2D-imaging of sampling-probe perturbations in laminar premixed flames using Kr X-ray fluorescence

Abstract

The perturbation of the temperature field caused by a quartz sampling probe has been investigated in a fuel-rich low-pressure premixed ethylene/oxygen/argon/krypton flame using Xray fluorescence. The experiments were performed at the 7-BM beamline at the Advanced Photon Source (APS) at the Argonne National Laboratory where a continuous beam of X-rays at 15 keV was used to excite krypton atoms that were added to the unburnt flame gases in a concentration of 5% (by volume). The resulting krypton X-ray fluorescence at 12.65 keV was collected and the spatially resolved signal was subsequently converted into the local temperature of the imaged spot. One and two dimensional scans of the temperature field were obtained by translating the entire flame chamber through a pre-programmed sequence of positions on high precision translation stages and measuring the X-ray fluorescence at each location. Multiple measurements were performed at various separations between the burner surface and probe tip, representing sampling positions from the preheat, reaction, and postflame zones of the lowpressure flame. Distortions of up to 1000 K of the burner-probe centerline flame temperature were found with the tip of the probe in the preheat zone and distortions of up to 500 K were observed with it in the reaction and postflame zones. Furthermore, perturbations of the temperature field have been revealed that reach radially as far as 20 mm from the burner-probe centerline and about 3 mm in front of the probe tip. These results clearly reveal the limitations of one-dimensional models for predicting flame-sampling experiments and comments are made with regard to model developments and validations based on quantitative speciation data from low-pressure flames obtained via intrusive sampling techniques.

The fate of the OH radical in molecular beam sampling experiments

“…Recently, simulations of probe sampling by computational fluid dynamics codes and X-ray fluorescence measurements have helped to visualize and quantify the perturbation effects for selected flame conditions [14][15][16]. In addition, chemical relaxation, which is mainly based on a reaction between OH and H 2 during the expansion in the free jet [10,11], affects the OH mole fraction.…”

Combustion in the future: The importance of chemistry

“…Sampling techniques may influence the detection of important combustion intermediates such as the OH radical [286] , and they might contribute to the formation of low-temperature intermediates via cooling effects [287] . As clearly demonstrated by Hansen et al [281 , 282] , probe perturbations in flat laminar premixed flames are typically not one-dimensional. However, such deviations from one-dimensionality are typically not considered in modeling approaches for validation, and perfect agreement of experiment and model can thus not be expected [282] .…”

“…As clearly demonstrated by Hansen et al [281 , 282] , probe perturbations in flat laminar premixed flames are typically not one-dimensional. However, such deviations from one-dimensionality are typically not considered in modeling approaches for validation, and perfect agreement of experiment and model can thus not be expected [282] . As a superior strategy, the specific probe-sampling conditions could be included in two-dimensional model predictions.…”

“…Because of their widespread use for (full) speciation measurements, it is necessary to keep in mind that probe perturbations can significantly affect temperature and species profiles [281] , [282] , [283] , [284] , [285] , [286] , [287] . Sampling techniques may influence the detection of important combustion intermediates such as the OH radical [286] , and they might contribute to the formation of low-temperature intermediates via cooling effects [287] .…”

X-ray computed tomography for flame-structure analysis of laminar premixed flames

“…Synchrotron X-ray radiography has been employed to examine atomization and spraydynamics in liquid fuel injectors [9][10][11] and to investigate scalar mixing in turbulent jets [12]. More recently, X-ray fluorescence experiments have been performed to obtain quantitative measurements of density fields in turbulent shear-coaxial jet flames [13], sampling-probe perturbations [14], and heated miniature reactors [15], demonstrating the feasibility of using krypton and argon as fluorescent species. Laboratory X-ray sources have been employed for tomographic imaging of in-situ combustion for oil recovery [16], for probing the spray structure in a full-cone atomizer [17], for investigating the scalar mixing in turbulent jets [18], and for examining the interstitial flame structure and temperature field in porous media burners [19].…”