separated into two, one of which grows back towards the laser source. Eventually after a short period, the two flame kernels developed into two flame fronts propagating individually, which is unique in laser-induced spark ignition. For a given mixture, lower initial mixture pressure and higher spark energy resulted in shorter flame initiation period and faster flame propagation. The results of minimum ignition energies for laser ignition shows higher values than electric discharge results, however, the difference decreases toward lean and rich flammability limits.

A study of plasma jet ignition mechanisms

Having demonstrated the effectiveness of high-velocity jets of chemically active plasmas for igniting and promoting fast flame propagation in lean fuel-air mixtures, the contributory mechanisms are studied with the object of designing improved ignition plugs. Incendivity and flame spread are measured in apparatus provided with time-resolved Schlieren, C 2 emission and OH absorption measuring facilities. Theoretical and experimental studies indicate major contributions from chemical and fluid-mechanical effects. The former are prominent when the plasma medium is rich in H, and appear absent with Ar plasmas. When dominant, they enable sub-limit mixtures to be ignited and burnt, lead to rapid transition from an expanding plasma plume to a propagating flame and give rise to considerably increased burning velocities, persisting for appreciable times, during which the flame structure appears to be modified. The fluid mechanics determines the structure and turbulence of the jet and hence the initial location and surface area of the flame kernel. The increased flame area leads to increased flame speeds and conversion rates, even in the absence of chemically modified burning velocities. When propagating into quiescent reactants, the flame convolutions tend gradually to laminarise at a rate which increases with laminar burning velocity. The magnitude and persistence of fluid-mechanical effects thus depends on the chemical effectiveness of the plasma medium through both the formation and the rate of propagation of the flame. The two effects are complementary in that flame convolutions are most effective in chemically inactive plasmas. Either effect can be fostered by plug design. Liquid hydrocarbon fuelled plugs combine a plentiful supply of radicals with incompressibility of feedstock and suitability for practical applications.

Transition from Slow Burning to Detonation in Gaseous Explosives

High-speed photographic observations have been made of flame structure, flame velocity, and shock velocity during the transition from slow burning to detonation in acetylene-oxygen mixtures contained in glass pipe at reduced pressures. During intermediate stages the dome-shaped flame travels down the tube at about 800 m/sec and is preceded by a shock wave traveling slightly faster. During the final stages of formation of the detonation wave, the flame changes shape, accelerates, and advances along the walls in an asymmetric manner. The detonation first appears at, or slightly behind, the leading edge of the flame. The final flame acceleration occurs when the Reynolds number of the mass flow between the flame and the preceding shock, based upon the distance traveled by a mass element in the time interval between passage of the shock and the flame through it, is about 107. Under these conditions the development of turbulent regions in the boundary-layer flow ahead of flame appears likely. Changes in flame configuration which result in the final acceleration and the formation of the detonation wave then arise from the faster flame propagation into these turbulent areas than into the free-stream region.