The ignition of primary explosives by electric discharges

It has long been known that explosives can be ignited by electric sparks. Compounds such as lead styphnate are particularly sensitive, and electric discharges, arely visible to the naked eye, are capable of igniting them. This is a source of azard in the manufacture and handling of those primary explosives used as itiatory materials, and there is an extensive literature on methods of measurement and on experimental values of the minimum ignition energy of these materials Langevin & Bicquard 1934; Brown, Kusler & Gibson 1946; Morris 1947, 1953; Lathsburg & Schmitz 1949). A review of the literature followed by some experimental determinations, showed that widely varying values of ignition energy ould be obtained for the same substance by using different experimental test methods and conditions. A systematic investigation of this problem by Wyatt, Moore and Sumner at the Explosive Research and Development Establishment, Waltham Abbey, has revealed a number of factors which influence the ignition energy and this paper presents a brief summary of some aspects of their work. There are two types of test apparatus commonly used to measure the ease of ignition of initiator materials by electric sparks; first, a fixed-gap method in which e voltage is applied across two electrodes, one of which is covered by the material sted, the voltage being sufficient to break down the gap and cause a spark to ass; second, an approaching-electrode method in which the gap is initially too ide for a discharge to take place with the voltage applied, but a spark is produced moving one electrode towards the other. The energy dissipated in the discharge varied by changing the applied voltage or the capacity of the condenser used to ore the electrical energy.

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Ignition and flame quenching of quiescent fuel mists

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1978.0201 ◽

1978 ◽

Vol 364 (1717) ◽

pp. 277-294 ◽

Cited By ~ 41

Keyword(s):

Reaction Rates ◽

Ignition Energy ◽

Minimum Ignition Energy ◽

Proposed Model ◽

Theoretical Predictions ◽

Quenching Distance ◽

Experimental Values ◽

Sole Criterion ◽

Fuel Vapour ◽

Level Of Agreement

A model is proposed for the ignition of quiescent multidroplet fuel mists which assumes that chemical reaction rates are infinitely fast, and that the sole criterion for successful ignition is the generation, by the spark, of an adequate concentration of fuel vapour in the ignition zone. From analysis of the relevant heat transfer and evaporation processes involved, expressions are derived for the prediction of quenching distance and minimum ignition energy. Support for the model is demonstrated by a close level of agreement between theoretical predictions of minimum ignition energy and the corresponding experimental values obtained using a specially designed ignition apparatus in which ignition energies are measured for several different fuels, over wide ranges of pressure, mixture composition and mean drop size. The results show that both quenching distance and minimum ignition energy are strongly dependent on droplet size, and are also dependent, but to a lesser extent, on air density, equivalence ratio and fuel volatility. An expression is derived to indicate the range of drop sizes over which the proposed model is valid.

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Investigation of the Spark channel of Electrical Discharges Near the Minimum Ignition Energy

Plasma Physics and Technology Journal ◽

10.14311/ppt.2016.3.116 ◽

2016 ◽

Vol 3 (3) ◽

pp. 116-121 ◽

Cited By ~ 6

Author(s):

S. Essmann ◽

D. Markus ◽

U. Maas

Keyword(s):

Pressure Wave ◽

Discharge Energy ◽

Ignition Energy ◽

Minimum Ignition Energy ◽

Electrical Discharges ◽

One Dimensional ◽

Hot Gas ◽

Experimental Values ◽

Spark Channel ◽

Wave Induced

In this work, we investigate the expansion of the hot gas kernel and pressure wave induced by electrical discharges near the minimum ignition energy experimentally by means of a schlieren setup and numerically through one-dimensional simulations. The effects of discharge energy and energy density on the expansion are discussed. Via comparison of experimental values with numerical simulations, an estimate of the overall losses of the discharge is presented.

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Gaseous combustion in electric discharges.— Part I. The combustion of electrolytic gas in direct current discharges

Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character ◽

10.1098/rspa.1926.0067 ◽

1926 ◽

Vol 111 (757) ◽

pp. 257-280 ◽

Cited By ~ 9

Keyword(s):

Direct Current ◽

Electrical Energy ◽

External Source ◽

Systematic Investigation ◽

The Other ◽

Electric Discharges ◽

Ignition Point ◽

Gaseous Mixtures ◽

Other Hand ◽

Set Up

In connection with the researches upon various aspects of gaseous combustion which are being carried out in the Department of Chemical Technology, Imperial College of Science and Technology, South Kensington, one of us (C. I. F.) has undertaken a systematic investigation of the mechanism of combustion as initiated by, or occurring in, electric discharges. The present paper embodies the results of our experiments upon the combustion of electrolytic gas in direct-current discharges. Three phases have been distinguished in the process of gaseous combustion, namely, (i) a slow (non-self-propellant) combustion below the ignition point, (ii) a self-propellant flame propagation, and (iii) sometimes, also, a short preflame period during which combustion is self-propellant. The point at which he process becomes self-propel ant is sometimes termed the “ignition point.” Before combustion can be determined in it, however, energy in some form or other must be imparted to a given combustible system from an external source. Frequently, heat energy is so supplied, in which case it is customary to speak of an ignition temperature; on the other hand, electrical energy may be supplied, e. g ., by a spark, in which case we speak of a minimum igniting energy or minimum igniting current being necessary to make the combustion self propellant. In such conditions, whilst the self-propellant stage of combustion is usually determined by the purely electrical properties of the discharge employed (even though it may be affected by other influences, such as pressure waves emanating therefrom), yet, having once been set up, it is subsequently propagated through the system independently of such properties. On the other hand, the pre-ignition or non-self-propellant stage is throughout intimately associated with and determined by the discharge itself. Hence, an experimental examination of the influence of electric discharges upon the preignition stage of combustion in gaseous mixtures may be expected to reveal the nature and mechanism of electrical ignition, and indeed of the process of combustion as a whole.

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Correlating minimum ignition criteria

Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences ◽

10.1098/rspa.1994.0153 ◽

1994 ◽

Vol 447 (1931) ◽

pp. 513-526 ◽

Cited By ~ 8

Keyword(s):

Electrical Energy ◽

Laser Wavelength ◽

Ignition Energy ◽

Minimum Ignition Energy ◽

Power Flux ◽

Small Areas ◽

Quenching Distance ◽

Safety Considerations ◽

Short Time ◽

Target Combination

Potential hazards associated with the use of optical fibres carrying laser beams in flammable atmospheres have prompted a series of ignition studies. Investigations are described in which diverse target materials are exposed to radiation from various lasers in a variety of flammable mixtures. A distinctive feature of these systems is that ignition by a laser beam expanding from a fine optical fibre, interacting with particles of varying size and position, can, with minor adjustments to the geometry, give rise to wide variations in irradiated area and duration. This contrasts with the more customary circumstances in which the criterion is either a minimum ignition energy (short time, small dimensions) or an ignition temperature (long times, large dimensions). It is shown that the laser ignition criteria of a minimum power flux at large areas and minimum igniting power for small areas may be interpreted in terms of an igniting energy increasing linearly with time and area, but tending to constant values for dimensions smaller than the quenching distance. A general theory is developed, correlating the various ignition criteria with each other and with fundamental combustion parameters. The hazard may be related to corresponding safety considerations for the dissipation of electrical energy in flammable atmospheres by taking into account the absorptivity/emissivity characteristics of each particular laser wavelength/target combination.

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