In this work, we have first investigated
the explosion limit behaviors
from hydrogen to propane through numerical simulations and validated
with the available experimental data. The shape of the explosion limit
curves and the possible turning points (
P
1–2
,
T
1–2
), first to second limit
transition, and (
P
2–3
,
T
2–3
), second to third limit transition
that bound the second explosion limit as a function of the fuel carbon
number, have been examined. Results show that with an increase of
methane mole fraction in the hydrogen/methane system, the upper turning
point (
P
1–2
,
T
1–2
) remains almost unchanged and the lower transition
point (
P
2–3
,
T
2–3
) rotates counterclockwise around (
P
1–2
,
T
1–2
).
With a further increase of carbon number, (
P
1–2
,
T
1–2
) moves
to the lower-pressure and -temperature region and (
P
2–3
,
T
2–3
) gradually
moves to the lower-pressure and higher-temperature region. The slope
of the second explosion limit is inversely proportional to the carbon
number,
k
PT
= 0.0069 – 0.005/(
X
c
– 0.7), approximately. Second, a sensitivity
analysis has been conducted to study the elementary reaction on the
second explosion limits. The results show that the chain branching
and termination reactions governing the explosion limit of hydrogen
have a little effect on the second explosion limit of methane. The
C
2
H
5
O
2
H decomposition to form OH
radicals is dominant in controlling the nonmonotonic behavior of the
second explosion limit of C
2
H
6
. The second explosion
limit behavior of propane is governed by three sets of reactions in
the low-temperature oxidation process.
In this study, the effects of ozone
addition on the cool flame
and NTC (negative temperature coefficient) regions of stoichiometric
C
3
H
8
/O
2
mixtures are computationally
studied through the explosion limit profiles. The results show that
with minute quantities of ozone addition (the mole fraction of ozone
is 0.1%), the cool flame area is enlarged to the low-temperature region.
Further increases in the mole fraction of ozone gradually weaken the
NTC behavior, and a monotonic explosion limit is eventually achieved.
The sensitivity analysis of the main reactions involving ozone reveals
that the explosion limit is mainly controlled by the ozone unimolecular
decomposition reaction O
3
(+M) = O
2
+ O (+M).
However, as its reverse reaction is a third-body reaction, this reaction
will lose its effect on the explosion limit in the high-pressure region.
On the contrary, the reaction O
3
+ HO
2
= OH
+ O
2
+ O
2
has a significant effect on the explosion
limit in the high-pressure and low-temperature region, as the concentration
of HO
2
increases through the rapid third-body reaction
H + O
2
+ M = HO
2
+ M.
Rapid compression machines (RCMs), as prominent zero-dimensional homogeneous reactors, have been widely applied in autoignition chemistry investigations. However, species sampling in these reactors is challenged by quantitative accuracy and the limited time allowed for transient sampling. In this study, the uncertainty of quantitative species sampling during the autoignition of a typical fuel/oxidizer mixture was numerically evaluated. Results show that the sampled species profiles tend to lag behind the "true" values due to the dilution of gases outside of the "core" region. When sampling at a longer ignition delay time, increasing the sample duration over 2 ms has very limited improvement on the sampling accuracy, and adopting a 10 mm probe tube is long enough to minimize the errors caused by dilution. In the reaction chamber where nonideal gas disturbance occurs, the sampled gases can be seriously diluted, leading to large deviations in the results. Concentration uncertainties vary among species, while uncertainties in the normalized time are consistent, which can be limited to 6% with proper piston design and sampling setups.
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