Evaluation of LIF thermometry technique using Krypton as a tracer: Impact of laser lineshape and collisional bandwidth

In recent years, two-photon laser-induced fluorescence (TPLIF) of inert gas krypton (Kr) has received much attention as a potential non-intrusive flow diagnostic, thermometry, and velocimetry technique. However, the interaction between the laser bandwidth and the atomic excitation transition at varying pressures, and the variation of the fluorescence quantum yield due to spatially varying collisional quenching of Kr are not fully understood. The resulting lineshape and quenching corrections become especially important when the temporal scales of excitation laser pulses vary from nanosecond (ns) to femtosecond (fs) timescales. In this study, we report a comprehensive investigation of Kr line broadening and quenching effects, in particular, with respect to TPLIF-based temperature (T) measurements in flames. Experiments were conducted in both low-pressure and atmospheric-pressure flames and in two different burner configurations, using conventional ns and ultrafast fs duration laser sources. When using ns pulses, the results suggest no lineshape corrections are necessary at low pressure; however, a T-dependent empirical model was needed for atmospheric-pressure flames. In contrast, broadband fs pulses have linewidths in excess of 400 cm−1, which are significantly larger than sub-cm−1 spectral transitions, hence requiring no lineshape corrections at any pressure. Quenching measurements in low-pressure flames were found to be scaled by T−0.5 using previous low-pressure thermometry data and 1D flame model predictions. The Kr TPLIF temperature profiles measured using ns pulses (low and atmospheric pressures) and fs pulses (atmospheric pressures) agreed well with model predictions. In the absence of the necessity for lineshape corrections, fs Kr-TPLIF provides an easy way to extract T simply by taking the inverse square of the fluorescence signal. T-line imaging using fs-pulses are presented for several flame heights. The present study establishes a simplified approach for Kr TPLIF-based thermometry in flames.