Laser Diagnostics for Combustion Temperature and Species

Alan C. Eckbreth

• Publisher: CRC Press
• Year: 2022
• eBook ISBN:
  9781000159448
• Topic: Physical Sciences
• Subtopic: Electrical Engineering & Telecommunications

This book examines the variety of potential laser diagnostic techniques and presents a considerable theoretical foundation elucidating physics relevant to the laser diagnostics. It explains the Raman-based approaches for major species and temperature measurements.

Chapter 1 SURVEY OF LASER DIAGNOSTICS

DOI: 10.1201/9781003077251-1

1.1 INTRODUCTION

With the introduction and, now, the increasing availability and high reliability of lasers, laser spectroscopy is assuming an ever expanding role in the diagnostic probing of combustion processes. Laser-based techniques supply the combustion researcher with the capability for remote, nonintrusive, in-situ, spatially and temporally precise measurements of important chemical parameters. Laser diagnostics are facilitating improved understanding of a wide variety of combustion phenomena which, in turn, will lead to improved efficiency and cleanliness in the energy conversion devices so vital to modern day life.

Lasers provide the combustion scientist and engineer not only with more accurate measurements but with completely new capabilities. Spatially precise, instantaneous measurements at high rates permit the combustion process to be frozen and tracked with high frequency response. Measurements at many locations simultaneously along a line, over a plane or several planes permit spatial correlations to be obtained providing new phenomenological insight into fundamental behavior. Atoms and molecules difficult to detect by any other means become amenable to interrogation. Furthermore, the various internal energy states can be detected permitting state-specific studies and examination of nonequilibrium phenomena. Partitioning of energy among the various internal modes allows their influence to be studied and mode-specific temperatures to be determined. Laser techniques permit accurate interrogation in easily perturbed regions such as recirculation zones, boundary layers, flame fronts and supersonic streams. Measurements in confined vessels and hostile environments become feasible and laser spectroscopic probes have been applied to diesel engines, afterburning jet engine exhausts, furnaces and coal gasifiers. Laser approaches have experimentally confirmed important phenomena such as countergradient diffusion, superequilibrium radical concentration levels and differential species diffusion effects. The application of these new and exciting techniques promises to impact combustion science and technology in a very significant fashion.

This text will focus upon laser spectroscopic techniques appropriate to spatially precise measurements of temperature and species concentrations in combustion. The utilization of lasers for velocimetry (Durst et al., 1981; Adrian, 1986; Taylor, 1993) and droplet/particle sizing (Chigier, 1981; Gupta and Lilley, 1985) will not be discussed. These are highly developed areas already in widespread use in many combustion laboratories and considerable commercial instrumentation has been available for some time.

In this chapter, various laser diagnostic techniques potentially suitable for “point” temperature and species concentration measurements in combustion will be surveyed. From this screening, the various laser approaches to receive major emphasis in the book will emerge. These include: Rayleigh scattering for total density determinations and, thus, temperature under isobaric conditions; spontaneous Raman scattering and coherent anti-Stokes Raman spectroscopy (CARS) for major species concentrations, i.e., ~ 1% or greater, and temperature measurements; and laser-induced fluorescence spectroscopy (LIF) and degenerate four-wave mixing (DFWM) for thermometry and detection of reactive intermediates at low levels, O(ppm). In the infrared, DFWM is also applicable to the detection of major product species. Some attractive variants of these approaches will also be described. Before beginning the survey, we will examine the advantages of laser techniques and provide some useful background perspective. Much of the discussion here will become clearer with the detailed treatments found in later chapters.

