Ignition Of Flammable Mixtures Research Articles

Effect of the Electrode Material on Ignition of Ethylene–Air Mixtures

This article presents an experimental quantification of the ignition risk of flammable mixtures in conditions relevant to industry and aviation environment. More specifically, the effect of resistive material on ignition of flammable mixtures was investigated by applying a 500-ns duration high-voltage pulse between pin-to-pin electrodes located across a jet flow of ethylene and air, at atmospheric pressure. Fuel-rich, stoichiometric, and fuel-lean equivalence ratios were considered. A parametric study of the effect of the applied voltage and electrode material was performed. The electrical energy deposited in the discharges was determined from current and voltage measurements. Direct visualization of the sparks and the ignition kernel was performed in order to better understand the early stages of the ignition process. The results showed that the use of highly resistive electrodes compared to metallic electrodes: 1) could significantly reduce the energy deposition in the interelectrode area; and 2) could increase the minimum ignition energy. Therefore, the use of highly resistive material wires instead of metallic wires could offer safer conditions with respect to the ignition hazard in flammable environments.

Effects of Particle Size on the Ignition of Static CH4/Air and H2/Air Mixtures by Hot Particles

ABSTRACT Understanding and characterizing the non-homogenous ignition of flammable mixtures by hot particles are important for industry safety. In this study, one-dimensional transient simulations considering detailed chemistry and transport are conducted for the ignition of static methane/air and hydrogen/air mixtures by hot particles with different radii. The objective is to assess the effects of particle size on the ignition delay time for the hot particle-induced ignition process. Unlike the nearly homogenous ignition in shock tubes or rapid compression machines, the non-homogeneous ignition caused by a hot particle depends on not only chemical kinetics but also transport of reactive species. It is found that 1D hot particle ignition is much slower than the 0D homogenous ignition (in which the mixture temperature is the same as the fixed particle temperature) since there is radical loss near the particle surface caused by mass diffusion. The ignition delay time for hot particle ignition is found to strongly depend on the particle radius and temperature. This is interpreted through the change of radical loss with particle radius and temperature. Moreover, to characterize the hot particle ignition, we introduce a Damköhler number which is the ratio between two characteristic times for chemical reaction and mass diffusion. The change of the normalized ignition delay time for 1D hot particle with the Damköhler number is discussed; and a linear relationship is observed for certain conditions.

Optical and laser diagnostic investigation of flame stabilization in a novel, ultra-lean, non-premixed model GT burner

The flame dynamics and stabilization mechanism in a novel, ultra-lean, non-premixed, model GT burner is experimentally investigated with optical and laser diagnostics. The burner operating with methane as fuel exhibits a convergent-divergent flow field and is capable of stabilizing a non-sooty flame at ultra-low global equivalence ratio (ϕglob) down to 0.1. At 1.0 > ϕglob > 0.6, the flame stabilizes in the diverging section where the flow field is characterized predominantly by the swirl. The flame stabilizes at locations of low velocity and low turbulence where the mean flow strain rates are found to be well below the extinctions strain rates for turbulent non-premixed flames. With decreasing ϕglob, the flame is located progressively near the burner lip, where a large recirculation zone of the central bluff body in the burner exists. The transition from swirl stabilized to bluffbody stabilized region occurs between 0.6 > ϕglob > 0.4 and bi-stability of the flame with low-frequency oscillation between the two stabilization locations was observed. The upstream marching of the flame, despite a decreasing ϕglob, and a corresponding decrease in heat input, is explained in terms of the interaction of the flame front with the evolution of the scalar profile upstream, in presence of the recirculating hot gases. The results support flame stabilization theories based on partial premixing and flow modification through heat release upstream of the flame. For the leaner conditions, 0.4 > ϕglob > 0.1, the flame is stabilizing in a region characterized by bluffbody induced toroidal flow structures of high strain rates, where the presence of recirculating hot burned gases aid in flame stabilization by the ignition of flammable mixture.

The contribution of homogeneous reactions in catalytic oxidation processes: safety and selectivity aspects

After a brief summary of the main aspects concerning the formation and ignition of flammable mixtures, attention is given mainly to: (i) the identification of risk location in oxidation processes; (ii) how homogeneous phenomena may affect the products distribution in heterogeneously catalyzed gas-phase oxidations; (iii) how the flow regime in the oxidation reactor can affect the flammability limits of the process gas; (iv) the preventive measures for minimizing selectivity degradation and maximizing safety in oxidation processes.

Large-scale turbulent structures and the stabilization of lifted turbulent jet diffusion flames

The physical mechanisms responsible for the stabilization of lifted axisymmetric turbulent jet diffusion flames remain uncharacterized. Flame stabilization has been shown to take place at radial locations in the corresponding isothermal jets where the flow has an intermittent character. Recently developed flow diagnostics capable of simultaneous multi-point concentration and velocity measurements are used to investigated large-scale turbulent structures in the intermittent region of an isothermal jet of propane. The findings, in conjunction with earlier literature results, show that large-scale structures form as the results of organized turbulent motion during which strong outward ejections of fluid form central regions of the jet flow into the ambient surroundings. Once formed, the large-scale structures are convected downstream for long distances. The downstream edges of the structures are regions of significantly higher concentration gradients and shear than regions further upstream. Occasionally, the jet fluid loses its turbulent energy and is no longer transported by the flow. This fluid can be reentrained by the passage of later large-scale structures. It is concluded that the processes responsible for flame stabilization must occur in these large-scale turbulent structures. Flame extinction is most probable on the downstream edges of the structures while the ignition and stabilization of the flame is likely on the upstream sides. Reentrained fluid provides a possible mechanism for ignition of flammable mixtures on the upstream sides of the structures.

Fuel Systems Problems in High Speed Aircraft

SummaryThe lecture discusses the extent to which the problems met in the fuel systems of subsonic aircraft are aggravated by flight at supersonic speeds and mentions some new problems of considerable severity. These arise from the relatively large fuel quantity required, its high rate of consumption and its heating by kinetic effects or by its use as a heat sink for the aircraft systems. The paper includes a discussion on fuel management and gauging and of the means used to transfer fuel to the engines—showing the relative merits of some of the available pumping techniques. A few comments only are made on the engine control system.The problem of thermal degradation of the fuel in the tanks and in components leading to the engine is discussed, with particular attention being paid to the difficulty of establishing a valid test method for differing aircraft configurations.Spontaneous ignition of flammable mixtures in contact with heated surfaces is a hazard that is likely to arise in a particularly acute form at a Mach number of about 3. The progress of research into this phenomenon is briefly outlined, and some of the measures which may be adopted to ensure safety are indicated. Associated with this is the necessity to develop instruments to detect hazardous conditions in various parts of an aircraft, on which some recommendations are made.