T H E CRITICAL IGNITION O F EXPLOSIVE HYDROGEN MIXTURES J. H.MAAS1 AND CHARLES EWING2 University of Cincinnati, Cincinnati, Ohio
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Thornton (1) observed that the least spark energy required to ignite an explosive mixture of a gas in air varies discontinuously as the composition of the gas is changed continuously. If this condition be represented graphically by plotting the least spark energy for explosion of mixtures with the proportion of explosive gas in air, the resulting ignition curve is composed of a series of distinct “steps.” This phenomenon, which is called “stepped” or “critical” ignition, is shown to depend upon the presence of nitrogen in the gas mixtures considered here. It was found that the ignition curve for hydrogen in air mixtures has two very distinct steps. On the other hand, smooth curves were found for mixtures of hydrogen and oxygen and also for mixtures of hydrogen, argon and oxygen wherein the argon was present in the same proportion as nitrogen in the air mixtures. The purpose of substituting argon for nitrogen was to determine if the action of nitrogen on ignition differed from that of an inert gas, and this seems to be the case. EXPERIMENTAL
The explosions were carried out in an iron bomb of about 100 cc. capacity with a spark gap consisting of a steel needle cathode and a plane anode also of steel. This bomb was conveniently made of ordinary pipe fitting, the main piece a one and one-fourth inch cross. Two one-half inch holes in the cross on opposite sides of the spark gap were fitted with rubber stoppers to provide a safe outlet for the exploding gas, which blows the stoppers out. Steel phonograph needles were used for the cathode. These were replaced for each explosion. The gas mixtures were made up in volumes of three liters by displacement of water from the gas container. The measurements were made on gases saturated with water vapor and a t room temperature. The bomb is filled with gas by mercury displacement. The electrical arrangement for discharging the spark in the gas consists 1
2
Department of Physics, University of Cincinnati. Department of Chemistry, University of Cincinnati. 13
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J. H. MAAS AX‘D CHARLES EWING
of charging condensers from a source of direct current with the electrodes wide apart, The direct current source is t,hen disconnected and the condensers discharged by approaching the electrodes. Not, more than three seconds need elapse between t,he t,ime the direct current connect,ion is broken and the time the electrodes are brought within sparking distance.
FIG. 1. EXPLOSION BOMB
FIG.2. WIRINGDIAGRAM S, spark gap; C,condenser; V, voltage of D. c. source
During this interval of time there is some leakage loss in the condensers, but it has been found that this loss is negligible u p t o about twenty seconds. The energy “E” expressed in terms of the capacity and voltage of the condensers discharged in the spark is expressed as follows: E
=
112 CV’
IGNITION OF EXPLOSIVE HYDROGEN MIXTURES
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The curves in figure 3 are for ignition a t 160 volts, spark potential. Similar curves have been obtained for the ignition of the same hydrogen mixtures a t 100 volts, but the least spark energy a t this potential is higher than the least spark energy for ignition at 160 volts. JOULES
per
SPARK
FIG.3. IGNITION CURVES FOR
IGNITION O F
HYDROGEN MIXTURE6
REFERENCE (1) THORNTON: Proc. Roy. SOC. London 91, 17 (1914). The Ignition of Gases by Condenser Discharge Sparks.
