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4Hydrogen transport: safety, economy
CHAIN-THERMAL EXPLOSION AND IONIZATION DEGREE OF HYDROGEN-AIR FLAME
V. F. Proskudin, P. G. Berezhko, E. N. Belyaev, V. N. Tarakanov, P. E. Polovinkin, A. G. Leshchinskaya
Russian Federal Nuclear Centre — All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF), 37 Mira pr., Sarov, Nizhniy Novgorod Region, 607188, Russia
It has been shown [1] that within the flame propagation concentration limits for hydrogen-air mixtures being for the initial normal conditions from 5 to 75 vol. % of hydrogen, there is an internal range (from ~18 to ~60 vol. % H2) where the gas mixture burning occurs with an increased intensity being explained by peculiarities of the chain-thermal mechanism of non-isothermal reactions passing at burning (chain-thermal explosion). Near the above-mentioned values of the hydrogen content in the air (i.e. ~18 and ~60 vol. %), significant changes should take place in the character and rate of passing of a number of elementary chemical reactions providing totally the whole spectrum of branching chain non-isothermal transformations at the burning of hydrogen-air mixtures [1], and it is not excluded that such changes can also be accompanied by the change of the flame ionization degree for hydrogen-air mixtures.
The goal of this report is to evaluate the concentration limits, within which the ionization degree rises rapidly at the burning of hydrogen-air mixtures, and to compare them with the concentration limits of the sharp change of burning intensity established in the report [1]. The experiments on gas mixture combustion have been carried out at the initial pressure of 1 atm and the initial room temperature in a steel cylindrical reactor with the internal diameter 35 mm. The gas mixture was ignited by a Nichrome bridgewire with the diameter 0.1 mm and the length 20 mm to be heated by a minimally possible electric current, in order to reduce a possibility of ion formation in the gas mixture ignition process and provide heating of the bridgewire up to the temperature only slightly exceeding the minimum ignition temperature for hydrogen-air mixtures by a Nichrome bridgewire with the diameter 0.1 mm, which is equal to ~ 1150-1260 K [2].
With the use of a passive probe method, which provides registration of the accumulation of signals for the «electrical noise» of flame [3, 4] on the measuring circuit capacitor plates at the burning of hydrogen-air mixtures, it has been established that within the concentration limits of the intensive burning of these mixtures [1], a considerable rise of the flame ionization degree is observed in burning mixtures (see the Fig. 1). In doing so it has been adopted that the integral intensity of the «electrical noise» of flame being evaluated by the maximum voltage of Umax on the measuring circuit capacitor plates can be a measure of the relative degree ionization of a burning gas mixture.
Fig. 1. Dependence of the maximum voltage of Umax (curve 1) and the maximum pressure Ap (curve 2 [1]) at the burning of hydrogen-air mixtures being ignited by a heated Nichrome wire, upon the initial hydrogen content in the gas mixture: A[HJ. are the limits of the ionization degree growth at the burning of hydrogen-air mixture (our own data); A[HJc are the flame propagation concentration limits in hydrogen-air mixtures [1].
Recognizing the decisive significance of the chain-thermal explosion in the explanation of a sharp rise of gas mixture burning intensity within the limits of hydrogen content in the mixture from ~18 to ~60 vol. % [1], one should however note that a certain role in the burning intensity rise being observed can also be played by a sharp rise of the flame ionization degree, which takes place within the same limits of hydrogen content in the air.
References
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2.Проскудин В.Ф., Бережко П.Г., Ярошенко B.B., Селезенев А.А., Буланников А.С., Беляев Е.Н. Физика горения и взрыва. 2001. Т.37, №4. С. 3-8.
3.Marsden R.S., Jr., Fourth Symposium (Intern.) on Combustion. - Baltimore: Williams and Wilkins Company. 1953. P.683-688; Марсден P. В сб. Вопросы горения и детонационных волн. Четвертый симпозиум (международный) по вопросам горения и детонационных волн. Пер. с англ. - М.: Гос. изд-во оборонной промышленности. 1958. С.468-472.
Фиалков Б.С., Плицын В.Т., Магун Я.И., Сенкевич Г.П. Физика горения и взрыва. 1971. Т.7, №3. С.383-392.
ISJAEE Special issue (2003)
