Two-line Laser-induced Fluorescence Research Articles

Comparison of line-peak and line-scanning excitation in two-color laser-induced-fluorescence thermometry of OH

Two-line laser-induced-fluorescence (LIF) thermometry is commonly employed to generate instantaneous planar maps of temperature in unsteady flames. The use of line scanning to extract the ratio of integrated intensities is less common because it precludes instantaneous measurements. Recent advances in the energy output of high-speed, ultraviolet, optical parameter oscillators have made possible the rapid scanning of molecular rovibrational transitions and, hence, the potential to extract information on gas-phase temperatures. In the current study, two-line OH LIF thermometry is performed in a well-calibrated reacting flow for the purpose of comparing the relative accuracy of various line-pair selections from the literature and quantifying the differences between peak-intensity and spectrally integrated line ratios. Investigated are the effects of collisional quenching, laser absorption, and the integration width for partial scanning of closely spaced lines on the measured temperatures. Data from excitation scans are compared with theoretical line shapes, and experimentally derived temperatures are compared with numerical predictions that were previously validated using coherent anti-Stokes-Raman scattering. Ratios of four pairs of transitions in the A2Sigma+<--X2Pi (1,0) band of OH are collected in an atmospheric-pressure, near-adiabatic hydrogen-air flame over a wide range of equivalence ratios--from 0.4 to 1.4. It is observed that measured temperatures based on the ratio of Q1(14)/Q1(5) transition lines result in the best accuracy and that line scanning improves the measurement accuracy by as much as threefold at low-equivalence-ratio, low-temperature conditions. These results provide a comprehensive analysis of the procedures required to ensure accurate two-line LIF measurements in reacting flows over a wide range of conditions.

Two Dimensional Rotational Temperature Measurement by Multiline Laser Induced Fluorescence of Nitric Oxide in Combustion Flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A2∑+Π;X2II(0,0) Q1 and Q2 lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q1(31.5)/Q1(16.5) and Q1(18.5)/Q1(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q1(31.5)/Q1(16.5) excitation. However, for Q1(18.5)/Q1(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.

A detailed rate equation model for the simulation of energy transfer in OH laser-induced fluorescence

A rate equation model (the LASKIN program packet) has been developed for the detailed computation of energy transfer in Laser-Induced Fluorescence (LIF). Calculations of this type are necessary for analysis of the influence of energy transfer processes [e.g., electronic quenching and Rotational Energy Transfer (RET)] on the fluorescence signal. The model has been utilized to examine linear LIF in the OHA 2Σ+-X 2 Π (0, 0) band. Available data on quenching, RET and spontaneous emission rates for the (A, v′ = 0) state have been reviewed, and models for the state-specific RET and quenching rates have been developed. The accuracy of the calculations has been confirmed by comparison with experimental data, and the LASKIN program has been applied to the analysis of potential error sources in the widely applied two-line LIF temperature-measurement technique. Extensions of the model to the examination of saturated LIF, OHA–X (1, 0) and (3, 0) excitation and LIF of other species (e.g., O2 and NO) are discussed.

Spatially resolved temperature measurements in a flame using laser-excited two-line atomic fluorescence and diode-array detection

Spatially resolved temperature measurements in a CH(4)-air flame have been achieved using two-line laser-induced fluorescence from indium atoms seeded to the flame.