Quantitative concentration measurements in atmospheric-pressure flames by picosecond pump/probe absorption spectroscopy

Gregory John Fiechtner, Purdue University

Abstract

Measurement of flame radical concentrations is important to the understanding of the chemical kinetics involved in flames. Application of optical techniques allows for non-intrusive determination of the radical concentration. One of the most challenging problems for investigators is obtaining data in high-pressure flames that is independent of the collisional environment. An added complication is turbulence, which requires that data be taken in less than one millisecond. This thesis reports on a new method for obtaining quenching-independent concentrations based on picosecond pump/probe absorption spectroscopy. The asynchronous optical sampling (ASOPS) method is chosen, in which beams from two independent mode-locked lasers are crossed in the flame. Verification of the method is obtained through concentration measurements of atomic sodium. Using the ASOPS method, data are taken at a rate of 155.7 kHz with only 128 averages. A corresponding detection limit of 5 $\times\ 10\sp9$ cm$\sp{-3}$ is measured for atomic sodium in an atmospheric methane-air flame. For the first time, ASOPS measurements are made on a quantitative basis. This is accomplished by calibration of the Na concentration using atomic absorption spectroscopy. Two distinct types of anomalous ASOPS signals are reported. These are studied over a wide range of sodium concentrations. No attempt is made to mathematically model either type of anomaly. Rather, enough experimental detail is reported in a well-characterized flame so that models can be developed during future investigations. A picosecond pump/probe absorption model is developed using a rate-equation analysis. For a three-level model, this development has led to a technique with which the ASOPS instrument can be used to obtain both the electronic quenching rate coefficient and the doublet mixing rate coefficient during a single measurement. The technique, called the dual-beam ASOPS method, is successfully used to measure subnanosecond lifetimes of the 3P level of atomic sodium. Using the picosecond pump/probe absorption model, a detection limit of 2 $\times\ 10\sp{17}$ cm$\sp{-3}$ is predicted for Q$\sb1$(9) OH at 2000 K. Although this value is too large for practical flame studies, a number of improvements are suggested that will substantially reduce the ASOPS detection limit.

Degree

Ph.D.

Advisors

Laurendeau, Purdue University.

Subject Area

Mechanical engineering

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