On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames

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.

Dynamics of lean blowout of a swirl-stabilized flame in a gas turbine model combustor

“…For a premixed opposed jet flame, Ju et al found a decrease of the extinction stretch rate from ≈1800 s −1 at φ=0.75 to ≈460 s −1 at φ=0.55 [28]. Principal strain rates were determined for the present flow fields, and the lowest absolute values (<500 s −1 ) were found in the vortex centers, where solid-body rotation occurs.…”

“…6, the 2D principal strain rates at the stagnation point were determined as 5620 s −1 (extensive) and 4058 s −1 (compressive). These values are well above the stretch extinction limit of ≈460 s −1 at φ=0.55 [28]. The flame root can, to some extent, withstand the high strain rates because of the locally relatively fuel rich and hot conditions (see Fig.…”

“…Several experimental and numerical studies have shown that the stretch extinction limit of lean premixed methane-air flames decreases considerably when the equivalence ratio is reduced [27,28]. For a premixed opposed jet flame, Ju et al found a decrease of the extinction stretch rate from ≈1800 s −1 at φ=0.75 to ≈460 s −1 at φ=0.55 [28].…”

Studies of radiation absorption on flame speed and flammability limit of CO2 diluted methane flames at elevated pressures

“…This concern becomes more serious when the ambient pressure increases. Recently, it has become clear that spectral radiation is the dominant loss mechanism for flammability limit [1][2][3][4][5] and it leads to radiation extinction [6][7][8][9], flame bifurcations [10][11][12] and radiation induced instability [13]. Therefore, the effect of radiation heat loss on near limit flames has been extensively studied [1][2][3][4][5][14][15][16][17].…”

A Numerical Study on the Extinguishing Effects of CO2in Counterflow Diffusion Flames with the Concept of Local Application System

“…낮은 화염신장율의 조건에서 중요시되는 복사 열손실 및 첨가된 CO 2 의 추가적인 복사 열손실을 고려하기 위하 여, 간략화된 모델로서 화염 및 부근의 가스가 광학적으로 매우 얇아 복사에너지를 흡수하지 않는다는 가정이 적용 된 Optically Thin Model(OTM) (15) 이 사용되었다.…”