Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures

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Abstract

An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a "neck". The flame fronts merge at the "neck" location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame "neck" which leads to the flame fronts merging and to the incomplete reaction.
LanguageEnglish
Pages565-576
Number of pages12
JournalCombustion and Flame
Volume160
Issue number3
DOIs
StatePublished - 2013

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Methane
Hydrogen
flames
methane
air
hydrogen
Air
Flammability
flame holders
Stabilization
flammability
Merging
flame propagation
equivalence
Heat transfer
extinction
stabilization
flame temperature

Cite this

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title = "Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures",
abstract = "An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a {"}neck{"}. The flame fronts merge at the {"}neck{"} location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame {"}neck{"} which leads to the flame fronts merging and to the incomplete reaction.",
author = "Y. Shoshyn and R.J.M. Bastiaans and {Goey, de}, L.P.H.",
year = "2013",
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language = "English",
volume = "160",
pages = "565--576",
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Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures. / Shoshyn, Y.; Bastiaans, R.J.M.; Goey, de, L.P.H.

In: Combustion and Flame, Vol. 160, No. 3, 2013, p. 565-576.

Research output: Contribution to journalArticleAcademicpeer-review

TY - JOUR

T1 - Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures

AU - Shoshyn,Y.

AU - Bastiaans,R.J.M.

AU - Goey, de,L.P.H.

PY - 2013

Y1 - 2013

N2 - An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a "neck". The flame fronts merge at the "neck" location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame "neck" which leads to the flame fronts merging and to the incomplete reaction.

AB - An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a "neck". The flame fronts merge at the "neck" location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame "neck" which leads to the flame fronts merging and to the incomplete reaction.

U2 - 10.1016/j.combustflame.2012.11.012

DO - 10.1016/j.combustflame.2012.11.012

M3 - Article

VL - 160

SP - 565

EP - 576

JO - Combustion and Flame

T2 - Combustion and Flame

JF - Combustion and Flame

SN - 0010-2180

IS - 3

ER -