In:
Journal of Fluid Mechanics, Cambridge University Press (CUP), Vol. 342 ( 1997-07-10), p. 315-334
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.
Type of Medium:
Online Resource
ISSN:
0022-1120
,
1469-7645
DOI:
10.1017/S0022112097005636
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
1997
detail.hit.zdb_id:
1472346-3
detail.hit.zdb_id:
218334-1
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