Laser‐Based Combustion Diagnostics

Abstract

In laser spectroscopy the interaction of light emitted from various types of laser sources – tunable or nontunable in their output frequency – with the atomic or molecular species of interest is used to probe the sample through a variety of spectral responses. In order to perform laser spectroscopy suitable laser sources must be selected which meet the requirements of the chosen spectroscopic method. This means that the laser has to provide radiation which is in the wavelength range of interest, has the appropriate emission characteristics (lineshape) and a suitable energy to perform the measurements. Further requirements are pulse length (milliseconds to femtoseconds or continuous wave), repetition rate and beam profile. Nowadays, laser radiation can be generated with most of the required parameters necessary for the respective spectroscopic application, either directly or by generating new radiation frequencies by frequency mixing of one or several laser beams in a nonlinear medium (gas, liquid, solid). As an example, the most direct probe is absorption of laser radiation (LAS, laser absorption spectroscopy) by suitable spectroscopically allowed transitions in atoms or molecules which are known from conventional spectroscopic methods. The increase or decrease in the laser radiation transmitted through the sample is then a measure of the amount of substance probed in the sample which characteristically absorbs at the required wavelength. Laser light scattering methods, elastic (Rayleigh scattering (RS)) or inelastic (spontaneous Raman scattering (SRS)), are other techniques to probe the medium. In the first method, which is nonspecies specific, the density of the medium can be interrogated, whereas the second is able to probe all species with Raman‐active vibration–rotation transitions. There are several advantages in using laser spectroscopy instead of conventional spectroscopic techniques using conventional thermal light sources. The spectral brightness of laser beams is many orders of magnitude higher than that of thermal radiation sources, which correspondingly increase the detection sensitivity of laser spectroscopic techniques. In addition, the small linewidth of the emitted radiation increases dramatically the spectral resolution such that minor details of the spectroscopic branch investigated can be resolved. This enables more quantitative interpretations of all parameters influencing the lineshape and intensity of the probed transition, and as such the physical and chemical environment of the probed species: temperature, pressure, velocity, chemical species and so on. It makes laser spectroscopic techniques much more selective than conventional methods, which often are not able to separate closely spaced spectral features from different species. A third advantage of laser spectroscopic techniques is connected with the variable pulse duration and repetition frequency of lasers: the very short pulse lengths can be used successfully to probe the sample within time periods which are short compared to any other physical or chemical time development – flow, chemical reaction, pressure changes and so on. Finally, the small spatial regions which can be probed by focusing diffraction limited laser beams makes laser spectroscopic techniques ideally suited for applications where high spatial resolution is required. All these advantages of laser spectroscopy are beneficial when the various techniques are applied as a diagnostic tool in combustion processes: flames constitute a complex interaction of fast chemistry with flow fields and surfaces and, therefore, a detailed understanding of combustion events often needs in situ, species‐specific optical diagnostics with high spatial and temporal resolution. In many applications laser spectroscopy is a developed technique and can even be applied by untrained people. However, numerous laser spectroscopic techniques require detailed theoretical knowledge of the spectroscopy underlying the respective technique, and use sophisticated equipment in order to obtain meaningful results. Future development is aimed towards simplifying experimental set‐ups, data evaluation and maintenance. This development runs parallel to the breathtaking development in laser technology.

Laser-spectrometric gas analysis: CO2–TDLAS at 2 µm

“…Carbon dioxide and its 13 C isotopes, for instance, have been found to be important molecules in atmospheric monitoring, breath analysis and car exhaust emission measurements [6,15,[23][24][25][26][27][28][29][30][31]. Carbon dioxide (CO 2 ) and other species have been measured, for isotope ratio analysis, industrial applications and trace gas monitoring, using different laser spectrometric techniques by several groups [32][33][34][35][36][37] with detection limits down to the pmol mol −1 level [3].…”

Cavity-based absorption spectroscopy techniques

“…A prime example is photoacoustic spectroscopy (PAS), in which thermal relaxation of the excited molecules generates sound waves (Sigrist, 1994b;Harren et al , 2000Harren et al , , 2010Tittel et al , 2003;Ngai et al , 2008;Patel, 2008Patel, , 2010Zelinger and Strizik, 2008;Elia et al , 2009). Likewise, luminescence from electronically excited molecules results in laser-induced fl uorescence (LIF) spectroscopy (Naik et al , 2008), with particular applications to combustion diagnostics (Lucht, 1987;Wolfrum et al ., 2000;Wolfrum, 2001;Kaminski, 2005;Schulz, 2005;Hult, 2008). Other examples of indirectly measured optical absorption spectroscopy for gas-phase molecules (Demtr ö der, 2003;Svanberg, 2004;Svanberg and Demtr ö der, 2012) include: optothermal spectroscopy, detected by lensing or deposition of energy on a bolometer; resonance-enhanced multiphoton ionisation (REMPI); laser-induced photofragmentation or photoionisation, detected by mass spectrometry or optogalvanically or in molecular beams.…”

Diffractive/refractive (hybrid) UV-imaging system for minimally invasive metrology: design, performance, and application experiments

“…Laser-based measurements are powerful techniques for the observation of in-cylinder processes in internal combustion (IC) engines such as fuel/air mixing and combustion [21,22]. For example, in laser-induced fluorescence (LIF) a pulsed laser-light sheet, often in the UV, is used to excite fluorescence from molecules such as the hydroxyl radical OH or components added to the fuel, so-called tracers.…”