Determination of Burning Velocity and Flammability Limit of Methane/Air Mixture Using Counterflow Flames

Abstract

The burning velocity and the flammability limit of a methane/air mixture are investigated numerically using counterflow premixed flames. Two different methods, the minimum velocity method and the upstream temperature boundary method, for the determination of burning velocity are compared. The results show that the minimum velocity method fails when the chemical heat release is weak. A velocity gradient method is presented and examined. Furthermore, at low equivalence ratio, the results show that flame thickness increases exponentially with the decrease of stretch rate. This flame thickening results in a radiation extinction. An extrapolation of burning velocity to zero stretch is shown to be inaccurate. At a high equivalence ratio, it is found that two kinds of stable flames exist, normal flame and weak flame, with distinct burning velocities. A Gshaped curve showing the flammable regions is obtained by plotting the burning velocities at extinction limits as a function of equivalence ratio. The results indicate that a simple linear extrapolation of the extinction limits of stretched flames to zero stretch rate does not give the standard flammability limit.

Numerical study on effect of CO2 addition in flame structure and NOx formation of CH4-air counterflow diffusion flames

“…The present results may have stressed only the side of positive e!ect in the reduction of NO V formation. By the way, it should be noted that the addition of a diluent induces the #ame destabilization such as #ame extinction even at low-strain rates (Ju et al, 1999).…”

“…Recently, considering radiation e!ects Ju et al (1997Ju et al ( ,1999) examined the laminar burning velocity and #ame extinction for several fuel combinations in premixed counter#ow #ames and Chan et al (1998) analysed the change of #ame structure systematically with radiation. Study on NO V formation in fuels with the addition of CO, N and H O has been actively performed (Li et al, 1997).…”

Flammability Limits: A Comprehensive Review of Theory, Experiments, and Estimation Methods

“…The maximum temperatures reached near the normally stretched lean premixed flames at different stretch rates are shown in Figure b, where the radiation extinction limit is at point d and the stretch extinction limit is at point c. The existence of the radiation extinction limit is related to the increase in flame volume, which leads to larger radiation heat losses from the flame …”

“…The maximum temperatures reached near the normally stretched lean premixed flames at different stretch rates are shown in Figure 6b, where the radiation extinction limit is at point d and the stretch extinction limit is at point c. The existence of the radiation extinction limit is related to the increase in flame volume, which leads to larger radiation heat losses from the flame. 58 The stretch and radiation extinction limits were determined by Ju et al 1,57 for various equivalence ratios, and the results are shown in Figure 6c. The lower burning limit of a stretched flame is the point where the stretch and radiation extinction branches meet, which is denoted as point B in Figure 6c.…”

Numerical and Experimental Assessment of a Novel Multinozzle Burner with CO2 Diluent to Improve the Emissions from a Swirling Flame in a Combustion Chamber

“…The effects of carbon dioxide on natural gas combustion have been the focus of many studies. In general, when CO 2 is added as a diluent into the fuel or oxidizer, its influence may be considered similar to that of radiation effects. − The thermal and diffusive effects of CO 2 on the laminar flame speed for hydrogen and hydrocarbons have also been studied. − …”