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.

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 + O2	2810	2821
   H2 + N2O	2284	2305
   CH4 + 2O2	2287	2322
   C2H4 + 3O2	2210	2364
   2C2H2 + 5O2	2482	2391
   C2N2 + 2O2	2195	2321

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.
   200	2627	. . .	. . .
   300	2705	390	2697
   500	2775	500	2738
   760	2821	760	2790
   1100	2856	1000	2828
   1500	2872	1450	2842

   
   
   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-vapour	At 10° C	1.2	1676
   	20° C	2.3	1703
   	28° C	3.7	1713
   	35° C	5.6	1738
   	45° C	9.5	1693
   	55° C	15.6	1666
   	65° C	24.9	1526
   	75° C	38.4	1266

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 Mixture	Velocity. Metres/Sec.	Gaseous Mixture	Velocity. Metres/Sec.	Gaseous Mixture	Velocity. Metres/Sec.	Gaseous Mixture	Velocity. Metres/Sec.
   2H2 + O2	2821	2H2 + O2	2821	C2H4 + 2O2	2581	C2H4 + 2O2	2581
   2H2 + O2 + O2	2328	2H2 + O2 + N2	2426	C2H4 + 2O2 + O2	2368	C2H4 + 2O2 + N2	2413
   2H2 + O2 + 3O2	1927	2H2 + O2 + 3N2	2055	C2H4 + 2O2 + 2O2	2247	C2H4 + 2O2 + 2N2	2211
   2H2 + O2 + 5O2	1707	2H2 + O2 + 5N2	1822	C2H4 + 2O2 + 4O2	2118	C2H4 + 2O2 + 4N2	2024
   2H2 + O2 + 7O2	1281	2H2 + O2 + 7N2	none	C2H4 + 2O2 + 6O2	1980	C2H4 + 2O2 + 6N2	1878
   				C2H4 + 2O2 + 8O2	1856	C2H4 + 2O2 + 8N2	1734

   
   
   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 mixture	2H2 + O2	2H2 + O2 + 2H2	2H2 + O2 + 4H2	2H2 + O2 + 6H2
   Explosion velocity, metres/sec	2821	3268	3527	3532

   
   
   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.