ON A POSSIBILITY TO REDUCE THE IGNITION THRESHOLD
9
forming a molecule in the rth chemical reaction, were
calculated in the same manner as in [4].
logγi
At low temperatures (T0 ≤ 600 K), there are marked
differences in the mechanism of the combustion initia-
tion of the H2 + O2 mixture as compared to higher val-
ues of T0 even in the case of the absence of the prelim-
inary excitation of the H2 or O2 molecules. At the initial
stage, as for high values of T0 , the principal reaction
initiating the formation of active OH radicals is the
(9C) reaction. Furthermore, OH radicals dissociate [the
(4C) reverse reaction] and also react with H2, which is
accompanied by the formation of H2O molecules and H
atoms [the (8C) reverse reaction]. These products par-
ticipate in two different processes. The former of them
results in formation of O atoms [the (6C) reaction] and
initiates the chain-combustion mechanism. The latter
process results in the disappearance of H atoms and for-
mation of HO2 radicals [the (10C) reverse reaction]. At
T ≥ 800 K, the rate of the former process becomes
higher than that of the latter one, but the situation alters
to the opposite one at T ≤ 600 K.At T ≤ 600 K, an impor-
tant source of O atoms is the dissociation of O3 [the
reaction (24C)], which is preliminary formed as a result
of the interaction between HO2 and O2 [the (27C)
reverse reaction]. The rate of the (24C) reaction is com-
parable to that of formation of O atoms in the reaction
of the chain initiation. The (5C) reaction closes the
chain mechanism for the combustion of H2 + O2 mix-
tures.
H2
O2
–1
–2
–3
–4
–5
H
O
OH
HO2
–6
–5
–4
logt, s
Fig. 4. Variation in concentrations for the OH, O, H, HO ,
H , and O components with time in the case of combustion
2
2
2
of the 2H + O mixture at P = 10 kPa, T = 600 and 300 K
2
2
0
0
(solid and dashed lines) in the case of exciting molecular
vibrations in H (T = 3000 K).
2
40
cient source of H atoms becomes the reaction of disso-
ciation of OH and H2 [the (4C) and (2C) reactions,
respectively]. An increase in the concentration of H
atoms results in accelerating the formation of HO2 rad-
icals in the (10C) reaction. This process initiates
increasing the rate of the (15C) reverse reaction, the
growth in the concentration of OH radicals and, as a
consequence, of H and O atoms as well.
The preliminary excitation of H2 or O2 molecules
significantly change the mechanism of the combustion
initiation and results in decreasing τin at the same T0. In
Fig. 2, we show the dependences τin(T0) in the case of
the absence (Tξ0 = T0) and of the presence of the excita-
tion of H2 for various vibrational temperatures T40 in H2
and the dependence τHD (T0) for Ra = 0.1 m. As is seen,
the condition τin > τHD is realized at lower T0 in the case
With decreasing T0 from 600 to 300 K, the forma-
tion rate for HO2 radicals in the (10C) recombination
reaction increases. This fact, as was already noted
above, results in increasing the concentration of OH
radicals and H and O atoms, and thus, in intensifying
the chain mechanism of the process and in reducing the
period of induction. This can be clearly seen from
Fig. 4, in which the dependences γi(t), i = OH, H, O,
HO2, H2 and O2 are shown for T0 = 600 and 300 K.
of the excitation of the H2 molecules. Thus, at T40
=
1000 K, the self-ignition is possible at T0 = 500 K,
while at T40 = T0—only at T0 = 550 K. For a significant
excitation of H2 (T40 ≥ 2000 K), the character of the
dependence τin(T0) changes in the range T0 = 600–
300 K; namely, the value of τin decreases with the
reduction of T0. In the case of a strong excitation of H2
molecules, variation in the form of the dependence
τin(T0) is caused by a change in the mechanisms of pro-
duction of O and H atoms, as well as OH radicals com-
pared to unexcited gas. This is illustrated in Fig. 3 in
which we show the variation of the OH, H, and O for-
mation rates in the process of combustion of the hydro-
gen–oxygen mixture with and without the excitation of
H2 at T0 = 600 K. It is seen that the excitation of H2
leads primarily to a significant increase in the forma-
tion rate for OH radicals in the (9C) reaction and H
atoms in the (8C) reverse reaction. In this case, an effi-
The results obtained indicate a possibility of a sig-
nificant (by the factor of two or higher) reducing the
ignition threshold for combustible mixtures when
exciting the molecular vibrations in initial reagents. In
the case of the low-temperature initiation of combus-
tion by means of a selective excitation of vibrational
degrees of freedom in the reacting molecules, new
mechanisms of formation of active atoms and radicals
appear. These mechanisms can lead to a qualitative
change in the dependence for the induction period on
the initial temperature of a mixture.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, grant no. 99-01-01165.
DOKLADY PHYSICS Vol. 45
No. 1
2000