Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

Abstract

Laser-based methods have transformed combustion diagnostics in the past few decades. The high intensity, coherence, high spectral resolution and frequency tunability available from laser radiation has provided powerful tools for studying microscopic processes and macroscopic phenomena in combustion by linear and nonlinear optical processes. This review focuses on nonlinear optical techniques based on resonant four-wave mixing for non-intrusive measurements of minor species in combustion. The importance of minor species such as reaction intermediates is outlined together with the challenges they present for detection and measurement in the hostile environments of flames, technical combustors, and engines. The limitations of conventional optical methods for such measurements are described and the particular advantages of coherent methods using nonlinear optical techniques are discussed.The basic physics underlying four-wave wave mixing processes and theoretical models for signal calculation are then presented together with a discussion of how combustion parameters may be derived from analysis of signals generated in various four-wave mixing processes. The most important four-wave mixing processes, in this context, are then reviewed:Degenerate Four-Wave Mixing (DFWM), Coherent Anti-Stokes Raman Scattering (CARS), Laser Induced Grating Spectroscopy (LIGS) and Polarization Spectroscopy (PS). In each case the fundamental physics is outlined to explain the particular properties and diagnostic advantages of each technique. The application of the methods mentioned to molecular physics studies of combustion species is then reviewed along with their application in measurement of concentration, temperature and other combustion parameters. Related nonlinear techniques and recent extensions to the ultra-fast regime are briefly reviewed. Finally practical considerations relevant to multi-dimensional and multi-species measurements, as well as applications in technical combustion systems are discussed.Keywords: nonlinear optical spectroscopy, four-wave mixing, combustion diagnostics, minor species detection, laser-induced gratings, polarization spectroscopy IntroductionEnergy from renewable sources such as solar and wind power is the goal of much current research and technological development. Combustion, however, is and is likely to remain for the foreseeable future, the most important energy source for heat, electrical power and especially for transportation. In technical combustion systems the chemical energy of fossil fuels such as coal, crude oil and natural gas is converted into heat through oxidation with oxygen from air. However, fossil fuel resources are limited and alternatives like biofuels [1] or novel technologies such as fuel cells [2] are not yet able to meet the demand. Consequently, for a sustainable use of fossil fuels it is desirable to further improve the efficiency, maintainability and reliability of existing combustion devices. Achieving such improvements depends upon an improved understanding of t...

Mixing characterization of highly underexpanded fluid jets with real gas expansion

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Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

“…Further details of the theoretical principles of nonlinear phenomena can be found in a number of textbooks 17,62,94 and review articles. 95,96 The list of FWM techniques for advanced optical diagnostics is quite long and includes degenerate four-wave mixing (DFWM), [97][98][99][100][101] two-color FWM (TC-FWM), [102][103][104][105] polarization spectroscopy (PS), [106][107][108][109][110][111] CARS, 96 and LIGS also known as laser-induced thermal acoustics (LITA). [112][113][114] In the following, we will focus on the developments of the two latter, CARS and LIGS, and eventually review combined applications of linear and nonlinear methods.…”

Time-resolved gas thermometry by laser-induced grating spectroscopy with a high-repetition rate laser system

“…However, it is also important to ensure that the temperature is not appreciably perturbed by the measurement process itself (Kiefer and Ewart 2011). Of critical importance is that the pulse duration is short relative to the transit time of the induced acoustic waves to cross the fringe spacing of the grating.…”

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