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Gaseous Explosions

When two or more gases interact with ever increasing velocity until a high maximum speed is attained, a Gaseous Explosions is said to result. The velocity is many thousand times greater than of the slow, uniform propagation of flame dealt with in the previous section, and its accurate determination is a problem of considerable experimental difficulty.

In 1880 an explosion of coal gas occurred in Tottenham Court Road in London, and during the legal investigations subsequent thereto, the attention of scientists was directed to the fact that practically nothing was known of the rate at which an explosion-wave could travel. The following year Mallard and Le Chatelier gave the results of an investigation on the subject carried out by themselves, and this was followed in 1882 by the memoirs of Berthelot and Vieille. In 1893 Dixon reopened the question, and as his researches were carried out with such consummate skill and yielded results so concordant in their values, brief reference may here be made to his method of experiment. The explosion tubes consisted of leaden pipes ranging in length from 55 to 100 metres, and in diameter from 8 to 13 mm. As no appreciable difference could be detected in the velocity of the explosion-wave through the tubes when lying straight on the floor and when coiled on a drum about 2 feet in diameter, coiled tubes were used most frequently as their temperature admitted of easy control by immersion in a thermostat. It was found impossible to coil a small leaden pipe without stretching it somewhat - about 2 or 3 cm. The outside of the tube was therefore measured after each coil was wound on the drum, and the length of the axis of the pipe calculated.

velocity of explosion-wave
Dixon's apparatus for determining the velocity of an explosion-wave (1893).
Each end of the leaden tube was connected with a short, wider tube carrying a bridge of silver foil W1 W2 (fig.), and one of the tubes was fitted with a platinum spark gap S. The explosive mixture was admitted in a thoroughly dry condition - unless otherwise stated – the pressure determined, and the spark passed. The explosion-wave ruptured the silver-foil bridge W1 passed through the leaden coil and, upon emerging at the other end, ruptured W2. As these bridges were connected electrically with a chronometer, the time-interval between the ruptures was recorded, and from this the velocity of the explosion- wave was readily calculated.

The foregoing researches have sufficed to establish the following facts:
  1. Both the composition and the diameter of the explosion tube are immaterial, provided the latter is above a certain minimum - about 5 mm.
  2. The velocity of the explosion-wave after the passage of the spark increases rapidly until a high maximum is reached, after which it remains constant.

    The results obtained by Berthelot and by Dixon for several typical gaseous mixtures are given in the following table. They exhibit a remarkably close agreement:

    Velocity of Gaseous Explosions at room temperature

    Gaseous Mixture.Berthelot (m/c).Dixon (m/c).
    2H2 + O228102821
    H2 + N2O22842305
    CH4 + 2O222872322
    C2H4 + 3O222102364
    2C2H2 + 5O224822391
    C2N2 + 2O221952321
  3. Although Berthelot concluded that the velocity of the explosion- wave is independent of the initial pressure of the gases, Dixon found that this is approximately true only after a certain minimum pressure - about 1½ atmospheres - has been exceeded. In other cases the explosion velocity rises with the pressure. This is abundantly evident from the following data:

    Influence of pressure on the explosion velocity
    Gaseous mixture - 2H2 + O2.
    Velocity - metres per second.

    Pressure, mm.Velocity at 10° C.Pressure, mm.Velocity at 100° C.
    2002627. . .. . .

    Reduction of pressure reduces the intensity of explosion, and for each gaseous mixture there appears to be a critical pressure, below which explosions will not take place. This pressure is a function of the chemical composition and proportions of the gases, the moisture content and the initial spark impulse. The completeness of combustion likewise falls with the pressure. Thus, for example, in one series of experiments with mixtures of methane and air, it was observed that under a pressure of 40 mm. of mercury, 6 per cent, of the gas combined, whereas under 61 mm. 30 per cent, combined.
  4. The foregoing table also illustrates the retarding influence of rise of temperature upon the velocity of explosion, and similar effects were observed with other gases.
  5. The influence of wrater-vapour upon the rate of explosion of oxygen and carbon monoxide was studied with interesting results. The maximum velocity was attained when the mixture was saturated at 35° C., i.e. it contained some 5.6 per cent, of water-vapour. Further addition of steam is seen to retard the reaction.

    Influence of water-vapour upon the velocity of explosion

    Gaseous Mixture.Water-Vapour. Per cent.Velocity. Metres per Second.
    Dried with P2O5. . .1264
    Moderately dry. . .1305
    Saturated with water-vapourAt 10° C1.21676
    20° C2.31703
    28° C3.71713
    35° C5.61738
    45° C9.51693
    55° C15.61666
    65° C24.91526
    75° C38.41266
  6. Addition of an inert gas to an explosive mixture usually results in a retardation of the explosion-wave. If one of the combustible gases is in excess, it is liable to behave like an inert gas of similar volume and density. This is evident from a consideration of the data in the following table:

    Influence of Inert gases upon the velocity of Explosion at ordinary temperature

    Gaseous MixtureVelocity. Metres/Sec.Gaseous MixtureVelocity. Metres/Sec.Gaseous MixtureVelocity. Metres/Sec.Gaseous MixtureVelocity. Metres/Sec.
    2H2 + O228212H2 + O22821C2H4 + 2O22581C2H4 + 2O22581
    2H2 + O2 + O223282H2 + O2 + N22426C2H4 + 2O2 + O22368C2H4 + 2O2 + N22413
    2H2 + O2 + 3O219272H2 + O2 + 3N22055C2H4 + 2O2 + 2O22247C2H4 + 2O2 + 2N22211
    2H2 + O2 + 5O217072H2 + O2 + 5N21822C2H4 + 2O2 + 4O22118C2H4 + 2O2 + 4N22024
    2H2 + O2 + 7O212812H2 + O2 + 7N2noneC2H4 + 2O2 + 6O21980C2H4 + 2O2 + 6N21878
    C2H4 + 2O2 + 8O21856C2H4 + 2O2 + 8N21734

    Excess of hydrogen, on the other hand, actually accelerates the explosion velocity unless present in too great a quantity. This is evident from the following data:

    Rate of explosion of electrolytic gas with excess of Hydrogen

    Gaseous mixture2H2 + O22H2 + O2 + 2H22H2 + O2 + 4H22H2 + O2 + 6H2
    Explosion velocity, metres/sec2821326835273532

    A similar acceleration has been observed on addition of excess of hydrogen to mixtures of nitrous oxide and hydrogen.
  7. Finally, it has been established that in the explosion-wave, the combustion of electrolytic gas is not complete. In the explosion of carbon monoxide and oxygen a residuum of unburned gas is likewise found.

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