Experimental and numerical investigations of the relation between OH* emission, flame speed and mass consumption rate

6 PagesWe investigate the relation between the intensity of the luminous emission of the excited OH* radical and the mass consumption rate of lean planar premixed methane-air flames. The flames were maintained perfectly flat using parametric acoustic stabilization in an imposed acoustic field. The consumption rate of the flames was varied by changing both the equivalence ratio and the temperature of the unburned mixture. We also compare our experimental measurements to the results of numerical simulations, using detailed chemical kinetics. For lean flames, we find that the OH* emission intensity is linear with mass consumption rate but, not proportional. Consequently, the relative fluctuation in mass consumption rate is not linearly related to the relative fluctuation in OH* emission intensity

The response of premixed flames to pressure oscillations

International audienceRecent measurements of the direct response of premixed flames to acoustic pressure fluctuations have shed doubt on the validity of analytical models that use irreversible one-step chemistry, and suggest that more realistic chemical kinetic models are needed to fully describe the unsteady dynamics of premixed flames. However, in the analysis of the experimental results some doubts subsisted concerning the exact relation between the intensity of emission from the excited OH* radical, used to determine the flame response, and the unsteady reaction rate given by the theoretical analyses. Combining experimental and numerical approaches on premixed methane-air flames, we propose corrections to give more confidence in the experimental results and to confirm the need for further investigations on the dynamics of unsteady premixed flames

Markstein numbers in counterflow, methane- and propane-air flames: a computational study

The concept of Markstein numbers with respect to unburned and burned gases is first reviewed. We next present numerical simulations of methane–air and propane–air flames in the counterflow configuration using a detailed chemical kinetic model consisting of 469 reactions and 71 species. Markstein numbers of these flames, as a function of equivalence ratio, are computed with respect to both the unburned and burned gases. It is shown that the values of Markstein number relative to the unburned and burned gases are not equal and may even have opposite signs, as supported by asymptotic theory. The values from these numerical simulations are then compared to the values published in the literature for the same mixtures. We show that experimental values in the literature, obtained from the expansion rates of spherical flames, are very close to our numerical values of the Markstein numbers with respect to the burned gases, simply re-normalized by the gas density ratio. These results support the idea that flames respond similarly for equal values of a small characteristic stretch. We also find close agreement with an experimental measurement made in the unburned gases on a counterflow propane flame. However, some of our values evaluated in the unburned gases are significantly different from those in the literature, obtained indirectly using measurements of the growth rate of unstable structures on planar flames. We present evidence to suggest that the results of asymptotic flame theory, used to obtain the indirect measurements of Markstein numbers, are not quantitatively applicable when the effective Lewis number of the mixture is not close to unity

A numerical investigation of stretch effects in counterflow premixed laminar flames

We use direct numerical simulation of propane/air flames with full chemistry in the geometry of stagnation flow to investigate the effect of different definitions of local flame stretch in the presence of spatially varying velocity gradients. Specifically, we compare simulations with potential- and plug-flow inlet conditions, and show that the widely used definition of upstream stretch leads to unphysical results for flames having the ‘same' stretch. We then show that a reasonable re-definition of local stretch allows us to produce the ‘same' flame in the presence of the ‘same' stretch

Determination of Markstein numbers in counterflow premixed flames

This paper attempts to settle a long-standing issue concerning the differences in Markstein numbers measured by different experimental protocols. Numerical simulations of a counterflow flame with full transport coefficients, but using a fictitious reactive mixture having properties close to those assumed in asymptotic laminar flame analysis, are used to show how to correctly identify the burning velocity and stretch of a stretched flame with a finite width chemical zone. We show how to measure the Markstein number of the flame with respect to both the unburned and burned gases. The numerical values of these numbers differ by a quantity that depends on the internal flame structure. The physical origin of this difference is made evident. Our numerical results are in close agreement with the predictions of asymptotic theory. We show that laboratory experiments on counterflow flames give Markstein numbers related to the unburned gas, whereas laboratory experiments on spherical expanding flames give Markstein numbers related to the burned gases. The range of validity of asymptotic theory, for Lewis numbers departing from unity, is also examined. We conjecture that the so-called consumption velocity of a flame (normal integral of the species consumption rate) may allow a measure of the Lewis number dependent part of the Markstein number, even for realistic flames with complex chemistry and an effective Lewis number not close to unity

Some problems in analysing premixed flames by infrared thermography

International audienceQuantitative studies of flames by IR thermography can be achieved if we solve properly two problems. At first, there is a low thermal level but corresponding to very high temperatures at wich calibration curves are not available, therefore we are concerned with the question of the extrapolability of these curves. Then burned gases are semi-transparent media so that we have to account for 3D effects. Our approach is proved by comparing thermal images of permixed flames in simple configurations and the images calculated by simulation of the complete acquisition system

La thermographie infrarouge appliquée à l'étude des flammes

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Thermographie des flammes accrochées, comparaison expérience-modélisation

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