Measurement and simulation of rotationally-resolved chemiluminescence spectra in flames

Abstract

In recent years, there has been renewed interest in chemiluminescence, since it has been shown that these emissions can be used to determine flame parameters such as stoichiometry and heat release under some conditions. Even though the origin of these emissions has been known for a long time, little attention has been paid to the detailed analysis of the spectral structure.In this contribution, we present rotationally-resolved spectra of all important chemiluminescent emissions OH A-X, CH B-X, CH A-X, and C 2 d-a in CH 4 /air flames. A numerical model based on the LASKINν 2 code has been developed that allows, for the first time, to accurately predict the shape of the measured spectra for all of these transitions. Reabsorption of chemiluminescence within the emitting flame is shown to be a major factor, affecting both intensity and structure of OH * spectra. Even in lab-scale flames, it might change the intensity of individual lines by a factor of 5. The shape of chemiluminescence spectra depends on several processes including initial state distribution and rotational and vibrational energy transfer (which, in turn, depend on the collisional environment and the temperature). It is shown that chemical reactions form OH * in highly excited states and that the number of collisions is not sufficient to equilibrate the initial distribution. Therefore, high apparent temperatures are necessary to describe the shape of the measured spectra. In contrast, CH * is formed with less excess energy and the spectral shape is very close to thermal. The rotational structure of C * 2 is close to thermal equilibrium as well. Vibrational temperatures are, however, signif-A. Brockhinke ( ) · J. Krüger · M. Heusing · M. Letzgus icantly higher than the flame temperature. Implications and perspectives for flame measurements are discussed.

On the quantification of OH*, CH*, and C2* chemiluminescence in flames

“…Therefore, the electronic A and B states of CH seem to be formed close to thermal equilibrium. This finding is in accord with our recent analysis of the spectral structure of chemiluminescence signals, where we found a near-thermal distribution of the rotational states in CH* spectra [9].…”

“…This suggests that CH is formed in thermal equilibrium between the two electronic states, a finding which is in accord with our recent analysis of the spectral structure of chemiluminescence signals [9]. Introduction of different rate constants for reactions leading to CH-A and CH-B has been suggested before [27].…”

“…More recent information on the rotational fine structure of chemiluminescence spectra and their simulation may be found in [9]. Most important in hydrocarbon combustion is the OH* emission around 310 nm, which originates from the A 2 Σ + to the ground state.…”

Experimental analysis of thermo-acoustic instabilities in a generic gas turbine combustor by phase-correlated PIV, chemiluminescence, and laser Raman scattering measurements

“…turbulence level [Ayoola 2006] or signal trapping [Brockhinke 2012, Sadanandan 2011. Thus, in the current study, only qualitative information is derived from the chemiluminescence images, such as phase-dependent variations in heat release and the location and extension of the flame zone.…”

Simultaneous measurement of localized heat release with OH/CH2O-LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

“…However, for the flames investigated in our studies this would be a rare situation to occur. Spectrally resolved measurements and detailed simulations of CL emissions in these flames will also help in quantifying such effects [26].…”