Effects of radiative emission and absorption on the propagation and extinction of premixed gas flames

Abstract

Premixed gas flames in mixtures of CH 4 , O 2 , N 2 and CO 2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively-induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically-thin radiation models, particularly when CO 2 , a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H 2 O and CO 2 , most of the radiation from product H 2 O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO 2 is broader at flame temperatures than ambient temperature, thus some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO 2 . Via both mechanisms some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison to experiment demonstrates the importance of reabsorption in CO 2 -diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emissionabsorption spectra of the reactant and product gases and their temperature dependence, and cannot be predicted using gray-gas or optically-thin model parameters. Applications to practical flames at high pressure, in large combustion chambers and with exhaust-gas or flue-gas recirculation are discussed.

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

“…Rather, it is spectral dependent and has temperature and pressure broadening. Ju and his coworkers [20,21] first used the non-gray statistical narrow band (SNB) model in studying the CO 2 diluted propagating flames and showed that a ''fundamental'' flammability limit exists because of the emission-absorption spectra difference between the reactants and products and band broadening. This model was later used in flame balls [22] and high pressure planar flames [23].…”

“…However, even the prediction was better than that of optically thin model [8,9], it still could not reproduce the experimental data [7]. Moreover, because of the limitation of computation time, radiation modeling [20] still used the gray assumption on each spectral line and was limited to one-dimensional (1D) planar flame geometry although many experimental data are from spherical flames. More importantly, experimental data of radiation absorption was not available to validate the modeling.…”

Numerical Study of an In-line Pre-Heating Treatment along the Axis on the Propagation of a Laminar Premixed Flame in a Channel

“…However, a little attempt is made on the issue of flame propagation in the tube/channel with the addition of local heating. It is reported by Ju et al [17] and Chen et al [18] that the self-absorption of radiation emitted from the flame has significant effect on burning velocity and flammability limits. Obviously, this selfabsorption of emitted radiation from the flame is mild while the absorption of an external radiation is strong in nature.…”

Thermal radiation properties of turbulent lean premixed methane air flames

“…[8,9] and their citations). These papers contain interesting findings regarding radiation-induced bifurcations [8] and effects of self-absorption of radiation [9]. Premixed flames are more prone to instability and their radiation signals may be used in a control strategy.…”

“…[8,9] and their citations). These papers contain interesting findings regarding radiation-induced bifurcations [8] and effects of self-absorption of radiation [9].…”