Laser Raman and Fluorescence Techniques for Practical Combustion Diagnostics

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  • This article was downloaded by: ["Queen's University Libraries,Kingston"]On: 07 October 2013, At: 01:59Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK

    Applied SpectroscopyReviewsPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/laps20

    Laser Raman andFluorescence Techniquesfor Practical CombustionDiagnosticsA. C. Eckbreth a , P. A. Bonczyk a & J. F.Verdieck aa United Technologies Research Center EastHartford , Connecticut, 06108Published online: 05 Dec 2006.

    To cite this article: A. C. Eckbreth , P. A. Bonczyk & J. F. Verdieck (1977) LaserRaman and Fluorescence Techniques for Practical Combustion Diagnostics, AppliedSpectroscopy Reviews, 13:1, 15-164

    To link to this article: http://dx.doi.org/10.1080/05704927708060383

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  • APPLIED SPECTROSCOPY REVIEWS. 13(1). 15-164 (1977)

    Laser Raman and Fluorescence Techniques for Practical Combustion Diagnostics

    A . C . ECKBRETH. P . A . BONCZYK. and J . F . VERDIECK United Technologies Research Center Eas t Hartford. Connecticut 06108

    I . INTRODUCTION . . . . . . . . . . . . . . . 16 I1 . REVIEW O F POTENTIAL IN-SITU. POINT. COMBUSTION

    DIAGNOSTIC TECHNIQUES . . . . . . . . . . . 18 A . Elastic Scattering P r o c e s s e s . . . . . . . . . . 19 B . Inelastic Scattering Processes . . . . . . . . . 19 D . Nonlinear Optical P rocesses . . . . . . . . . . 23 E . Selected Techniques . . . . . . . . . . . . . 26

    111 . SPECIES SPECTROSCOPY . . . . . . . . . . . . 26 A . Atomic Species of In te res t in Combustion . . . . . 27 B . Diatomic Molecules . . . . . . . . . . . . . 31 C . Polyatomic Molecules . . . . . . . . . . . . 39 D . Raman Cross Sections . . . . . . . . . . . . 51 E . Interferences . . . . . . . . . . . . . . . 51 F . Applicability of Techniques . . . . . . . . . . 54

    IV . PRACTICAL CONSIDERATIONS . . . . . . . . . . 54 A . Sources of Noise . . . . . . . . . . . . . . 56 B . Perturbations . . . . . . . . . . . . . . . 65 C . Laser/Signal Transmission . . . . . . . . . . 69

    C . Absorption . . . . . . . . . . . . . . . 22

    15

    Copyright 0 1978 by Marcel Dekker. Inc . All Rights Reserved . Neither this work nor any part may be reproduced or transmitted in any form or by any means. electronic or mechanical. including photocopying. microfilming. and recording. or by any information storage and retrieval system. without permission in writing from the publisher .

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  • 16 ECKBRETH. BONCZYK. AND VERDIECK

    D . Signal Averaging . . . . . . . . . . . . . 72 E . Summary . . . . . . . . . . . . . . . 72

    V . RAMANSCATTERING . . . . . . . . . . . . . 73 A . Introduction . . . . . . . . . . . . . . . 73 B . Theory . . . . . . . . . . . . . . . . 75 C . Preferred Raman Approaches . . . . . . . . . 80' D . Pulsed Laser Raman Signal-to-Noise Calculations . . 86 E . Required S / N and Signal Averaging . . . . . . . 90 G . Clean Flame Diagnostics . . . . . . . . . . . 94 F . Practical Combustion Device Applicability . . . . . 92 H . Near-Resonant Raman Scattering . . . . . . . . 94

    VI . LASER FLUORESCENCE . . . . . . . . . . . . 98 A . Introduction . . . . . . . . . . . . . . . 98 B . Fluorescence Theory . . . . . . . . . . . . 102 C . Signal-to-Noise Estimates and Detectability Limits . . 114 D . Measurement Considerations . . . . . . . . . 124 E . Summary . . . . . . . . . . . . . . . . 126

