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 X-ray 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 15keV 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 12.65keV 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 low-pressure flame. Distortions of up to 1000K of the burner-probe centerline flame temperature were found with the tip of the probe in the preheat zone and distortions of up to 500K 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 20mm from the burner-probe centerline and about 3mm 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.