Author:
JU YIGUANG,GUO HONGSHENG,MARUTA KAORU,LIU FENGSHAN
Abstract
Extinction limits and the lean flammability limit of
non-adiabatic stretched premixed
methane–air flames are investigated numerically with detailed chemistry and two
different Planck mean absorption coefficient models. Attention is paid to
the combined
effect of radiative heat loss and stretch at low stretch rate. It
is found that for a
mixture at an equivalence ratio lower than the standard lean flammability limit, a
moderate stretch can strengthen the combustion and allow burning. The flame is
extinguished at a high stretch rate due to stretch and is quenched at a
low stretch rate
due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with
two distinct extinction limits, a radiation extinction limit and
a stretch extinction limit
respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing
the flammability limit of the stretched methane–air flame
is obtained by plotting these
two extinction limits in the mixture strength coordinate. A good agreement is shown
on comparing the predicted results with the experimental data. For equivalence ratio
larger than a critical value, it is found that the O-shaped
temperature curve opens up
in the middle of the stable branch, so that the stable branch divides into two stable
flame branches; a weak flame branch and a normal flame branch. The weak flame
can survive between the radiation extinction limit and the opening point (jump limit)
while the normal flame branch can survive from its stretch extinction limit to zero
stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits
of stretched methane–air flames is presented. It is found
that the critical equivalence
ratio for opening up corresponds to the standard flammability limit measured in
microgravity. Furthermore, the results show that the flammability
limit (inferior limit)
of the stretched methane–air flame is lower than the standard
flammability limit
because flames are strengthened by a moderate stretch at Lewis number less than unity.
Publisher
Cambridge University Press (CUP)
Subject
Mechanical Engineering,Mechanics of Materials,Condensed Matter Physics
Cited by
258 articles.
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