    VII . COHERENT ANTI-STOKES RAMAN SCATTERING (CARS) . 127 A . Introduction . . . . . . . . . . . . . . . 127 B . Review . . . . . . . . . . . . . . . . 128 D . Medium Property Measurements . . . . . . . . 133 E . CARSVariants . . . . . . . . . . . . . . 137 F . Practical Applicability . . . . . . . . . . . 139

    VIII . SYSTEMS CONSIDERATIONS . . . . . . . . . . 140 A . Raman Scattering . . . . . . . . . . . . . 141 B . Laser Fluorescence . . . . . . . . . . . . 147 C . CARS . . . . . . . . . . . . . . . . . 150 D . Systems Integration . . . . . . . . . . . . 153

    IX . CONCLUSIONS . . . . . . . . . . . . . . . 155 References . . . . . . . . . . . . . . . . 157

    C . CARS Signal Strength and S / N Calculations . . . . 131

    I . INTRODUCTION With the advent of laser light sources. light-scattering spectro-

    scopic diagnostic techniques a re assuming an ever-increasing role in a broad spectrum of physical investigations . Of particular importance is the potential application of laser spectroscopy to the hostile. yet sensitive. environments characteristic of those in which combustion occurs . These diagnostic techniques should facilitate greatly improved

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  • LASER RAMAN AND FLUORESCENCE TECHNIQUES 17

    understanding of a variety of combustion processes which, in turn, should lead to enhanced efficiencies and cleanliness in energy, propul- sion, and waste disposal systems.

    Recently, exciting experimental demonstrations of the potential of a variety of Raman processes (spontaneous, near-resonant, coherent, stimulated) and laser fluorescence techniques have appeared in studies dealing primarily with laboratory flames. Unlike the situation pre- vailing in the field of the remote detection of atmospheric pollutants where several comprehensive reviews have appeared comparing the capabilities and systems aspects of various diagnostic approaches, little work of a similar nature has appeared in connection with the remote, localized probing of practical combustion devices, e. g. , furnaces, gas turbine combustors. Although t h e pollutant detection review studies can be drawn upon, the measurement requirements and potential problem areas in practical flame diagnosis a r e sufficiently different to require a fresh perspective and review of measurement techniques more aptly suited for the extraction of species and tempera- ture information from combustion devices. The objective of this paper is to provide such a review. Realistically, such a review of diagnostic techniques must focus keenly on t h e problems and sources of noise which must be circumvented for successful application to practical devices. Since it i s unlikely that any one technique will provide the species and temperature measurements over the range desired, systems considerations become important in ascertaining how the various approaches can be integrated together with maximum measure- ment ra te and minimal redundancy. Such systems studies should also illuminate the tradeoffs between systems complexity and cost on the one hand, and probability of successful measurements on the other.

    In the next section of th i s paper, various laser diagnostic tech- niques potentially suitable for "point" temperature and species concentration measurements in flames a re reviewed. From this list, four techniques a re selected for detailed evaluation including: (1) spontaneous Raman scattering, (2) near-resonant Raman scattering, (3) laser fluorescence, and (4) coherent anti-Stokes Raman scattering (CARS). The spectroscopy of species of combustion interest is dis- cussed and the applicability of the foregoing techniques to detection of the various species is examined. Practical device considerations a re reviewed with emphasis on sources of noise (e. g. , luminosity, particulates), medium perturbations, laser and signal transmission, and signal averaging in temporally fluctuating media. Each diagnostic technique is then addressed in some detail in the order previously stated. Basic physics, species sensitivity, thermometry applicability, signal to noise, problem areas , and new variations of the techniques a re

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  • 18 ECKBRETH, BONCZYK, AND VERDIECK

    included in these treatments. Measurement systems approaches are described together with probability of success assessments and r isk assignments. An integrated measurement system i s described.