1.2 ADVANTAGES OF LASER DIAGNOSTICS

1.2.1 Disadvantages of Physical Probing

Laser techniques offer a number of advantages over the physical probing methods which have been traditionally employed to investigate and characterize combustion phenomena (Goulard, 1976; Heitor and Moreira, 1992; Thylor, 1993). Combustion processes, despite their hostility, are easily perturbed. Physical probes, due to their intrusion, can markedly alter the fundamental flame behavior they seek to investigate. In the presence of the probe, the combustion process may be fundamentally affected, either through flow disturbance, thermally or catalytically, and behave quite differently than had the probe not been present. This is particularly true in practical devices where the flame is stabilized by flow recirculation established in various ways. The presence of a probe locally disturbs the recirculation and, in many instances, acts as a flameholder itself. In addition, physical probes are generally limited in both their spatial resolution and temporal response. Although thermocouples, when bare, can be extremely fine, their applicability is generally restricted to relatively benign flames. Under high aerodynamic loads and in . the presence of particles, e.g., fuel droplets and soot, they must be protected and, as a result, their spatial resolution is reduced, their temporal response diminished and their accuracy degraded. Closely related is the matter of probe survival at high pressures and temperatures. As the environment of interest becomes more severe from a heat transfer standpoint, the probes must be made more physically robust. This increases the chance for perturbation and degrades spatial and temporal precision. Furthermore, physical probes do not perform measurements in situ and need to be “corrected,” a process which can introduce ambiguity into a measurement. A thermocouple, for example, measures its own temperature, not the gas temperature. Radiation, convection and conduction corrections have to be made and these vary depending on the environment of the measurement. Gas sampling probes generally measure chemical composition far removed from the sampling location. Great care must be exercised in their design and use to avoid chemical transformation at the sampling orifice and in the lines to the detection equipment.

1.2.2 Laser Probing

Advantages

Laser optical techniques, for the most part, circumvent all of the foregoing disadvantages. The techniques are remote and, almost always, nonperturbing. Thus, they are ideally suited for application to recirculation zones, boundary layers, and confined locations such as the combustion zone of internal combustion engines. The measurements are performed in situ, i.e., at the measurement location, and thus they are usually unambiguous. Temperatures and species concentrations are measured directly without the requirement for elaborate and often uncertain “corrections.” They are capable of simultaneous high spatial and temporal resolution. Measurement volumes are typically cylindrical with diameters of the order of 50 to 500 μm and lengths down to 100 μm with several millimeters being typical. Single pulse measurements are possible with measurement times of the order of 10⁻⁸ to 10⁻⁶ sec depending on the type of laser employed. With recently emergent femtosecond (10⁻¹⁵ sec) laser technology, subpicosecond (≤ 10⁻¹² sec) resolution can be obtained to study very fast chemical reactions or energy transfer processes, albeit time averaged over many experimental measurement cycles. With continuous lasers, i.e., cw or continuous wave, high temporal response may accrue from appropriate time resolution of the signal. In practice, compromises in either or both the spare and time scales may be necessary to ensure adequately high signal levels. Obviously, because laser techniques are nonintrusive, there is no upper temperature limit in their applicability. Furthermore, the techniques are not limited to equilibrium situations and therefore can be employed to diagnose nonequilibrium phenomena.

Disadvantages

Of course, laser techniques are not without their disadvantages. A major limitation is the requirement for optical access to the test volume. This often necessitates the addition of windows to an enclosure. In some cases, these can be eliminated by the use of fiber optics or small uncovered apertures if one can tolerate the attendent pressure loss incurred. Nevertheless, some geometries, e.g., annular gas turbine combustors, can prove challenging. No single laser technique is capable of measuring all species and temperature simultaneously; various approaches are complementary and several may have to be combined to characterize the medium to the extent desired. Laser spectroscopic techniques tend to be most applicable to small molecules, i.e., ≤ six atoms. For large molecules, the spectroscopy is often very complicated depending on symmetry class and the s...

Table of contents

• Cover Page
• Half Title Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Preface to the First Edition
• Nomenclature
• Chapter 1 Survey of Laser Diagnostics
• Chapter 2 Background Physics
• Chapter 3 Experimental Considerations
• Chapter 4 Application Considerations
• Chapter 5 Spontaneous Raman and Rayleigh Scattering
• Chapter 6 Coherent Anti-Stokes Raman Spectroscopy (CARS)
• Chapter 7 Laser-Induced Fluorescence (LIF)
• Chapter 8 Coherent Methods for Minor Species
• Chapter 9 Field Techniques
• Subject Index