    11. REVIEW OF POTENTIAL IN-SITU, POINT, COMBUSTION DIAGNOSTIC TECHNIQUES

    There a re a large variety of diagnostic processes potentially applicable to the remote, nonintrusive, point probing of combustion phenomena. These various processes will be briefly reviewed and from them, a list of the most promising techniques for flame probing will be selected for further detailed study. In th i s review, attention will be directly ultimately only to those laser techniques which permit the determination of local species concentrations and temperature. Velocity measurements and emission spectroscopy techniques will not be con- sidered.

    attention in the past few years. In the summer of 1974, the American The subject of combustion diagnostics has received a great deal of

    TABLE 1

    Potential Combustion Diagnostic Techniques

    A. Elastic Scattering Processes 1. Rayleigh 2. Mie

    B. Inelastic Scattering Processes 1. Raman 2. Near-resonant Raman 3. Fluorescence

    C. Absorption Processes 1. Resonant, Line of Sight 2. Differential Absorption

    1. Inverse Raman Scattering 2. Raman-Induced Kerr Effect (RIKES) 3. Stimulated Raman Scattering 4. Hyper-Raman Scattering 5. Coherent Anti-Stokes Raman Scattering (CARS) 6. Higher-order Raman Spectral Excitation Spectroscopy

    (HORSES)

    D. Nonlinear Optical Processes Dow

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  • LASER RAMAN AND FLUORESCENCE TECHNIQUES 19

    Physical Society conducted a 1-month summer study to evaluate the role of physics in combustion [ l ] . Diagnostics for experimental combustion research received considerable attention. In May 1975, Project SQUID conducted a several day workshop devoted exclusively to combustion measurements in jet propulsion systems [2]. In January 1976, the combustion sessions of the AIAA 14th Aerospace Sciences Meeting emphasized combustion diagnostics [31.

    In Table 1 potential flame diagnostic techniques a re listed: these have been drawn from a listing of techniques applicable to a i r pollution measurements [41, gas dynamics 151, and analytical chemistry [61 . Image formation and tracer techniques [ l ] will not be considered.

    A. Elastic Scattering Processes

    1. Rayleigh Scattering

    Rayleigh scattering (Fig. 1) and is the phenomenon giving rise to the blue appearance of the sky. Because the scattering process is elastic, the scattered light is unshifted in frequency and, hence, not specific to the molecule causing the scattering. Thus the technique can be used for total density measurements but not for individual species concentra- tions. Temperature measurements can be made by resolving the Doppler linewidth of the scattering [71. From a practical viewpoint, Rayleigh diagnostics suffer from Mie interferences and spuriously scattered laser light and have been employed in only very clean situations. The technique has seen very limited utilization and is not suitable for prac- tical combustor device probing.

    The elastic scattering of light quanta from molecules i s termed

    2. Mie Scattering

    Mie scattering. It is not dependent on molecular number density or temperature and, hence, cannot be used to provide such information. It i s the basic effect underlying laser Doppler velocimetry and differ- ential absorption backscattering I41 measurements. It can be a very strong process depending on particle number density and particle size, and i s a potential source of interference as will be described later.

    Elastic scattering of light quanta from particulate matter is termed

    B. Inelastic Scattering Processes

    1. Raman and Near-Resonant Raman Scattering

    Raman scattering is the inelastic scattering of light from molecules as illustrated in Fig. 1 , and is termed rotational, vibrational, o r

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  • LASER RAMAN AND FLUORESCENCE TECHNIQUES 21

    electronic depending on the nature of the energy change which occurs in the molecule. The process is essentially instantaneous, occurring within a time of 10-12sec o r less. The molecule may either become excited o r deexcited depending on its original state prior to the inter- action. Raman scattering is ideally suited to combustion diagnostics and has been widely applied in clean flames [81. Visible wavelength lasers a re typically employed since the strength of the scattering scales as the fourth power of the Raman frequency, but no specific wavelength is required. Due to the quantization of the molecular energy states, the Raman spectrum resides at fixed frequency separations from the laser line characteristic of the molecule from which the scattering emanates. Thus t h e Raman scattering is species specific and linearly proportional to species number density. Furthermore, spectral interferences between vibrational Raman bands in gases a r e rare. Temperature measurements a re readily made from the distribution of the s...

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